perlre(1)



PERLRE(1)              Perl Programmers Reference Guide              PERLRE(1)

NAME
       perlre - Perl regular expressions

DESCRIPTION
       This page describes the syntax of regular expressions in Perl.

       If you haven't used regular expressions before, a tutorial introduction
       is available in perlretut.  If you know just a little about them, a
       quick-start introduction is available in perlrequick.

       Except for "The Basics" section, this page assumes you are familiar
       with regular expression basics, like what is a "pattern", what does it
       look like, and how it is basically used.  For a reference on how they
       are used, plus various examples of the same, see discussions of "m//",
       "s///", "qr//" and "??" in "Regexp Quote-Like Operators" in perlop.

       New in v5.22, "use re 'strict'" applies stricter rules than otherwise
       when compiling regular expression patterns.  It can find things that,
       while legal, may not be what you intended.

   The Basics
       Regular expressions are strings with the very particular syntax and
       meaning described in this document and auxiliary documents referred to
       by this one.  The strings are called "patterns".  Patterns are used to
       determine if some other string, called the "target", has (or doesn't
       have) the characteristics specified by the pattern.  We call this
       "matching" the target string against the pattern.  Usually the match is
       done by having the target be the first operand, and the pattern be the
       second operand, of one of the two binary operators "=~" and "!~",
       listed in "Binding Operators" in perlop; and the pattern will have been
       converted from an ordinary string by one of the operators in "Regexp
       Quote-Like Operators" in perlop, like so:

        $foo =~ m/abc/

       This evaluates to true if and only if the string in the variable $foo
       contains somewhere in it, the sequence of characters "a", "b", then
       "c".  (The "=~ m", or match operator, is described in
       "m/PATTERN/msixpodualngc" in perlop.)

       Patterns that aren't already stored in some variable must be
       delimitted, at both ends, by delimitter characters.  These are often,
       as in the example above, forward slashes, and the typical way a pattern
       is written in documentation is with those slashes.  In most cases, the
       delimitter is the same character, fore and aft, but there are a few
       cases where a character looks like it has a mirror-image mate, where
       the opening version is the beginning delimiter, and the closing one is
       the ending delimiter, like

        $foo =~ m<abc>

       Most times, the pattern is evaluated in double-quotish context, but it
       is possible to choose delimiters to force single-quotish, like

        $foo =~ m'abc'

       If the pattern contains its delimiter within it, that delimiter must be
       escaped.  Prefixing it with a backslash (e.g., "/foo\/bar/") serves
       this purpose.

       Any single character in a pattern matches that same character in the
       target string, unless the character is a metacharacter with a special
       meaning described in this document.  A sequence of non-metacharacters
       matches the same sequence in the target string, as we saw above with
       "m/abc/".

       Only a few characters (all of them being ASCII punctuation characters)
       are metacharacters.  The most commonly used one is a dot ".", which
       normally matches almost any character (including a dot itself).

       You can cause characters that normally function as metacharacters to be
       interpreted literally by prefixing them with a "\", just like the
       pattern's delimiter must be escaped if it also occurs within the
       pattern.  Thus, "\." matches just a literal dot, "." instead of its
       normal meaning.  This means that the backslash is also a metacharacter,
       so "\\" matches a single "\".  And a sequence that contains an escaped
       metacharacter matches the same sequence (but without the escape) in the
       target string.  So, the pattern "/blur\\fl/" would match any target
       string that contains the sequence "blur\fl".

       The metacharacter "|" is used to match one thing or another.  Thus

        $foo =~ m/this|that/

       is TRUE if and only if $foo contains either the sequence "this" or the
       sequence "that".  Like all metacharacters, prefixing the "|" with a
       backslash makes it match the plain punctuation character; in its case,
       the VERTICAL LINE.

        $foo =~ m/this\|that/

       is TRUE if and only if $foo contains the sequence "this|that".

       You aren't limited to just a single "|".

        $foo =~ m/fee|fie|foe|fum/

       is TRUE if and only if $foo contains any of those 4 sequences from the
       children's story "Jack and the Beanstalk".

       As you can see, the "|" binds less tightly than a sequence of ordinary
       characters.  We can override this by using the grouping metacharacters,
       the parentheses "(" and ")".

        $foo =~ m/th(is|at) thing/

       is TRUE if and only if $foo contains either the sequence "thisthing" or
       the sequence "thatthing".  The portions of the string that match the
       portions of the pattern enclosed in parentheses are normally made
       available separately for use later in the pattern, substitution, or
       program.  This is called "capturing", and it can get complicated.  See
       "Capture groups".

       The first alternative includes everything from the last pattern
       delimiter ("(", "(?:" (described later), etc. or the beginning of the
       pattern) up to the first "|", and the last alternative contains
       everything from the last "|" to the next closing pattern delimiter.
       That's why it's common practice to include alternatives in parentheses:
       to minimize confusion about where they start and end.

       Alternatives are tried from left to right, so the first alternative
       found for which the entire expression matches, is the one that is
       chosen. This means that alternatives are not necessarily greedy. For
       example: when matching "foo|foot" against "barefoot", only the "foo"
       part will match, as that is the first alternative tried, and it
       successfully matches the target string. (This might not seem important,
       but it is important when you are capturing matched text using
       parentheses.)

       Besides taking away the special meaning of a metacharacter, a prefixed
       backslash changes some letter and digit characters away from matching
       just themselves to instead have special meaning.  These are called
       "escape sequences", and all such are described in perlrebackslash.  A
       backslash sequence (of a letter or digit) that doesn't currently have
       special meaning to Perl will raise a warning if warnings are enabled,
       as those are reserved for potential future use.

       One such sequence is "\b", which matches a boundary of some sort.
       "\b{wb}" and a few others give specialized types of boundaries.  (They
       are all described in detail starting at "\b{}, \b, \B{}, \B" in
       perlrebackslash.)  Note that these don't match characters, but the
       zero-width spaces between characters.  They are an example of a zero-
       width assertion.  Consider again,

        $foo =~ m/fee|fie|foe|fum/

       It evaluates to TRUE if, besides those 4 words, any of the sequences
       "feed", "field", "Defoe", "fume", and many others are in $foo.  By
       judicious use of "\b" (or better (because it is designed to handle
       natural language) "\b{wb}"), we can make sure that only the Giant's
       words are matched:

        $foo =~ m/\b(fee|fie|foe|fum)\b/
        $foo =~ m/\b{wb}(fee|fie|foe|fum)\b{wb}/

       The final example shows that the characters "{" and "}" are
       metacharacters.

       Another use for escape sequences is to specify characters that cannot
       (or which you prefer not to) be written literally.  These are described
       in detail in "Character Escapes" in perlrebackslash, but the next three
       paragraphs briefly describe some of them.

       Various control characters can be written in C language style: "\n"
       matches a newline, "\t" a tab, "\r" a carriage return, "\f" a form
       feed, etc.

       More generally, "\nnn", where nnn is a string of three octal digits,
       matches the character whose native code point is nnn.  You can easily
       run into trouble if you don't have exactly three digits.  So always use
       three, or since Perl 5.14, you can use "\o{...}" to specify any number
       of octal digits.

       Similarly, "\xnn", where nn are hexadecimal digits, matches the
       character whose native ordinal is nn.  Again, not using exactly two
       digits is a recipe for disaster, but you can use "\x{...}" to specify
       any number of hex digits.

       Besides being a metacharacter, the "." is an example of a "character
       class", something that can match any single character of a given set of
       them.  In its case, the set is just about all possible characters.
       Perl predefines several character classes besides the "."; there is a
       separate reference page about just these, perlrecharclass.

       You can define your own custom character classes, by putting into your
       pattern in the appropriate place(s), a list of all the characters you
       want in the set.  You do this by enclosing the list within "[]" bracket
       characters.  These are called "bracketed character classes" when we are
       being precise, but often the word "bracketed" is dropped.  (Dropping it
       usually doesn't cause confusion.)  This means that the "[" character is
       another metacharacter.  It doesn't match anything just by itelf; it is
       used only to tell Perl that what follows it is a bracketed character
       class.  If you want to match a literal left square bracket, you must
       escape it, like "\[".  The matching "]" is also a metacharacter; again
       it doesn't match anything by itself, but just marks the end of your
       custom class to Perl.  It is an example of a "sometimes metacharacter".
       It isn't a metacharacter if there is no corresponding "[", and matches
       its literal self:

        print "]" =~ /]/;  # prints 1

       The list of characters within the character class gives the set of
       characters matched by the class.  "[abc]" matches a single "a" or "b"
       or "c".  But if the first character after the "[" is "^", the class
       matches any character not in the list.  Within a list, the "-"
       character specifies a range of characters, so that "a-z" represents all
       characters between "a" and "z", inclusive.  If you want either "-" or
       "]" itself to be a member of a class, put it at the start of the list
       (possibly after a "^"), or escape it with a backslash.  "-" is also
       taken literally when it is at the end of the list, just before the
       closing "]".  (The following all specify the same class of three
       characters: "[-az]", "[az-]", and "[a\-z]".  All are different from
       "[a-z]", which specifies a class containing twenty-six characters, even
       on EBCDIC-based character sets.)

       There is lots more to bracketed character classes; full details are in
       "Bracketed Character Classes" in perlrecharclass.

       Metacharacters

       "The Basics" introduced some of the metacharacters.  This section gives
       them all.  Most of them have the same meaning as in the egrep command.

       Only the "\" is always a metacharacter.  The others are metacharacters
       just sometimes.  The following tables lists all of them, summarizes
       their use, and gives the contexts where they are metacharacters.
       Outside those contexts or if prefixed by a "\", they match their
       corresponding punctuation character.  In some cases, their meaning
       varies depending on various pattern modifiers that alter the default
       behaviors.  See "Modifiers".

                   PURPOSE                                  WHERE
        \   Escape the next character                    Always, except when
                                                         escaped by another \
        ^   Match the beginning of the string            Not in []
              (or line, if /m is used)
        ^   Complement the [] class                      At the beginning of []
        .   Match any single character except newline    Not in []
              (under /s, includes newline)
        $   Match the end of the string                  Not in [], but can
              (or before newline at the end of the       mean interpolate a
              string; or before any newline if /m is     scalar
              used)
        |   Alternation                                  Not in []
        ()  Grouping                                     Not in []
        [   Start Bracketed Character class              Not in []
        ]   End Bracketed Character class                Only in [], and
                                                           not first
        *   Matches the preceding element 0 or more      Not in []
              times
        +   Matches the preceding element 1 or more      Not in []
              times
        ?   Matches the preceding element 0 or 1         Not in []
              times
        {   Starts a sequence that gives number(s)       Not in []
              of times the preceding element can be
              matched
        {   when following certain escape sequences
              starts a modifier to the meaning of the
              sequence
        }   End sequence started by {
        -   Indicates a range                            Only in [] interior

       Notice that most of the metacharacters lose their special meaning when
       they occur in a bracketed character class, except "^" has a different
       meaning when it is at the beginning of such a class.  And "-" and "]"
       are metacharacters only at restricted positions within bracketed
       character classes; while "}" is a metacharacter only when closing a
       special construct started by "{".

       In double-quotish context, as is usually the case,  you need to be
       careful about "$" and the non-metacharacter "@".  Those could
       interpolate variables, which may or may not be what you intended.

       These rules were designed for compactness of expression, rather than
       legibility and maintainability.  The "/x and /xx" pattern modifiers
       allow you to insert white space to improve readability.  And use of
       "re'strict'" adds extra checking to catch some typos that might
       silently compile into something unintended.

       By default, the "^" character is guaranteed to match only the beginning
       of the string, the "$" character only the end (or before the newline at
       the end), and Perl does certain optimizations with the assumption that
       the string contains only one line.  Embedded newlines will not be
       matched by "^" or "$".  You may, however, wish to treat a string as a
       multi-line buffer, such that the "^" will match after any newline
       within the string (except if the newline is the last character in the
       string), and "$" will match before any newline.  At the cost of a
       little more overhead, you can do this by using the ""/m"" modifier on
       the pattern match operator.  (Older programs did this by setting $*,
       but this option was removed in perl 5.10.)

       To simplify multi-line substitutions, the "." character never matches a
       newline unless you use the "/s" modifier, which in effect tells Perl to
       pretend the string is a single line--even if it isn't.

   Modifiers
       Overview

       The default behavior for matching can be changed, using various
       modifiers.  Modifiers that relate to the interpretation of the pattern
       are listed just below.  Modifiers that alter the way a pattern is used
       by Perl are detailed in "Regexp Quote-Like Operators" in perlop and
       "Gory details of parsing quoted constructs" in perlop.

       "m" Treat the string being matched against as multiple lines.  That is,
           change "^" and "$" from matching the start of the string's first
           line and the end of its last line to matching the start and end of
           each line within the string.

       "s" Treat the string as single line.  That is, change "." to match any
           character whatsoever, even a newline, which normally it would not
           match.

           Used together, as "/ms", they let the "." match any character
           whatsoever, while still allowing "^" and "$" to match,
           respectively, just after and just before newlines within the
           string.

       "i" Do case-insensitive pattern matching.  For example, "A" will match
           "a" under "/i".

           If locale matching rules are in effect, the case map is taken from
           the current locale for code points less than 255, and from Unicode
           rules for larger code points.  However, matches that would cross
           the Unicode rules/non-Unicode rules boundary (ords 255/256) will
           not succeed, unless the locale is a UTF-8 one.  See perllocale.

           There are a number of Unicode characters that match a sequence of
           multiple characters under "/i".  For example, "LATIN SMALL LIGATURE
           FI" should match the sequence "fi".  Perl is not currently able to
           do this when the multiple characters are in the pattern and are
           split between groupings, or when one or more are quantified.  Thus

            "\N{LATIN SMALL LIGATURE FI}" =~ /fi/i;          # Matches
            "\N{LATIN SMALL LIGATURE FI}" =~ /[fi][fi]/i;    # Doesn't match!
            "\N{LATIN SMALL LIGATURE FI}" =~ /fi*/i;         # Doesn't match!

            # The below doesn't match, and it isn't clear what $1 and $2 would
            # be even if it did!!
            "\N{LATIN SMALL LIGATURE FI}" =~ /(f)(i)/i;      # Doesn't match!

           Perl doesn't match multiple characters in a bracketed character
           class unless the character that maps to them is explicitly
           mentioned, and it doesn't match them at all if the character class
           is inverted, which otherwise could be highly confusing.  See
           "Bracketed Character Classes" in perlrecharclass, and "Negation" in
           perlrecharclass.

       "x" and "xx"
           Extend your pattern's legibility by permitting whitespace and
           comments.  Details in "/x and  /xx"

       "p" Preserve the string matched such that "${^PREMATCH}", "${^MATCH}",
           and "${^POSTMATCH}" are available for use after matching.

           In Perl 5.20 and higher this is ignored. Due to a new copy-on-write
           mechanism, "${^PREMATCH}", "${^MATCH}", and "${^POSTMATCH}" will be
           available after the match regardless of the modifier.

       "a", "d", "l", and "u"
           These modifiers, all new in 5.14, affect which character-set rules
           (Unicode, etc.) are used, as described below in "Character set
           modifiers".

       "n" Prevent the grouping metacharacters "()" from capturing. This
           modifier, new in 5.22, will stop $1, $2, etc... from being filled
           in.

             "hello" =~ /(hi|hello)/;   # $1 is "hello"
             "hello" =~ /(hi|hello)/n;  # $1 is undef

           This is equivalent to putting "?:" at the beginning of every
           capturing group:

             "hello" =~ /(?:hi|hello)/; # $1 is undef

           "/n" can be negated on a per-group basis. Alternatively, named
           captures may still be used.

             "hello" =~ /(?-n:(hi|hello))/n;   # $1 is "hello"
             "hello" =~ /(?<greet>hi|hello)/n; # $1 is "hello", $+{greet} is
                                               # "hello"

       Other Modifiers
           There are a number of flags that can be found at the end of regular
           expression constructs that are not generic regular expression
           flags, but apply to the operation being performed, like matching or
           substitution ("m//" or "s///" respectively).

           Flags described further in "Using regular expressions in Perl" in
           perlretut are:

             c  - keep the current position during repeated matching
             g  - globally match the pattern repeatedly in the string

           Substitution-specific modifiers described in
           "s/PATTERN/REPLACEMENT/msixpodualngcer" in perlop are:

             e  - evaluate the right-hand side as an expression
             ee - evaluate the right side as a string then eval the result
             o  - pretend to optimize your code, but actually introduce bugs
             r  - perform non-destructive substitution and return the new value

       Regular expression modifiers are usually written in documentation as
       e.g., "the "/x" modifier", even though the delimiter in question might
       not really be a slash.  The modifiers "/imnsxadlup" may also be
       embedded within the regular expression itself using the "(?...)"
       construct, see "Extended Patterns" below.

       Details on some modifiers

       Some of the modifiers require more explanation than given in the
       "Overview" above.

       "/x" and  "/xx"

       A single "/x" tells the regular expression parser to ignore most
       whitespace that is neither backslashed nor within a bracketed character
       class.  You can use this to break up your regular expression into more
       readable parts.  Also, the "#" character is treated as a metacharacter
       introducing a comment that runs up to the pattern's closing delimiter,
       or to the end of the current line if the pattern extends onto the next
       line.  Hence, this is very much like an ordinary Perl code comment.
       (You can include the closing delimiter within the comment only if you
       precede it with a backslash, so be careful!)

       Use of "/x" means that if you want real whitespace or "#" characters in
       the pattern (outside a bracketed character class, which is unaffected
       by "/x"), then you'll either have to escape them (using backslashes or
       "\Q...\E") or encode them using octal, hex, or "\N{}" escapes.  It is
       ineffective to try to continue a comment onto the next line by escaping
       the "\n" with a backslash or "\Q".

       You can use "(?#text)" to create a comment that ends earlier than the
       end of the current line, but "text" also can't contain the closing
       delimiter unless escaped with a backslash.

       A common pitfall is to forget that "#" characters begin a comment under
       "/x" and are not matched literally.  Just keep that in mind when trying
       to puzzle out why a particular "/x" pattern isn't working as expected.

       Starting in Perl v5.26, if the modifier has a second "x" within it, it
       does everything that a single "/x" does, but additionally non-
       backslashed SPACE and TAB characters within bracketed character classes
       are also generally ignored, and hence can be added to make the classes
       more readable.

           / [d-e g-i 3-7]/xx
           /[ ! @ " # $ % ^ & * () = ? <> ' ]/xx

       may be easier to grasp than the squashed equivalents

           /[d-eg-i3-7]/
           /[!@"#$%^&*()=?<>']/

       Taken together, these features go a long way towards making Perl's
       regular expressions more readable.  Here's an example:

           # Delete (most) C comments.
           $program =~ s {
               /\*     # Match the opening delimiter.
               .*?     # Match a minimal number of characters.
               \*/     # Match the closing delimiter.
           } []gsx;

       Note that anything inside a "\Q...\E" stays unaffected by "/x".  And
       note that "/x" doesn't affect space interpretation within a single
       multi-character construct.  For example in "\x{...}", regardless of the
       "/x" modifier, there can be no spaces.  Same for a quantifier such as
       "{3}" or "{5,}".  Similarly, "(?:...)" can't have a space between the
       "{", "?", and ":".  Within any delimiters for such a construct, allowed
       spaces are not affected by "/x", and depend on the construct.  For
       example, "\x{...}" can't have spaces because hexadecimal numbers don't
       have spaces in them.  But, Unicode properties can have spaces, so in
       "\p{...}" there can be spaces that follow the Unicode rules, for which
       see "Properties accessible through \p{} and \P{}" in perluniprops.

       The set of characters that are deemed whitespace are those that Unicode
       calls "Pattern White Space", namely:

        U+0009 CHARACTER TABULATION
        U+000A LINE FEED
        U+000B LINE TABULATION
        U+000C FORM FEED
        U+000D CARRIAGE RETURN
        U+0020 SPACE
        U+0085 NEXT LINE
        U+200E LEFT-TO-RIGHT MARK
        U+200F RIGHT-TO-LEFT MARK
        U+2028 LINE SEPARATOR
        U+2029 PARAGRAPH SEPARATOR

       Character set modifiers

       "/d", "/u", "/a", and "/l", available starting in 5.14, are called the
       character set modifiers; they affect the character set rules used for
       the regular expression.

       The "/d", "/u", and "/l" modifiers are not likely to be of much use to
       you, and so you need not worry about them very much.  They exist for
       Perl's internal use, so that complex regular expression data structures
       can be automatically serialized and later exactly reconstituted,
       including all their nuances.  But, since Perl can't keep a secret, and
       there may be rare instances where they are useful, they are documented
       here.

       The "/a" modifier, on the other hand, may be useful.  Its purpose is to
       allow code that is to work mostly on ASCII data to not have to concern
       itself with Unicode.

       Briefly, "/l" sets the character set to that of whatever Locale is in
       effect at the time of the execution of the pattern match.

       "/u" sets the character set to Unicode.

       "/a" also sets the character set to Unicode, BUT adds several
       restrictions for ASCII-safe matching.

       "/d" is the old, problematic, pre-5.14 Default character set behavior.
       Its only use is to force that old behavior.

       At any given time, exactly one of these modifiers is in effect.  Their
       existence allows Perl to keep the originally compiled behavior of a
       regular expression, regardless of what rules are in effect when it is
       actually executed.  And if it is interpolated into a larger regex, the
       original's rules continue to apply to it, and only it.

       The "/l" and "/u" modifiers are automatically selected for regular
       expressions compiled within the scope of various pragmas, and we
       recommend that in general, you use those pragmas instead of specifying
       these modifiers explicitly.  For one thing, the modifiers affect only
       pattern matching, and do not extend to even any replacement done,
       whereas using the pragmas gives consistent results for all appropriate
       operations within their scopes.  For example,

        s/foo/\Ubar/il

       will match "foo" using the locale's rules for case-insensitive
       matching, but the "/l" does not affect how the "\U" operates.  Most
       likely you want both of them to use locale rules.  To do this, instead
       compile the regular expression within the scope of "use locale".  This
       both implicitly adds the "/l", and applies locale rules to the "\U".
       The lesson is to "use locale", and not "/l" explicitly.

       Similarly, it would be better to use "use feature 'unicode_strings'"
       instead of,

        s/foo/\Lbar/iu

       to get Unicode rules, as the "\L" in the former (but not necessarily
       the latter) would also use Unicode rules.

       More detail on each of the modifiers follows.  Most likely you don't
       need to know this detail for "/l", "/u", and "/d", and can skip ahead
       to /a.

       /l

       means to use the current locale's rules (see perllocale) when pattern
       matching.  For example, "\w" will match the "word" characters of that
       locale, and "/i" case-insensitive matching will match according to the
       locale's case folding rules.  The locale used will be the one in effect
       at the time of execution of the pattern match.  This may not be the
       same as the compilation-time locale, and can differ from one match to
       another if there is an intervening call of the setlocale() function.

       Prior to v5.20, Perl did not support multi-byte locales.  Starting
       then, UTF-8 locales are supported.  No other multi byte locales are
       ever likely to be supported.  However, in all locales, one can have
       code points above 255 and these will always be treated as Unicode no
       matter what locale is in effect.

       Under Unicode rules, there are a few case-insensitive matches that
       cross the 255/256 boundary.  Except for UTF-8 locales in Perls v5.20
       and later, these are disallowed under "/l".  For example, 0xFF (on
       ASCII platforms) does not caselessly match the character at 0x178,
       "LATIN CAPITAL LETTER Y WITH DIAERESIS", because 0xFF may not be "LATIN
       SMALL LETTER Y WITH DIAERESIS" in the current locale, and Perl has no
       way of knowing if that character even exists in the locale, much less
       what code point it is.

       In a UTF-8 locale in v5.20 and later, the only visible difference
       between locale and non-locale in regular expressions should be tainting
       (see perlsec).

       This modifier may be specified to be the default by "use locale", but
       see "Which character set modifier is in effect?".

       /u

       means to use Unicode rules when pattern matching.  On ASCII platforms,
       this means that the code points between 128 and 255 take on their
       Latin-1 (ISO-8859-1) meanings (which are the same as Unicode's).
       (Otherwise Perl considers their meanings to be undefined.)  Thus, under
       this modifier, the ASCII platform effectively becomes a Unicode
       platform; and hence, for example, "\w" will match any of the more than
       100_000 word characters in Unicode.

       Unlike most locales, which are specific to a language and country pair,
       Unicode classifies all the characters that are letters somewhere in the
       world as "\w".  For example, your locale might not think that "LATIN
       SMALL LETTER ETH" is a letter (unless you happen to speak Icelandic),
       but Unicode does.  Similarly, all the characters that are decimal
       digits somewhere in the world will match "\d"; this is hundreds, not
       10, possible matches.  And some of those digits look like some of the
       10 ASCII digits, but mean a different number, so a human could easily
       think a number is a different quantity than it really is.  For example,
       "BENGALI DIGIT FOUR" (U+09EA) looks very much like an "ASCII DIGIT
       EIGHT" (U+0038).  And, "\d+", may match strings of digits that are a
       mixture from different writing systems, creating a security issue.
       "num()" in Unicode::UCD can be used to sort this out.  Or the "/a"
       modifier can be used to force "\d" to match just the ASCII 0 through 9.

       Also, under this modifier, case-insensitive matching works on the full
       set of Unicode characters.  The "KELVIN SIGN", for example matches the
       letters "k" and "K"; and "LATIN SMALL LIGATURE FF" matches the sequence
       "ff", which, if you're not prepared, might make it look like a
       hexadecimal constant, presenting another potential security issue.  See
       <http://unicode.org/reports/tr36> for a detailed discussion of Unicode
       security issues.

       This modifier may be specified to be the default by "use feature
       'unicode_strings", "use locale ':not_characters'", or "use 5.012" (or
       higher), but see "Which character set modifier is in effect?".

       /d

       This modifier means to use the "Default" native rules of the platform
       except when there is cause to use Unicode rules instead, as follows:

       1.  the target string is encoded in UTF-8; or

       2.  the pattern is encoded in UTF-8; or

       3.  the pattern explicitly mentions a code point that is above 255 (say
           by "\x{100}"); or

       4.  the pattern uses a Unicode name ("\N{...}");  or

       5.  the pattern uses a Unicode property ("\p{...}" or "\P{...}"); or

       6.  the pattern uses a Unicode break ("\b{...}" or "\B{...}"); or

       7.  the pattern uses ""(?[ ])""

       Another mnemonic for this modifier is "Depends", as the rules actually
       used depend on various things, and as a result you can get unexpected
       results.  See "The "Unicode Bug"" in perlunicode.  The Unicode Bug has
       become rather infamous, leading to yet another (printable) name for
       this modifier, "Dodgy".

       Unless the pattern or string are encoded in UTF-8, only ASCII
       characters can match positively.

       Here are some examples of how that works on an ASCII platform:

        $str =  "\xDF";      # $str is not in UTF-8 format.
        $str =~ /^\w/;       # No match, as $str isn't in UTF-8 format.
        $str .= "\x{0e0b}";  # Now $str is in UTF-8 format.
        $str =~ /^\w/;       # Match! $str is now in UTF-8 format.
        chop $str;
        $str =~ /^\w/;       # Still a match! $str remains in UTF-8 format.

       This modifier is automatically selected by default when none of the
       others are, so yet another name for it is "Default".

       Because of the unexpected behaviors associated with this modifier, you
       probably should only explicitly use it to maintain weird backward
       compatibilities.

       /a (and /aa)

       This modifier stands for ASCII-restrict (or ASCII-safe).  This modifier
       may be doubled-up to increase its effect.

       When it appears singly, it causes the sequences "\d", "\s", "\w", and
       the Posix character classes to match only in the ASCII range.  They
       thus revert to their pre-5.6, pre-Unicode meanings.  Under "/a",  "\d"
       always means precisely the digits "0" to "9"; "\s" means the five
       characters "[ \f\n\r\t]", and starting in Perl v5.18, the vertical tab;
       "\w" means the 63 characters "[A-Za-z0-9_]"; and likewise, all the
       Posix classes such as "[[:print:]]" match only the appropriate ASCII-
       range characters.

       This modifier is useful for people who only incidentally use Unicode,
       and who do not wish to be burdened with its complexities and security
       concerns.

       With "/a", one can write "\d" with confidence that it will only match
       ASCII characters, and should the need arise to match beyond ASCII, you
       can instead use "\p{Digit}" (or "\p{Word}" for "\w").  There are
       similar "\p{...}" constructs that can match beyond ASCII both white
       space (see "Whitespace" in perlrecharclass), and Posix classes (see
       "POSIX Character Classes" in perlrecharclass).  Thus, this modifier
       doesn't mean you can't use Unicode, it means that to get Unicode
       matching you must explicitly use a construct ("\p{}", "\P{}") that
       signals Unicode.

       As you would expect, this modifier causes, for example, "\D" to mean
       the same thing as "[^0-9]"; in fact, all non-ASCII characters match
       "\D", "\S", and "\W".  "\b" still means to match at the boundary
       between "\w" and "\W", using the "/a" definitions of them (similarly
       for "\B").

       Otherwise, "/a" behaves like the "/u" modifier, in that case-
       insensitive matching uses Unicode rules; for example, "k" will match
       the Unicode "\N{KELVIN SIGN}" under "/i" matching, and code points in
       the Latin1 range, above ASCII will have Unicode rules when it comes to
       case-insensitive matching.

       To forbid ASCII/non-ASCII matches (like "k" with "\N{KELVIN SIGN}"),
       specify the "a" twice, for example "/aai" or "/aia".  (The first
       occurrence of "a" restricts the "\d", etc., and the second occurrence
       adds the "/i" restrictions.)  But, note that code points outside the
       ASCII range will use Unicode rules for "/i" matching, so the modifier
       doesn't really restrict things to just ASCII; it just forbids the
       intermixing of ASCII and non-ASCII.

       To summarize, this modifier provides protection for applications that
       don't wish to be exposed to all of Unicode.  Specifying it twice gives
       added protection.

       This modifier may be specified to be the default by "use re '/a'" or
       "use re '/aa'".  If you do so, you may actually have occasion to use
       the "/u" modifier explicitly if there are a few regular expressions
       where you do want full Unicode rules (but even here, it's best if
       everything were under feature "unicode_strings", along with the "use re
       '/aa'").  Also see "Which character set modifier is in effect?".

       Which character set modifier is in effect?

       Which of these modifiers is in effect at any given point in a regular
       expression depends on a fairly complex set of interactions.  These have
       been designed so that in general you don't have to worry about it, but
       this section gives the gory details.  As explained below in "Extended
       Patterns" it is possible to explicitly specify modifiers that apply
       only to portions of a regular expression.  The innermost always has
       priority over any outer ones, and one applying to the whole expression
       has priority over any of the default settings that are described in the
       remainder of this section.

       The "use re '/foo'" pragma can be used to set default modifiers
       (including these) for regular expressions compiled within its scope.
       This pragma has precedence over the other pragmas listed below that
       also change the defaults.

       Otherwise, "use locale" sets the default modifier to "/l"; and "use
       feature 'unicode_strings", or "use 5.012" (or higher) set the default
       to "/u" when not in the same scope as either "use locale" or "use
       bytes".  ("use locale ':not_characters'" also sets the default to "/u",
       overriding any plain "use locale".)  Unlike the mechanisms mentioned
       above, these affect operations besides regular expressions pattern
       matching, and so give more consistent results with other operators,
       including using "\U", "\l", etc. in substitution replacements.

       If none of the above apply, for backwards compatibility reasons, the
       "/d" modifier is the one in effect by default.  As this can lead to
       unexpected results, it is best to specify which other rule set should
       be used.

       Character set modifier behavior prior to Perl 5.14

       Prior to 5.14, there were no explicit modifiers, but "/l" was implied
       for regexes compiled within the scope of "use locale", and "/d" was
       implied otherwise.  However, interpolating a regex into a larger regex
       would ignore the original compilation in favor of whatever was in
       effect at the time of the second compilation.  There were a number of
       inconsistencies (bugs) with the "/d" modifier, where Unicode rules
       would be used when inappropriate, and vice versa.  "\p{}" did not imply
       Unicode rules, and neither did all occurrences of "\N{}", until 5.12.

   Regular Expressions
       Quantifiers

       Quantifiers are used when a particular portion of a pattern needs to
       match a certain number (or numbers) of times.  If there isn't a
       quantifier the number of times to match is exactly one.  The following
       standard quantifiers are recognized:

           *           Match 0 or more times
           +           Match 1 or more times
           ?           Match 1 or 0 times
           {n}         Match exactly n times
           {n,}        Match at least n times
           {n,m}       Match at least n but not more than m times

       (If a non-escaped curly bracket occurs in a context other than one of
       the quantifiers listed above, where it does not form part of a
       backslashed sequence like "\x{...}", it is either a fatal syntax error,
       or treated as a regular character, generally with a deprecation warning
       raised.  To escape it, you can precede it with a backslash ("\{") or
       enclose it within square brackets  ("[{]").  This change will allow for
       future syntax extensions (like making the lower bound of a quantifier
       optional), and better error checking of quantifiers).

       The "*" quantifier is equivalent to "{0,}", the "+" quantifier to
       "{1,}", and the "?" quantifier to "{0,1}".  n and m are limited to non-
       negative integral values less than a preset limit defined when perl is
       built.  This is usually 32766 on the most common platforms.  The actual
       limit can be seen in the error message generated by code such as this:

           $_ **= $_ , / {$_} / for 2 .. 42;

       By default, a quantified subpattern is "greedy", that is, it will match
       as many times as possible (given a particular starting location) while
       still allowing the rest of the pattern to match.  If you want it to
       match the minimum number of times possible, follow the quantifier with
       a "?".  Note that the meanings don't change, just the "greediness":

           *?        Match 0 or more times, not greedily
           +?        Match 1 or more times, not greedily
           ??        Match 0 or 1 time, not greedily
           {n}?      Match exactly n times, not greedily (redundant)
           {n,}?     Match at least n times, not greedily
           {n,m}?    Match at least n but not more than m times, not greedily

       Normally when a quantified subpattern does not allow the rest of the
       overall pattern to match, Perl will backtrack. However, this behaviour
       is sometimes undesirable. Thus Perl provides the "possessive"
       quantifier form as well.

        *+     Match 0 or more times and give nothing back
        ++     Match 1 or more times and give nothing back
        ?+     Match 0 or 1 time and give nothing back
        {n}+   Match exactly n times and give nothing back (redundant)
        {n,}+  Match at least n times and give nothing back
        {n,m}+ Match at least n but not more than m times and give nothing back

       For instance,

          'aaaa' =~ /a++a/

       will never match, as the "a++" will gobble up all the "a"'s in the
       string and won't leave any for the remaining part of the pattern. This
       feature can be extremely useful to give perl hints about where it
       shouldn't backtrack. For instance, the typical "match a double-quoted
       string" problem can be most efficiently performed when written as:

          /"(?:[^"\\]++|\\.)*+"/

       as we know that if the final quote does not match, backtracking will
       not help. See the independent subexpression ""(?>pattern)"" for more
       details; possessive quantifiers are just syntactic sugar for that
       construct. For instance the above example could also be written as
       follows:

          /"(?>(?:(?>[^"\\]+)|\\.)*)"/

       Note that the possessive quantifier modifier can not be be combined
       with the non-greedy modifier. This is because it would make no sense.
       Consider the follow equivalency table:

           Illegal         Legal
           ------------    ------
           X??+            X{0}
           X+?+            X{1}
           X{min,max}?+    X{min}

       Escape sequences

       Because patterns are processed as double-quoted strings, the following
       also work:

        \t          tab                   (HT, TAB)
        \n          newline               (LF, NL)
        \r          return                (CR)
        \f          form feed             (FF)
        \a          alarm (bell)          (BEL)
        \e          escape (think troff)  (ESC)
        \cK         control char          (example: VT)
        \x{}, \x00  character whose ordinal is the given hexadecimal number
        \N{name}    named Unicode character or character sequence
        \N{U+263D}  Unicode character     (example: FIRST QUARTER MOON)
        \o{}, \000  character whose ordinal is the given octal number
        \l          lowercase next char (think vi)
        \u          uppercase next char (think vi)
        \L          lowercase until \E (think vi)
        \U          uppercase until \E (think vi)
        \Q          quote (disable) pattern metacharacters until \E
        \E          end either case modification or quoted section, think vi

       Details are in "Quote and Quote-like Operators" in perlop.

       Character Classes and other Special Escapes

       In addition, Perl defines the following:

        Sequence   Note    Description
         [...]     [1]  Match a character according to the rules of the
                          bracketed character class defined by the "...".
                          Example: [a-z] matches "a" or "b" or "c" ... or "z"
         [[:...:]] [2]  Match a character according to the rules of the POSIX
                          character class "..." within the outer bracketed
                          character class.  Example: [[:upper:]] matches any
                          uppercase character.
         (?[...])  [8]  Extended bracketed character class
         \w        [3]  Match a "word" character (alphanumeric plus "_", plus
                          other connector punctuation chars plus Unicode
                          marks)
         \W        [3]  Match a non-"word" character
         \s        [3]  Match a whitespace character
         \S        [3]  Match a non-whitespace character
         \d        [3]  Match a decimal digit character
         \D        [3]  Match a non-digit character
         \pP       [3]  Match P, named property.  Use \p{Prop} for longer names
         \PP       [3]  Match non-P
         \X        [4]  Match Unicode "eXtended grapheme cluster"
         \1        [5]  Backreference to a specific capture group or buffer.
                          '1' may actually be any positive integer.
         \g1       [5]  Backreference to a specific or previous group,
         \g{-1}    [5]  The number may be negative indicating a relative
                          previous group and may optionally be wrapped in
                          curly brackets for safer parsing.
         \g{name}  [5]  Named backreference
         \k<name>  [5]  Named backreference
         \K        [6]  Keep the stuff left of the \K, don't include it in $&
         \N        [7]  Any character but \n.  Not affected by /s modifier
         \v        [3]  Vertical whitespace
         \V        [3]  Not vertical whitespace
         \h        [3]  Horizontal whitespace
         \H        [3]  Not horizontal whitespace
         \R        [4]  Linebreak

       [1] See "Bracketed Character Classes" in perlrecharclass for details.

       [2] See "POSIX Character Classes" in perlrecharclass for details.

       [3] See "Backslash sequences" in perlrecharclass for details.

       [4] See "Misc" in perlrebackslash for details.

       [5] See "Capture groups" below for details.

       [6] See "Extended Patterns" below for details.

       [7] Note that "\N" has two meanings.  When of the form "\N{NAME}", it
           matches the character or character sequence whose name is "NAME";
           and similarly when of the form "\N{U+hex}", it matches the
           character whose Unicode code point is hex.  Otherwise it matches
           any character but "\n".

       [8] See "Extended Bracketed Character Classes" in perlrecharclass for
           details.

       Assertions

       Besides "^" and "$", Perl defines the following zero-width assertions:

        \b{}   Match at Unicode boundary of specified type
        \B{}   Match where corresponding \b{} doesn't match
        \b     Match a \w\W or \W\w boundary
        \B     Match except at a \w\W or \W\w boundary
        \A     Match only at beginning of string
        \Z     Match only at end of string, or before newline at the end
        \z     Match only at end of string
        \G     Match only at pos() (e.g. at the end-of-match position
               of prior m//g)

       A Unicode boundary ("\b{}"), available starting in v5.22, is a spot
       between two characters, or before the first character in the string, or
       after the final character in the string where certain criteria defined
       by Unicode are met.  See "\b{}, \b, \B{}, \B" in perlrebackslash for
       details.

       A word boundary ("\b") is a spot between two characters that has a "\w"
       on one side of it and a "\W" on the other side of it (in either order),
       counting the imaginary characters off the beginning and end of the
       string as matching a "\W".  (Within character classes "\b" represents
       backspace rather than a word boundary, just as it normally does in any
       double-quoted string.)  The "\A" and "\Z" are just like "^" and "$",
       except that they won't match multiple times when the "/m" modifier is
       used, while "^" and "$" will match at every internal line boundary.  To
       match the actual end of the string and not ignore an optional trailing
       newline, use "\z".

       The "\G" assertion can be used to chain global matches (using "m//g"),
       as described in "Regexp Quote-Like Operators" in perlop.  It is also
       useful when writing "lex"-like scanners, when you have several patterns
       that you want to match against consequent substrings of your string;
       see the previous reference.  The actual location where "\G" will match
       can also be influenced by using "pos()" as an lvalue: see "pos" in
       perlfunc. Note that the rule for zero-length matches (see "Repeated
       Patterns Matching a Zero-length Substring") is modified somewhat, in
       that contents to the left of "\G" are not counted when determining the
       length of the match. Thus the following will not match forever:

            my $string = 'ABC';
            pos($string) = 1;
            while ($string =~ /(.\G)/g) {
                print $1;
            }

       It will print 'A' and then terminate, as it considers the match to be
       zero-width, and thus will not match at the same position twice in a
       row.

       It is worth noting that "\G" improperly used can result in an infinite
       loop. Take care when using patterns that include "\G" in an
       alternation.

       Note also that "s///" will refuse to overwrite part of a substitution
       that has already been replaced; so for example this will stop after the
       first iteration, rather than iterating its way backwards through the
       string:

           $_ = "123456789";
           pos = 6;
           s/.(?=.\G)/X/g;
           print;      # prints 1234X6789, not XXXXX6789

       Capture groups

       The grouping construct "( ... )" creates capture groups (also referred
       to as capture buffers). To refer to the current contents of a group
       later on, within the same pattern, use "\g1" (or "\g{1}") for the
       first, "\g2" (or "\g{2}") for the second, and so on.  This is called a
       backreference.

       There is no limit to the number of captured substrings that you may
       use.  Groups are numbered with the leftmost open parenthesis being
       number 1, etc.  If a group did not match, the associated backreference
       won't match either. (This can happen if the group is optional, or in a
       different branch of an alternation.)  You can omit the "g", and write
       "\1", etc, but there are some issues with this form, described below.

       You can also refer to capture groups relatively, by using a negative
       number, so that "\g-1" and "\g{-1}" both refer to the immediately
       preceding capture group, and "\g-2" and "\g{-2}" both refer to the
       group before it.  For example:

               /
                (Y)            # group 1
                (              # group 2
                   (X)         # group 3
                   \g{-1}      # backref to group 3
                   \g{-3}      # backref to group 1
                )
               /x

       would match the same as "/(Y) ( (X) \g3 \g1 )/x".  This allows you to
       interpolate regexes into larger regexes and not have to worry about the
       capture groups being renumbered.

       You can dispense with numbers altogether and create named capture
       groups.  The notation is "(?<name>...)" to declare and "\g{name}" to
       reference.  (To be compatible with .Net regular expressions, "\g{name}"
       may also be written as "\k{name}", "\k<name>" or "\k'name'".)  name
       must not begin with a number, nor contain hyphens.  When different
       groups within the same pattern have the same name, any reference to
       that name assumes the leftmost defined group.  Named groups count in
       absolute and relative numbering, and so can also be referred to by
       those numbers.  (It's possible to do things with named capture groups
       that would otherwise require "(??{})".)

       Capture group contents are dynamically scoped and available to you
       outside the pattern until the end of the enclosing block or until the
       next successful match, whichever comes first.  (See "Compound
       Statements" in perlsyn.)  You can refer to them by absolute number
       (using "$1" instead of "\g1", etc); or by name via the "%+" hash, using
       "$+{name}".

       Braces are required in referring to named capture groups, but are
       optional for absolute or relative numbered ones.  Braces are safer when
       creating a regex by concatenating smaller strings.  For example if you
       have "qr/$a$b/", and $a contained "\g1", and $b contained "37", you
       would get "/\g137/" which is probably not what you intended.

       The "\g" and "\k" notations were introduced in Perl 5.10.0.  Prior to
       that there were no named nor relative numbered capture groups.
       Absolute numbered groups were referred to using "\1", "\2", etc., and
       this notation is still accepted (and likely always will be).  But it
       leads to some ambiguities if there are more than 9 capture groups, as
       "\10" could mean either the tenth capture group, or the character whose
       ordinal in octal is 010 (a backspace in ASCII).  Perl resolves this
       ambiguity by interpreting "\10" as a backreference only if at least 10
       left parentheses have opened before it.  Likewise "\11" is a
       backreference only if at least 11 left parentheses have opened before
       it.  And so on.  "\1" through "\9" are always interpreted as
       backreferences.  There are several examples below that illustrate these
       perils.  You can avoid the ambiguity by always using "\g{}" or "\g" if
       you mean capturing groups; and for octal constants always using "\o{}",
       or for "\077" and below, using 3 digits padded with leading zeros,
       since a leading zero implies an octal constant.

       The "\digit" notation also works in certain circumstances outside the
       pattern.  See "Warning on \1 Instead of $1" below for details.

       Examples:

           s/^([^ ]*) *([^ ]*)/$2 $1/;     # swap first two words

           /(.)\g1/                        # find first doubled char
                and print "'$1' is the first doubled character\n";

           /(?<char>.)\k<char>/            # ... a different way
                and print "'$+{char}' is the first doubled character\n";

           /(?'char'.)\g1/                 # ... mix and match
                and print "'$1' is the first doubled character\n";

           if (/Time: (..):(..):(..)/) {   # parse out values
               $hours = $1;
               $minutes = $2;
               $seconds = $3;
           }

           /(.)(.)(.)(.)(.)(.)(.)(.)(.)\g10/   # \g10 is a backreference
           /(.)(.)(.)(.)(.)(.)(.)(.)(.)\10/    # \10 is octal
           /((.)(.)(.)(.)(.)(.)(.)(.)(.))\10/  # \10 is a backreference
           /((.)(.)(.)(.)(.)(.)(.)(.)(.))\010/ # \010 is octal

           $a = '(.)\1';        # Creates problems when concatenated.
           $b = '(.)\g{1}';     # Avoids the problems.
           "aa" =~ /${a}/;      # True
           "aa" =~ /${b}/;      # True
           "aa0" =~ /${a}0/;    # False!
           "aa0" =~ /${b}0/;    # True
           "aa\x08" =~ /${a}0/;  # True!
           "aa\x08" =~ /${b}0/;  # False

       Several special variables also refer back to portions of the previous
       match.  $+ returns whatever the last bracket match matched.  $& returns
       the entire matched string.  (At one point $0 did also, but now it
       returns the name of the program.)  "$`" returns everything before the
       matched string.  "$'" returns everything after the matched string. And
       $^N contains whatever was matched by the most-recently closed group
       (submatch). $^N can be used in extended patterns (see below), for
       example to assign a submatch to a variable.

       These special variables, like the "%+" hash and the numbered match
       variables ($1, $2, $3, etc.) are dynamically scoped until the end of
       the enclosing block or until the next successful match, whichever comes
       first.  (See "Compound Statements" in perlsyn.)

       NOTE: Failed matches in Perl do not reset the match variables, which
       makes it easier to write code that tests for a series of more specific
       cases and remembers the best match.

       WARNING: If your code is to run on Perl 5.16 or earlier, beware that
       once Perl sees that you need one of $&, "$`", or "$'" anywhere in the
       program, it has to provide them for every pattern match.  This may
       substantially slow your program.

       Perl uses the same mechanism to produce $1, $2, etc, so you also pay a
       price for each pattern that contains capturing parentheses.  (To avoid
       this cost while retaining the grouping behaviour, use the extended
       regular expression "(?: ... )" instead.)  But if you never use $&, "$`"
       or "$'", then patterns without capturing parentheses will not be
       penalized.  So avoid $&, "$'", and "$`" if you can, but if you can't
       (and some algorithms really appreciate them), once you've used them
       once, use them at will, because you've already paid the price.

       Perl 5.16 introduced a slightly more efficient mechanism that notes
       separately whether each of "$`", $&, and "$'" have been seen, and thus
       may only need to copy part of the string.  Perl 5.20 introduced a much
       more efficient copy-on-write mechanism which eliminates any slowdown.

       As another workaround for this problem, Perl 5.10.0 introduced
       "${^PREMATCH}", "${^MATCH}" and "${^POSTMATCH}", which are equivalent
       to "$`", $& and "$'", except that they are only guaranteed to be
       defined after a successful match that was executed with the "/p"
       (preserve) modifier.  The use of these variables incurs no global
       performance penalty, unlike their punctuation character equivalents,
       however at the trade-off that you have to tell perl when you want to
       use them.  As of Perl 5.20, these three variables are equivalent to
       "$`", $& and "$'", and "/p" is ignored.

   Quoting metacharacters
       Backslashed metacharacters in Perl are alphanumeric, such as "\b",
       "\w", "\n".  Unlike some other regular expression languages, there are
       no backslashed symbols that aren't alphanumeric.  So anything that
       looks like "\\", "\(", "\)", "\[", "\]", "\{", or "\}" is always
       interpreted as a literal character, not a metacharacter.  This was once
       used in a common idiom to disable or quote the special meanings of
       regular expression metacharacters in a string that you want to use for
       a pattern. Simply quote all non-"word" characters:

           $pattern =~ s/(\W)/\\$1/g;

       (If "use locale" is set, then this depends on the current locale.)
       Today it is more common to use the "quotemeta()" function or the "\Q"
       metaquoting escape sequence to disable all metacharacters' special
       meanings like this:

           /$unquoted\Q$quoted\E$unquoted/

       Beware that if you put literal backslashes (those not inside
       interpolated variables) between "\Q" and "\E", double-quotish backslash
       interpolation may lead to confusing results.  If you need to use
       literal backslashes within "\Q...\E", consult "Gory details of parsing
       quoted constructs" in perlop.

       "quotemeta()" and "\Q" are fully described in "quotemeta" in perlfunc.

   Extended Patterns
       Perl also defines a consistent extension syntax for features not found
       in standard tools like awk and lex.  The syntax for most of these is a
       pair of parentheses with a question mark as the first thing within the
       parentheses.  The character after the question mark indicates the
       extension.

       A question mark was chosen for this and for the minimal-matching
       construct because 1) question marks are rare in older regular
       expressions, and 2) whenever you see one, you should stop and
       "question" exactly what is going on.  That's psychology....

       "(?#text)"
           A comment.  The text is ignored.  Note that Perl closes the comment
           as soon as it sees a ")", so there is no way to put a literal ")"
           in the comment.  The pattern's closing delimiter must be escaped by
           a backslash if it appears in the comment.

           See "/x" for another way to have comments in patterns.

           Note that a comment can go just about anywhere, except in the
           middle of an escape sequence.   Examples:

            qr/foo(?#comment)bar/'  # Matches 'foobar'

            # The pattern below matches 'abcd', 'abccd', or 'abcccd'
            qr/abc(?#comment between literal and its quantifier){1,3}d/

            # The pattern below generates a syntax error, because the '\p' must
            # be followed immediately by a '{'.
            qr/\p(?#comment between \p and its property name){Any}/

            # The pattern below generates a syntax error, because the initial
            # '\(' is a literal opening parenthesis, and so there is nothing
            # for the  closing ')' to match
            qr/\(?#the backslash means this isn't a comment)p{Any}/

       "(?adlupimnsx-imnsx)"
       "(?^alupimnsx)"
           One or more embedded pattern-match modifiers, to be turned on (or
           turned off if preceded by "-") for the remainder of the pattern or
           the remainder of the enclosing pattern group (if any).

           This is particularly useful for dynamically-generated patterns,
           such as those read in from a configuration file, taken from an
           argument, or specified in a table somewhere.  Consider the case
           where some patterns want to be case-sensitive and some do not:  The
           case-insensitive ones merely need to include "(?i)" at the front of
           the pattern.  For example:

               $pattern = "foobar";
               if ( /$pattern/i ) { }

               # more flexible:

               $pattern = "(?i)foobar";
               if ( /$pattern/ ) { }

           These modifiers are restored at the end of the enclosing group. For
           example,

               ( (?i) blah ) \s+ \g1

           will match "blah" in any case, some spaces, and an exact (including
           the case!)  repetition of the previous word, assuming the "/x"
           modifier, and no "/i" modifier outside this group.

           These modifiers do not carry over into named subpatterns called in
           the enclosing group. In other words, a pattern such as
           "((?i)(?&NAME))" does not change the case-sensitivity of the "NAME"
           pattern.

           A modifier is overridden by later occurrences of this construct in
           the same scope containing the same modifier, so that

               /((?im)foo(?-m)bar)/

           matches all of "foobar" case insensitively, but uses "/m" rules for
           only the "foo" portion.  The "a" flag overrides "aa" as well;
           likewise "aa" overrides "a".  The same goes for "x" and "xx".
           Hence, in

               /(?-x)foo/xx

           both "/x" and "/xx" are turned off during matching "foo".  And in

               /(?x)foo/x

           "/x" but NOT "/xx" is turned on for matching "foo".  (One might
           mistakenly think that since the inner "(?x)" is already in the
           scope of "/x", that the result would effectively be the sum of
           them, yielding "/xx".  It doesn't work that way.)  Similarly, doing
           something like "(?xx-x)foo" turns off all "x" behavior for matching
           "foo", it is not that you subtract 1 "x" from 2 to get 1 "x"
           remaining.

           Any of these modifiers can be set to apply globally to all regular
           expressions compiled within the scope of a "use re".  See "'/flags'
           mode" in re.

           Starting in Perl 5.14, a "^" (caret or circumflex accent)
           immediately after the "?" is a shorthand equivalent to "d-imnsx".
           Flags (except "d") may follow the caret to override it.  But a
           minus sign is not legal with it.

           Note that the "a", "d", "l", "p", and "u" modifiers are special in
           that they can only be enabled, not disabled, and the "a", "d", "l",
           and "u" modifiers are mutually exclusive: specifying one de-
           specifies the others, and a maximum of one (or two "a"'s) may
           appear in the construct.  Thus, for example, "(?-p)" will warn when
           compiled under "use warnings"; "(?-d:...)" and "(?dl:...)" are
           fatal errors.

           Note also that the "p" modifier is special in that its presence
           anywhere in a pattern has a global effect.

       "(?:pattern)"
       "(?adluimnsx-imnsx:pattern)"
       "(?^aluimnsx:pattern)"
           This is for clustering, not capturing; it groups subexpressions
           like "()", but doesn't make backreferences as "()" does.  So

               @fields = split(/\b(?:a|b|c)\b/)

           matches the same field delimiters as

               @fields = split(/\b(a|b|c)\b/)

           but doesn't spit out the delimiters themselves as extra fields
           (even though that's the behaviour of "split" in perlfunc when its
           pattern contains capturing groups).  It's also cheaper not to
           capture characters if you don't need to.

           Any letters between "?" and ":" act as flags modifiers as with
           "(?adluimnsx-imnsx)".  For example,

               /(?s-i:more.*than).*million/i

           is equivalent to the more verbose

               /(?:(?s-i)more.*than).*million/i

           Note that any "()" constructs enclosed within this one will still
           capture unless the "/n" modifier is in effect.

           Like the "(?adlupimnsx-imnsx)" construct, "aa" and "a" override
           each other, as do "xx" and "x".  They are not additive.  So, doing
           something like "(?xx-x:foo)" turns off all "x" behavior for
           matching "foo".

           Starting in Perl 5.14, a "^" (caret or circumflex accent)
           immediately after the "?" is a shorthand equivalent to "d-imnsx".
           Any positive flags (except "d") may follow the caret, so

               (?^x:foo)

           is equivalent to

               (?x-imns:foo)

           The caret tells Perl that this cluster doesn't inherit the flags of
           any surrounding pattern, but uses the system defaults ("d-imnsx"),
           modified by any flags specified.

           The caret allows for simpler stringification of compiled regular
           expressions.  These look like

               (?^:pattern)

           with any non-default flags appearing between the caret and the
           colon.  A test that looks at such stringification thus doesn't need
           to have the system default flags hard-coded in it, just the caret.
           If new flags are added to Perl, the meaning of the caret's
           expansion will change to include the default for those flags, so
           the test will still work, unchanged.

           Specifying a negative flag after the caret is an error, as the flag
           is redundant.

           Mnemonic for "(?^...)":  A fresh beginning since the usual use of a
           caret is to match at the beginning.

       "(?|pattern)"
           This is the "branch reset" pattern, which has the special property
           that the capture groups are numbered from the same starting point
           in each alternation branch. It is available starting from perl
           5.10.0.

           Capture groups are numbered from left to right, but inside this
           construct the numbering is restarted for each branch.

           The numbering within each branch will be as normal, and any groups
           following this construct will be numbered as though the construct
           contained only one branch, that being the one with the most capture
           groups in it.

           This construct is useful when you want to capture one of a number
           of alternative matches.

           Consider the following pattern.  The numbers underneath show in
           which group the captured content will be stored.

               # before  ---------------branch-reset----------- after
               / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
               # 1            2         2  3        2     3     4

           Be careful when using the branch reset pattern in combination with
           named captures. Named captures are implemented as being aliases to
           numbered groups holding the captures, and that interferes with the
           implementation of the branch reset pattern. If you are using named
           captures in a branch reset pattern, it's best to use the same
           names, in the same order, in each of the alternations:

              /(?|  (?<a> x ) (?<b> y )
                 |  (?<a> z ) (?<b> w )) /x

           Not doing so may lead to surprises:

             "12" =~ /(?| (?<a> \d+ ) | (?<b> \D+))/x;
             say $+{a};    # Prints '12'
             say $+{b};    # *Also* prints '12'.

           The problem here is that both the group named "a" and the group
           named "b" are aliases for the group belonging to $1.

       Lookaround Assertions
           Lookaround assertions are zero-width patterns which match a
           specific pattern without including it in $&. Positive assertions
           match when their subpattern matches, negative assertions match when
           their subpattern fails. Lookbehind matches text up to the current
           match position, lookahead matches text following the current match
           position.

           "(?=pattern)"
               A zero-width positive lookahead assertion.  For example,
               "/\w+(?=\t)/" matches a word followed by a tab, without
               including the tab in $&.

           "(?!pattern)"
               A zero-width negative lookahead assertion.  For example
               "/foo(?!bar)/" matches any occurrence of "foo" that isn't
               followed by "bar".  Note however that lookahead and lookbehind
               are NOT the same thing.  You cannot use this for lookbehind.

               If you are looking for a "bar" that isn't preceded by a "foo",
               "/(?!foo)bar/" will not do what you want.  That's because the
               "(?!foo)" is just saying that the next thing cannot be
               "foo"--and it's not, it's a "bar", so "foobar" will match.  Use
               lookbehind instead (see below).

           "(?<=pattern)"
           "\K"
               A zero-width positive lookbehind assertion.  For example,
               "/(?<=\t)\w+/" matches a word that follows a tab, without
               including the tab in $&.  Works only for fixed-width
               lookbehind.

               There is a special form of this construct, called "\K"
               (available since Perl 5.10.0), which causes the regex engine to
               "keep" everything it had matched prior to the "\K" and not
               include it in $&. This effectively provides variable-length
               lookbehind. The use of "\K" inside of another lookaround
               assertion is allowed, but the behaviour is currently not well
               defined.

               For various reasons "\K" may be significantly more efficient
               than the equivalent "(?<=...)" construct, and it is especially
               useful in situations where you want to efficiently remove
               something following something else in a string. For instance

                 s/(foo)bar/$1/g;

               can be rewritten as the much more efficient

                 s/foo\Kbar//g;

           "(?<!pattern)"
               A zero-width negative lookbehind assertion.  For example
               "/(?<!bar)foo/" matches any occurrence of "foo" that does not
               follow "bar".  Works only for fixed-width lookbehind.

       "(?<NAME>pattern)"
       "(?'NAME'pattern)"
           A named capture group. Identical in every respect to normal
           capturing parentheses "()" but for the additional fact that the
           group can be referred to by name in various regular expression
           constructs (like "\g{NAME}") and can be accessed by name after a
           successful match via "%+" or "%-". See perlvar for more details on
           the "%+" and "%-" hashes.

           If multiple distinct capture groups have the same name then the
           $+{NAME} will refer to the leftmost defined group in the match.

           The forms "(?'NAME'pattern)" and "(?<NAME>pattern)" are equivalent.

           NOTE: While the notation of this construct is the same as the
           similar function in .NET regexes, the behavior is not. In Perl the
           groups are numbered sequentially regardless of being named or not.
           Thus in the pattern

             /(x)(?<foo>y)(z)/

           $+{foo} will be the same as $2, and $3 will contain 'z' instead of
           the opposite which is what a .NET regex hacker might expect.

           Currently NAME is restricted to simple identifiers only.  In other
           words, it must match "/^[_A-Za-z][_A-Za-z0-9]*\z/" or its Unicode
           extension (see utf8), though it isn't extended by the locale (see
           perllocale).

           NOTE: In order to make things easier for programmers with
           experience with the Python or PCRE regex engines, the pattern
           "(?P<NAME>pattern)" may be used instead of "(?<NAME>pattern)";
           however this form does not support the use of single quotes as a
           delimiter for the name.

       "\k<NAME>"
       "\k'NAME'"
           Named backreference. Similar to numeric backreferences, except that
           the group is designated by name and not number. If multiple groups
           have the same name then it refers to the leftmost defined group in
           the current match.

           It is an error to refer to a name not defined by a "(?<NAME>)"
           earlier in the pattern.

           Both forms are equivalent.

           NOTE: In order to make things easier for programmers with
           experience with the Python or PCRE regex engines, the pattern
           "(?P=NAME)" may be used instead of "\k<NAME>".

       "(?{ code })"
           WARNING: Using this feature safely requires that you understand its
           limitations.  Code executed that has side effects may not perform
           identically from version to version due to the effect of future
           optimisations in the regex engine.  For more information on this,
           see "Embedded Code Execution Frequency".

           This zero-width assertion executes any embedded Perl code.  It
           always succeeds, and its return value is set as $^R.

           In literal patterns, the code is parsed at the same time as the
           surrounding code. While within the pattern, control is passed
           temporarily back to the perl parser, until the logically-balancing
           closing brace is encountered. This is similar to the way that an
           array index expression in a literal string is handled, for example

               "abc$array[ 1 + f('[') + g()]def"

           In particular, braces do not need to be balanced:

               s/abc(?{ f('{'); })/def/

           Even in a pattern that is interpolated and compiled at run-time,
           literal code blocks will be compiled once, at perl compile time;
           the following prints "ABCD":

               print "D";
               my $qr = qr/(?{ BEGIN { print "A" } })/;
               my $foo = "foo";
               /$foo$qr(?{ BEGIN { print "B" } })/;
               BEGIN { print "C" }

           In patterns where the text of the code is derived from run-time
           information rather than appearing literally in a source code
           /pattern/, the code is compiled at the same time that the pattern
           is compiled, and for reasons of security, "use re 'eval'" must be
           in scope. This is to stop user-supplied patterns containing code
           snippets from being executable.

           In situations where you need to enable this with "use re 'eval'",
           you should also have taint checking enabled.  Better yet, use the
           carefully constrained evaluation within a Safe compartment.  See
           perlsec for details about both these mechanisms.

           From the viewpoint of parsing, lexical variable scope and closures,

               /AAA(?{ BBB })CCC/

           behaves approximately like

               /AAA/ && do { BBB } && /CCC/

           Similarly,

               qr/AAA(?{ BBB })CCC/

           behaves approximately like

               sub { /AAA/ && do { BBB } && /CCC/ }

           In particular:

               { my $i = 1; $r = qr/(?{ print $i })/ }
               my $i = 2;
               /$r/; # prints "1"

           Inside a "(?{...})" block, $_ refers to the string the regular
           expression is matching against. You can also use "pos()" to know
           what is the current position of matching within this string.

           The code block introduces a new scope from the perspective of
           lexical variable declarations, but not from the perspective of
           "local" and similar localizing behaviours. So later code blocks
           within the same pattern will still see the values which were
           localized in earlier blocks.  These accumulated localizations are
           undone either at the end of a successful match, or if the assertion
           is backtracked (compare "Backtracking"). For example,

             $_ = 'a' x 8;
             m<
                (?{ $cnt = 0 })               # Initialize $cnt.
                (
                  a
                  (?{
                      local $cnt = $cnt + 1;  # Update $cnt,
                                              # backtracking-safe.
                  })
                )*
                aaaa
                (?{ $res = $cnt })            # On success copy to
                                              # non-localized location.
              >x;

           will initially increment $cnt up to 8; then during backtracking,
           its value will be unwound back to 4, which is the value assigned to
           $res.  At the end of the regex execution, $cnt will be wound back
           to its initial value of 0.

           This assertion may be used as the condition in a

               (?(condition)yes-pattern|no-pattern)

           switch.  If not used in this way, the result of evaluation of
           "code" is put into the special variable $^R.  This happens
           immediately, so $^R can be used from other "(?{ code })" assertions
           inside the same regular expression.

           The assignment to $^R above is properly localized, so the old value
           of $^R is restored if the assertion is backtracked; compare
           "Backtracking".

           Note that the special variable $^N  is particularly useful with
           code blocks to capture the results of submatches in variables
           without having to keep track of the number of nested parentheses.
           For example:

             $_ = "The brown fox jumps over the lazy dog";
             /the (\S+)(?{ $color = $^N }) (\S+)(?{ $animal = $^N })/i;
             print "color = $color, animal = $animal\n";

       "(??{ code })"
           WARNING: Using this feature safely requires that you understand its
           limitations.  Code executed that has side effects may not perform
           identically from version to version due to the effect of future
           optimisations in the regex engine.  For more information on this,
           see "Embedded Code Execution Frequency".

           This is a "postponed" regular subexpression.  It behaves in exactly
           the same way as a "(?{ code })" code block as described above,
           except that its return value, rather than being assigned to $^R, is
           treated as a pattern, compiled if it's a string (or used as-is if
           its a qr// object), then matched as if it were inserted instead of
           this construct.

           During the matching of this sub-pattern, it has its own set of
           captures which are valid during the sub-match, but are discarded
           once control returns to the main pattern. For example, the
           following matches, with the inner pattern capturing "B" and
           matching "BB", while the outer pattern captures "A";

               my $inner = '(.)\1';
               "ABBA" =~ /^(.)(??{ $inner })\1/;
               print $1; # prints "A";

           Note that this means that  there is no way for the inner pattern to
           refer to a capture group defined outside.  (The code block itself
           can use $1, etc., to refer to the enclosing pattern's capture
           groups.)  Thus, although

               ('a' x 100)=~/(??{'(.)' x 100})/

           will match, it will not set $1 on exit.

           The following pattern matches a parenthesized group:

            $re = qr{
                       \(
                       (?:
                          (?> [^()]+ )  # Non-parens without backtracking
                        |
                          (??{ $re })   # Group with matching parens
                       )*
                       \)
                    }x;

           See also "(?PARNO)" for a different, more efficient way to
           accomplish the same task.

           Executing a postponed regular expression too many times without
           consuming any input string will also result in a fatal error.  The
           depth at which that happens is compiled into perl, so it can be
           changed with a custom build.

       "(?PARNO)" "(?-PARNO)" "(?+PARNO)" "(?R)" "(?0)"
           Recursive subpattern. Treat the contents of a given capture buffer
           in the current pattern as an independent subpattern and attempt to
           match it at the current position in the string. Information about
           capture state from the caller for things like backreferences is
           available to the subpattern, but capture buffers set by the
           subpattern are not visible to the caller.

           Similar to "(??{ code })" except that it does not involve executing
           any code or potentially compiling a returned pattern string;
           instead it treats the part of the current pattern contained within
           a specified capture group as an independent pattern that must match
           at the current position. Also different is the treatment of capture
           buffers, unlike "(??{ code })" recursive patterns have access to
           their caller's match state, so one can use backreferences safely.

           PARNO is a sequence of digits (not starting with 0) whose value
           reflects the paren-number of the capture group to recurse to.
           "(?R)" recurses to the beginning of the whole pattern. "(?0)" is an
           alternate syntax for "(?R)". If PARNO is preceded by a plus or
           minus sign then it is assumed to be relative, with negative numbers
           indicating preceding capture groups and positive ones following.
           Thus "(?-1)" refers to the most recently declared group, and
           "(?+1)" indicates the next group to be declared.  Note that the
           counting for relative recursion differs from that of relative
           backreferences, in that with recursion unclosed groups are
           included.

           The following pattern matches a function "foo()" which may contain
           balanced parentheses as the argument.

             $re = qr{ (                   # paren group 1 (full function)
                         foo
                         (                 # paren group 2 (parens)
                           \(
                             (             # paren group 3 (contents of parens)
                             (?:
                              (?> [^()]+ ) # Non-parens without backtracking
                             |
                              (?2)         # Recurse to start of paren group 2
                             )*
                             )
                           \)
                         )
                       )
                     }x;

           If the pattern was used as follows

               'foo(bar(baz)+baz(bop))'=~/$re/
                   and print "\$1 = $1\n",
                             "\$2 = $2\n",
                             "\$3 = $3\n";

           the output produced should be the following:

               $1 = foo(bar(baz)+baz(bop))
               $2 = (bar(baz)+baz(bop))
               $3 = bar(baz)+baz(bop)

           If there is no corresponding capture group defined, then it is a
           fatal error.  Recursing deeply without consuming any input string
           will also result in a fatal error.  The depth at which that happens
           is compiled into perl, so it can be changed with a custom build.

           The following shows how using negative indexing can make it easier
           to embed recursive patterns inside of a "qr//" construct for later
           use:

               my $parens = qr/(\((?:[^()]++|(?-1))*+\))/;
               if (/foo $parens \s+ \+ \s+ bar $parens/x) {
                  # do something here...
               }

           Note that this pattern does not behave the same way as the
           equivalent PCRE or Python construct of the same form. In Perl you
           can backtrack into a recursed group, in PCRE and Python the
           recursed into group is treated as atomic. Also, modifiers are
           resolved at compile time, so constructs like "(?i:(?1))" or
           "(?:(?i)(?1))" do not affect how the sub-pattern will be processed.

       "(?&NAME)"
           Recurse to a named subpattern. Identical to "(?PARNO)" except that
           the parenthesis to recurse to is determined by name. If multiple
           parentheses have the same name, then it recurses to the leftmost.

           It is an error to refer to a name that is not declared somewhere in
           the pattern.

           NOTE: In order to make things easier for programmers with
           experience with the Python or PCRE regex engines the pattern
           "(?P>NAME)" may be used instead of "(?&NAME)".

       "(?(condition)yes-pattern|no-pattern)"
       "(?(condition)yes-pattern)"
           Conditional expression. Matches "yes-pattern" if "condition" yields
           a true value, matches "no-pattern" otherwise. A missing pattern
           always matches.

           "(condition)" should be one of:

           an integer in parentheses
               (which is valid if the corresponding pair of parentheses
               matched);

           a lookahead/lookbehind/evaluate zero-width assertion;
           a name in angle brackets or single quotes
               (which is valid if a group with the given name matched);

           the special symbol "(R)"
               (true when evaluated inside of recursion or eval).
               Additionally the "R" may be followed by a number, (which will
               be true when evaluated when recursing inside of the appropriate
               group), or by &NAME, in which case it will be true only when
               evaluated during recursion in the named group.

           Here's a summary of the possible predicates:

           "(1)" "(2)" ...
               Checks if the numbered capturing group has matched something.
               Full syntax: "(?(1)then|else)"

           "(<NAME>)" "('NAME')"
               Checks if a group with the given name has matched something.
               Full syntax: "(?(<name>)then|else)"

           "(?=...)" "(?!...)" "(?<=...)" "(?<!...)"
               Checks whether the pattern matches (or does not match, for the
               "!"  variants).  Full syntax: "(?(?=lookahead)then|else)"

           "(?{ CODE })"
               Treats the return value of the code block as the condition.
               Full syntax: "(?(?{ code })then|else)"

           "(R)"
               Checks if the expression has been evaluated inside of
               recursion.  Full syntax: "(?(R)then|else)"

           "(R1)" "(R2)" ...
               Checks if the expression has been evaluated while executing
               directly inside of the n-th capture group. This check is the
               regex equivalent of

                 if ((caller(0))[3] eq 'subname') { ... }

               In other words, it does not check the full recursion stack.

               Full syntax: "(?(R1)then|else)"

           "(R&NAME)"
               Similar to "(R1)", this predicate checks to see if we're
               executing directly inside of the leftmost group with a given
               name (this is the same logic used by "(?&NAME)" to
               disambiguate). It does not check the full stack, but only the
               name of the innermost active recursion.  Full syntax:
               "(?(R&name)then|else)"

           "(DEFINE)"
               In this case, the yes-pattern is never directly executed, and
               no no-pattern is allowed. Similar in spirit to "(?{0})" but
               more efficient.  See below for details.  Full syntax:
               "(?(DEFINE)definitions...)"

           For example:

               m{ ( \( )?
                  [^()]+
                  (?(1) \) )
                }x

           matches a chunk of non-parentheses, possibly included in
           parentheses themselves.

           A special form is the "(DEFINE)" predicate, which never executes
           its yes-pattern directly, and does not allow a no-pattern. This
           allows one to define subpatterns which will be executed only by the
           recursion mechanism.  This way, you can define a set of regular
           expression rules that can be bundled into any pattern you choose.

           It is recommended that for this usage you put the DEFINE block at
           the end of the pattern, and that you name any subpatterns defined
           within it.

           Also, it's worth noting that patterns defined this way probably
           will not be as efficient, as the optimizer is not very clever about
           handling them.

           An example of how this might be used is as follows:

             /(?<NAME>(?&NAME_PAT))(?<ADDR>(?&ADDRESS_PAT))
              (?(DEFINE)
                (?<NAME_PAT>....)
                (?<ADDRESS_PAT>....)
              )/x

           Note that capture groups matched inside of recursion are not
           accessible after the recursion returns, so the extra layer of
           capturing groups is necessary. Thus $+{NAME_PAT} would not be
           defined even though $+{NAME} would be.

           Finally, keep in mind that subpatterns created inside a DEFINE
           block count towards the absolute and relative number of captures,
           so this:

               my @captures = "a" =~ /(.)                  # First capture
                                      (?(DEFINE)
                                          (?<EXAMPLE> 1 )  # Second capture
                                      )/x;
               say scalar @captures;

           Will output 2, not 1. This is particularly important if you intend
           to compile the definitions with the "qr//" operator, and later
           interpolate them in another pattern.

       "(?>pattern)"
           An "independent" subexpression, one which matches the substring
           that a standalone "pattern" would match if anchored at the given
           position, and it matches nothing other than this substring.  This
           construct is useful for optimizations of what would otherwise be
           "eternal" matches, because it will not backtrack (see
           "Backtracking").  It may also be useful in places where the "grab
           all you can, and do not give anything back" semantic is desirable.

           For example: "^(?>a*)ab" will never match, since "(?>a*)" (anchored
           at the beginning of string, as above) will match all characters "a"
           at the beginning of string, leaving no "a" for "ab" to match.  In
           contrast, "a*ab" will match the same as "a+b", since the match of
           the subgroup "a*" is influenced by the following group "ab" (see
           "Backtracking").  In particular, "a*" inside "a*ab" will match
           fewer characters than a standalone "a*", since this makes the tail
           match.

           "(?>pattern)" does not disable backtracking altogether once it has
           matched. It is still possible to backtrack past the construct, but
           not into it. So "((?>a*)|(?>b*))ar" will still match "bar".

           An effect similar to "(?>pattern)" may be achieved by writing
           "(?=(pattern))\g{-1}".  This matches the same substring as a
           standalone "a+", and the following "\g{-1}" eats the matched
           string; it therefore makes a zero-length assertion into an analogue
           of "(?>...)".  (The difference between these two constructs is that
           the second one uses a capturing group, thus shifting ordinals of
           backreferences in the rest of a regular expression.)

           Consider this pattern:

               m{ \(
                     (
                       [^()]+           # x+
                     |
                       \( [^()]* \)
                     )+
                  \)
                }x

           That will efficiently match a nonempty group with matching
           parentheses two levels deep or less.  However, if there is no such
           group, it will take virtually forever on a long string.  That's
           because there are so many different ways to split a long string
           into several substrings.  This is what "(.+)+" is doing, and
           "(.+)+" is similar to a subpattern of the above pattern.  Consider
           how the pattern above detects no-match on "((()aaaaaaaaaaaaaaaaaa"
           in several seconds, but that each extra letter doubles this time.
           This exponential performance will make it appear that your program
           has hung.  However, a tiny change to this pattern

               m{ \(
                     (
                       (?> [^()]+ )        # change x+ above to (?> x+ )
                     |
                       \( [^()]* \)
                     )+
                  \)
                }x

           which uses "(?>...)" matches exactly when the one above does
           (verifying this yourself would be a productive exercise), but
           finishes in a fourth the time when used on a similar string with
           1000000 "a"s.  Be aware, however, that, when this construct is
           followed by a quantifier, it currently triggers a warning message
           under the "use warnings" pragma or -w switch saying it "matches
           null string many times in regex".

           On simple groups, such as the pattern "(?> [^()]+ )", a comparable
           effect may be achieved by negative lookahead, as in "[^()]+ (?!
           [^()] )".  This was only 4 times slower on a string with 1000000
           "a"s.

           The "grab all you can, and do not give anything back" semantic is
           desirable in many situations where on the first sight a simple
           "()*" looks like the correct solution.  Suppose we parse text with
           comments being delimited by "#" followed by some optional
           (horizontal) whitespace.  Contrary to its appearance, "#[ \t]*" is
           not the correct subexpression to match the comment delimiter,
           because it may "give up" some whitespace if the remainder of the
           pattern can be made to match that way.  The correct answer is
           either one of these:

               (?>#[ \t]*)
               #[ \t]*(?![ \t])

           For example, to grab non-empty comments into $1, one should use
           either one of these:

               / (?> \# [ \t]* ) (        .+ ) /x;
               /     \# [ \t]*   ( [^ \t] .* ) /x;

           Which one you pick depends on which of these expressions better
           reflects the above specification of comments.

           In some literature this construct is called "atomic matching" or
           "possessive matching".

           Possessive quantifiers are equivalent to putting the item they are
           applied to inside of one of these constructs. The following
           equivalences apply:

               Quantifier Form     Bracketing Form
               ---------------     ---------------
               PAT*+               (?>PAT*)
               PAT++               (?>PAT+)
               PAT?+               (?>PAT?)
               PAT{min,max}+       (?>PAT{min,max})

       "(?[ ])"
           See "Extended Bracketed Character Classes" in perlrecharclass.

           Note that this feature is currently experimental; using it yields a
           warning in the "experimental::regex_sets" category.

   Backtracking
       NOTE: This section presents an abstract approximation of regular
       expression behavior.  For a more rigorous (and complicated) view of the
       rules involved in selecting a match among possible alternatives, see
       "Combining RE Pieces".

       A fundamental feature of regular expression matching involves the
       notion called backtracking, which is currently used (when needed) by
       all regular non-possessive expression quantifiers, namely "*", "*?",
       "+", "+?", "{n,m}", and "{n,m}?".  Backtracking is often optimized
       internally, but the general principle outlined here is valid.

       For a regular expression to match, the entire regular expression must
       match, not just part of it.  So if the beginning of a pattern
       containing a quantifier succeeds in a way that causes later parts in
       the pattern to fail, the matching engine backs up and recalculates the
       beginning part--that's why it's called backtracking.

       Here is an example of backtracking:  Let's say you want to find the
       word following "foo" in the string "Food is on the foo table.":

           $_ = "Food is on the foo table.";
           if ( /\b(foo)\s+(\w+)/i ) {
               print "$2 follows $1.\n";
           }

       When the match runs, the first part of the regular expression
       ("\b(foo)") finds a possible match right at the beginning of the
       string, and loads up $1 with "Foo".  However, as soon as the matching
       engine sees that there's no whitespace following the "Foo" that it had
       saved in $1, it realizes its mistake and starts over again one
       character after where it had the tentative match.  This time it goes
       all the way until the next occurrence of "foo". The complete regular
       expression matches this time, and you get the expected output of "table
       follows foo."

       Sometimes minimal matching can help a lot.  Imagine you'd like to match
       everything between "foo" and "bar".  Initially, you write something
       like this:

           $_ =  "The food is under the bar in the barn.";
           if ( /foo(.*)bar/ ) {
               print "got <$1>\n";
           }

       Which perhaps unexpectedly yields:

         got <d is under the bar in the >

       That's because ".*" was greedy, so you get everything between the first
       "foo" and the last "bar".  Here it's more effective to use minimal
       matching to make sure you get the text between a "foo" and the first
       "bar" thereafter.

           if ( /foo(.*?)bar/ ) { print "got <$1>\n" }
         got <d is under the >

       Here's another example. Let's say you'd like to match a number at the
       end of a string, and you also want to keep the preceding part of the
       match.  So you write this:

           $_ = "I have 2 numbers: 53147";
           if ( /(.*)(\d*)/ ) {                                # Wrong!
               print "Beginning is <$1>, number is <$2>.\n";
           }

       That won't work at all, because ".*" was greedy and gobbled up the
       whole string. As "\d*" can match on an empty string the complete
       regular expression matched successfully.

           Beginning is <I have 2 numbers: 53147>, number is <>.

       Here are some variants, most of which don't work:

           $_ = "I have 2 numbers: 53147";
           @pats = qw{
               (.*)(\d*)
               (.*)(\d+)
               (.*?)(\d*)
               (.*?)(\d+)
               (.*)(\d+)$
               (.*?)(\d+)$
               (.*)\b(\d+)$
               (.*\D)(\d+)$
           };

           for $pat (@pats) {
               printf "%-12s ", $pat;
               if ( /$pat/ ) {
                   print "<$1> <$2>\n";
               } else {
                   print "FAIL\n";
               }
           }

       That will print out:

           (.*)(\d*)    <I have 2 numbers: 53147> <>
           (.*)(\d+)    <I have 2 numbers: 5314> <7>
           (.*?)(\d*)   <> <>
           (.*?)(\d+)   <I have > <2>
           (.*)(\d+)$   <I have 2 numbers: 5314> <7>
           (.*?)(\d+)$  <I have 2 numbers: > <53147>
           (.*)\b(\d+)$ <I have 2 numbers: > <53147>
           (.*\D)(\d+)$ <I have 2 numbers: > <53147>

       As you see, this can be a bit tricky.  It's important to realize that a
       regular expression is merely a set of assertions that gives a
       definition of success.  There may be 0, 1, or several different ways
       that the definition might succeed against a particular string.  And if
       there are multiple ways it might succeed, you need to understand
       backtracking to know which variety of success you will achieve.

       When using lookahead assertions and negations, this can all get even
       trickier.  Imagine you'd like to find a sequence of non-digits not
       followed by "123".  You might try to write that as

           $_ = "ABC123";
           if ( /^\D*(?!123)/ ) {                # Wrong!
               print "Yup, no 123 in $_\n";
           }

       But that isn't going to match; at least, not the way you're hoping.  It
       claims that there is no 123 in the string.  Here's a clearer picture of
       why that pattern matches, contrary to popular expectations:

           $x = 'ABC123';
           $y = 'ABC445';

           print "1: got $1\n" if $x =~ /^(ABC)(?!123)/;
           print "2: got $1\n" if $y =~ /^(ABC)(?!123)/;

           print "3: got $1\n" if $x =~ /^(\D*)(?!123)/;
           print "4: got $1\n" if $y =~ /^(\D*)(?!123)/;

       This prints

           2: got ABC
           3: got AB
           4: got ABC

       You might have expected test 3 to fail because it seems to a more
       general purpose version of test 1.  The important difference between
       them is that test 3 contains a quantifier ("\D*") and so can use
       backtracking, whereas test 1 will not.  What's happening is that you've
       asked "Is it true that at the start of $x, following 0 or more non-
       digits, you have something that's not 123?"  If the pattern matcher had
       let "\D*" expand to "ABC", this would have caused the whole pattern to
       fail.

       The search engine will initially match "\D*" with "ABC".  Then it will
       try to match "(?!123)" with "123", which fails.  But because a
       quantifier ("\D*") has been used in the regular expression, the search
       engine can backtrack and retry the match differently in the hope of
       matching the complete regular expression.

       The pattern really, really wants to succeed, so it uses the standard
       pattern back-off-and-retry and lets "\D*" expand to just "AB" this
       time.  Now there's indeed something following "AB" that is not "123".
       It's "C123", which suffices.

       We can deal with this by using both an assertion and a negation.  We'll
       say that the first part in $1 must be followed both by a digit and by
       something that's not "123".  Remember that the lookaheads are zero-
       width expressions--they only look, but don't consume any of the string
       in their match.  So rewriting this way produces what you'd expect; that
       is, case 5 will fail, but case 6 succeeds:

           print "5: got $1\n" if $x =~ /^(\D*)(?=\d)(?!123)/;
           print "6: got $1\n" if $y =~ /^(\D*)(?=\d)(?!123)/;

           6: got ABC

       In other words, the two zero-width assertions next to each other work
       as though they're ANDed together, just as you'd use any built-in
       assertions:  "/^$/" matches only if you're at the beginning of the line
       AND the end of the line simultaneously.  The deeper underlying truth is
       that juxtaposition in regular expressions always means AND, except when
       you write an explicit OR using the vertical bar.  "/ab/" means match
       "a" AND (then) match "b", although the attempted matches are made at
       different positions because "a" is not a zero-width assertion, but a
       one-width assertion.

       WARNING: Particularly complicated regular expressions can take
       exponential time to solve because of the immense number of possible
       ways they can use backtracking to try for a match.  For example,
       without internal optimizations done by the regular expression engine,
       this will take a painfully long time to run:

           'aaaaaaaaaaaa' =~ /((a{0,5}){0,5})*[c]/

       And if you used "*"'s in the internal groups instead of limiting them
       to 0 through 5 matches, then it would take forever--or until you ran
       out of stack space.  Moreover, these internal optimizations are not
       always applicable.  For example, if you put "{0,5}" instead of "*" on
       the external group, no current optimization is applicable, and the
       match takes a long time to finish.

       A powerful tool for optimizing such beasts is what is known as an
       "independent group", which does not backtrack (see ""(?>pattern)"").
       Note also that zero-length lookahead/lookbehind assertions will not
       backtrack to make the tail match, since they are in "logical" context:
       only whether they match is considered relevant.  For an example where
       side-effects of lookahead might have influenced the following match,
       see ""(?>pattern)"".

   Special Backtracking Control Verbs
       These special patterns are generally of the form "(*VERB:ARG)". Unless
       otherwise stated the ARG argument is optional; in some cases, it is
       mandatory.

       Any pattern containing a special backtracking verb that allows an
       argument has the special behaviour that when executed it sets the
       current package's $REGERROR and $REGMARK variables. When doing so the
       following rules apply:

       On failure, the $REGERROR variable will be set to the ARG value of the
       verb pattern, if the verb was involved in the failure of the match. If
       the ARG part of the pattern was omitted, then $REGERROR will be set to
       the name of the last "(*MARK:NAME)" pattern executed, or to TRUE if
       there was none. Also, the $REGMARK variable will be set to FALSE.

       On a successful match, the $REGERROR variable will be set to FALSE, and
       the $REGMARK variable will be set to the name of the last
       "(*MARK:NAME)" pattern executed.  See the explanation for the
       "(*MARK:NAME)" verb below for more details.

       NOTE: $REGERROR and $REGMARK are not magic variables like $1 and most
       other regex-related variables. They are not local to a scope, nor
       readonly, but instead are volatile package variables similar to
       $AUTOLOAD.  They are set in the package containing the code that
       executed the regex (rather than the one that compiled it, where those
       differ).  If necessary, you can use "local" to localize changes to
       these variables to a specific scope before executing a regex.

       If a pattern does not contain a special backtracking verb that allows
       an argument, then $REGERROR and $REGMARK are not touched at all.

       Verbs
          "(*PRUNE)" "(*PRUNE:NAME)"
              This zero-width pattern prunes the backtracking tree at the
              current point when backtracked into on failure. Consider the
              pattern "/A (*PRUNE) B/", where A and B are complex patterns.
              Until the "(*PRUNE)" verb is reached, A may backtrack as
              necessary to match. Once it is reached, matching continues in B,
              which may also backtrack as necessary; however, should B not
              match, then no further backtracking will take place, and the
              pattern will fail outright at the current starting position.

              The following example counts all the possible matching strings
              in a pattern (without actually matching any of them).

                  'aaab' =~ /a+b?(?{print "$&\n"; $count++})(*FAIL)/;
                  print "Count=$count\n";

              which produces:

                  aaab
                  aaa
                  aa
                  a
                  aab
                  aa
                  a
                  ab
                  a
                  Count=9

              If we add a "(*PRUNE)" before the count like the following

                  'aaab' =~ /a+b?(*PRUNE)(?{print "$&\n"; $count++})(*FAIL)/;
                  print "Count=$count\n";

              we prevent backtracking and find the count of the longest
              matching string at each matching starting point like so:

                  aaab
                  aab
                  ab
                  Count=3

              Any number of "(*PRUNE)" assertions may be used in a pattern.

              See also "(?>pattern)" and possessive quantifiers for other ways
              to control backtracking. In some cases, the use of "(*PRUNE)"
              can be replaced with a "(?>pattern)" with no functional
              difference; however, "(*PRUNE)" can be used to handle cases that
              cannot be expressed using a "(?>pattern)" alone.

          "(*SKIP)" "(*SKIP:NAME)"
              This zero-width pattern is similar to "(*PRUNE)", except that on
              failure it also signifies that whatever text that was matched
              leading up to the "(*SKIP)" pattern being executed cannot be
              part of any match of this pattern. This effectively means that
              the regex engine "skips" forward to this position on failure and
              tries to match again, (assuming that there is sufficient room to
              match).

              The name of the "(*SKIP:NAME)" pattern has special significance.
              If a "(*MARK:NAME)" was encountered while matching, then it is
              that position which is used as the "skip point". If no "(*MARK)"
              of that name was encountered, then the "(*SKIP)" operator has no
              effect. When used without a name the "skip point" is where the
              match point was when executing the "(*SKIP)" pattern.

              Compare the following to the examples in "(*PRUNE)"; note the
              string is twice as long:

               'aaabaaab' =~ /a+b?(*SKIP)(?{print "$&\n"; $count++})(*FAIL)/;
               print "Count=$count\n";

              outputs

                  aaab
                  aaab
                  Count=2

              Once the 'aaab' at the start of the string has matched, and the
              "(*SKIP)" executed, the next starting point will be where the
              cursor was when the "(*SKIP)" was executed.

          "(*MARK:NAME)" "(*:NAME)"
              This zero-width pattern can be used to mark the point reached in
              a string when a certain part of the pattern has been
              successfully matched. This mark may be given a name. A later
              "(*SKIP)" pattern will then skip forward to that point if
              backtracked into on failure. Any number of "(*MARK)" patterns
              are allowed, and the NAME portion may be duplicated.

              In addition to interacting with the "(*SKIP)" pattern,
              "(*MARK:NAME)" can be used to "label" a pattern branch, so that
              after matching, the program can determine which branches of the
              pattern were involved in the match.

              When a match is successful, the $REGMARK variable will be set to
              the name of the most recently executed "(*MARK:NAME)" that was
              involved in the match.

              This can be used to determine which branch of a pattern was
              matched without using a separate capture group for each branch,
              which in turn can result in a performance improvement, as perl
              cannot optimize "/(?:(x)|(y)|(z))/" as efficiently as something
              like "/(?:x(*MARK:x)|y(*MARK:y)|z(*MARK:z))/".

              When a match has failed, and unless another verb has been
              involved in failing the match and has provided its own name to
              use, the $REGERROR variable will be set to the name of the most
              recently executed "(*MARK:NAME)".

              See "(*SKIP)" for more details.

              As a shortcut "(*MARK:NAME)" can be written "(*:NAME)".

          "(*THEN)" "(*THEN:NAME)"
              This is similar to the "cut group" operator "::" from Perl 6.
              Like "(*PRUNE)", this verb always matches, and when backtracked
              into on failure, it causes the regex engine to try the next
              alternation in the innermost enclosing group (capturing or
              otherwise) that has alternations.  The two branches of a
              "(?(condition)yes-pattern|no-pattern)" do not count as an
              alternation, as far as "(*THEN)" is concerned.

              Its name comes from the observation that this operation combined
              with the alternation operator ("|") can be used to create what
              is essentially a pattern-based if/then/else block:

                ( COND (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ )

              Note that if this operator is used and NOT inside of an
              alternation then it acts exactly like the "(*PRUNE)" operator.

                / A (*PRUNE) B /

              is the same as

                / A (*THEN) B /

              but

                / ( A (*THEN) B | C ) /

              is not the same as

                / ( A (*PRUNE) B | C ) /

              as after matching the A but failing on the B the "(*THEN)" verb
              will backtrack and try C; but the "(*PRUNE)" verb will simply
              fail.

          "(*COMMIT)" "(*COMMIT:args)"
              This is the Perl 6 "commit pattern" "<commit>" or ":::". It's a
              zero-width pattern similar to "(*SKIP)", except that when
              backtracked into on failure it causes the match to fail
              outright. No further attempts to find a valid match by advancing
              the start pointer will occur again.  For example,

               'aaabaaab' =~ /a+b?(*COMMIT)(?{print "$&\n"; $count++})(*FAIL)/;
               print "Count=$count\n";

              outputs

                  aaab
                  Count=1

              In other words, once the "(*COMMIT)" has been entered, and if
              the pattern does not match, the regex engine will not try any
              further matching on the rest of the string.

          "(*FAIL)" "(*F)" "(*FAIL:arg)"
              This pattern matches nothing and always fails. It can be used to
              force the engine to backtrack. It is equivalent to "(?!)", but
              easier to read. In fact, "(?!)" gets optimised into "(*FAIL)"
              internally. You can provide an argument so that if the match
              fails because of this "FAIL" directive the argument can be
              obtained from $REGERROR.

              It is probably useful only when combined with "(?{})" or
              "(??{})".

          "(*ACCEPT)" "(*ACCEPT:arg)"
              This pattern matches nothing and causes the end of successful
              matching at the point at which the "(*ACCEPT)" pattern was
              encountered, regardless of whether there is actually more to
              match in the string. When inside of a nested pattern, such as
              recursion, or in a subpattern dynamically generated via
              "(??{})", only the innermost pattern is ended immediately.

              If the "(*ACCEPT)" is inside of capturing groups then the groups
              are marked as ended at the point at which the "(*ACCEPT)" was
              encountered.  For instance:

                'AB' =~ /(A (A|B(*ACCEPT)|C) D)(E)/x;

              will match, and $1 will be "AB" and $2 will be "B", $3 will not
              be set. If another branch in the inner parentheses was matched,
              such as in the string 'ACDE', then the "D" and "E" would have to
              be matched as well.

              You can provide an argument, which will be available in the var
              $REGMARK after the match completes.

   Warning on "\1" Instead of $1
       Some people get too used to writing things like:

           $pattern =~ s/(\W)/\\\1/g;

       This is grandfathered (for \1 to \9) for the RHS of a substitute to
       avoid shocking the sed addicts, but it's a dirty habit to get into.
       That's because in PerlThink, the righthand side of an "s///" is a
       double-quoted string.  "\1" in the usual double-quoted string means a
       control-A.  The customary Unix meaning of "\1" is kludged in for
       "s///".  However, if you get into the habit of doing that, you get
       yourself into trouble if you then add an "/e" modifier.

           s/(\d+)/ \1 + 1 /eg;            # causes warning under -w

       Or if you try to do

           s/(\d+)/\1000/;

       You can't disambiguate that by saying "\{1}000", whereas you can fix it
       with "${1}000".  The operation of interpolation should not be confused
       with the operation of matching a backreference.  Certainly they mean
       two different things on the left side of the "s///".

   Repeated Patterns Matching a Zero-length Substring
       WARNING: Difficult material (and prose) ahead.  This section needs a
       rewrite.

       Regular expressions provide a terse and powerful programming language.
       As with most other power tools, power comes together with the ability
       to wreak havoc.

       A common abuse of this power stems from the ability to make infinite
       loops using regular expressions, with something as innocuous as:

           'foo' =~ m{ ( o? )* }x;

       The "o?" matches at the beginning of ""foo"", and since the position in
       the string is not moved by the match, "o?" would match again and again
       because of the "*" quantifier.  Another common way to create a similar
       cycle is with the looping modifier "/g":

           @matches = ( 'foo' =~ m{ o? }xg );

       or

           print "match: <$&>\n" while 'foo' =~ m{ o? }xg;

       or the loop implied by "split()".

       However, long experience has shown that many programming tasks may be
       significantly simplified by using repeated subexpressions that may
       match zero-length substrings.  Here's a simple example being:

           @chars = split //, $string;           # // is not magic in split
           ($whitewashed = $string) =~ s/()/ /g; # parens avoid magic s// /

       Thus Perl allows such constructs, by forcefully breaking the infinite
       loop.  The rules for this are different for lower-level loops given by
       the greedy quantifiers "*+{}", and for higher-level ones like the "/g"
       modifier or "split()" operator.

       The lower-level loops are interrupted (that is, the loop is broken)
       when Perl detects that a repeated expression matched a zero-length
       substring.   Thus

          m{ (?: NON_ZERO_LENGTH | ZERO_LENGTH )* }x;

       is made equivalent to

          m{ (?: NON_ZERO_LENGTH )* (?: ZERO_LENGTH )? }x;

       For example, this program

          #!perl -l
          "aaaaab" =~ /
            (?:
               a                 # non-zero
               |                 # or
              (?{print "hello"}) # print hello whenever this
                                 #    branch is tried
              (?=(b))            # zero-width assertion
            )*  # any number of times
           /x;
          print $&;
          print $1;

       prints

          hello
          aaaaa
          b

       Notice that "hello" is only printed once, as when Perl sees that the
       sixth iteration of the outermost "(?:)*" matches a zero-length string,
       it stops the "*".

       The higher-level loops preserve an additional state between iterations:
       whether the last match was zero-length.  To break the loop, the
       following match after a zero-length match is prohibited to have a
       length of zero.  This prohibition interacts with backtracking (see
       "Backtracking"), and so the second best match is chosen if the best
       match is of zero length.

       For example:

           $_ = 'bar';
           s/\w??/<$&>/g;

       results in "<>PERLRE(1)

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