unicode(3erl) Erlang Module Definition unicode(3erl)
NAME
unicode - Functions for converting Unicode characters.
DESCRIPTION
This module contains functions for converting between different charac-
ter representations. It converts between ISO Latin-1 characters and
Unicode characters, but it can also convert between different Unicode
encodings (like UTF-8, UTF-16, and UTF-32).
The default Unicode encoding in Erlang is in binaries UTF-8, which is
also the format in which built-in functions and libraries in OTP expect
to find binary Unicode data. In lists, Unicode data is encoded as inte-
gers, each integer representing one character and encoded simply as the
Unicode code point for the character.
Other Unicode encodings than integers representing code points or UTF-8
in binaries are referred to as "external encodings". The ISO Latin-1
encoding is in binaries and lists referred to as latin1-encoding.
It is recommended to only use external encodings for communication with
external entities where this is required. When working inside the Er-
lang/OTP environment, it is recommended to keep binaries in UTF-8 when
representing Unicode characters. ISO Latin-1 encoding is supported both
for backward compatibility and for communication with external entities
not supporting Unicode character sets.
Programs should always operate on a normalized form and compare canoni-
cal-equivalent Unicode characters as equal. All characters should thus
be normalized to one form once on the system borders. One of the fol-
lowing functions can convert characters to their normalized forms char-
acters_to_nfc_list/1, characters_to_nfc_binary/1, charac-
ters_to_nfd_list/1 or characters_to_nfd_binary/1. For general text
characters_to_nfc_list/1 or characters_to_nfc_binary/1 is preferred,
and for identifiers one of the compatibility normalization functions,
such as characters_to_nfkc_list/1, is preferred for security reasons.
The normalization functions where introduced in OTP 20. Additional in-
formation on normalization can be found in the Unicode FAQ.
DATA TYPES
encoding() =
latin1 | unicode | utf8 | utf16 |
{utf16, endian()} |
utf32 |
{utf32, endian()}
endian() = big | little
unicode_binary() = binary()
A binary() with characters encoded in the UTF-8 coding standard.
chardata() = charlist() | unicode_binary()
charlist() =
maybe_improper_list(char() | unicode_binary() | charlist(),
unicode_binary() | [])
external_unicode_binary() = binary()
A binary() with characters coded in a user-specified Unicode en-
coding other than UTF-8 (that is, UTF-16 or UTF-32).
external_chardata() =
external_charlist() | external_unicode_binary()
external_charlist() =
maybe_improper_list(char() |
external_unicode_binary() |
external_charlist(),
external_unicode_binary() | [])
latin1_binary() = binary()
A binary() with characters coded in ISO Latin-1.
latin1_char() = byte()
An integer() representing a valid ISO Latin-1 character (0-255).
latin1_chardata() = latin1_charlist() | latin1_binary()
Same as iodata().
latin1_charlist() =
maybe_improper_list(latin1_char() |
latin1_binary() |
latin1_charlist(),
latin1_binary() | [])
Same as iolist().
EXPORTS
bom_to_encoding(Bin) -> {Encoding, Length}
Types:
Bin = binary()
A binary() such that byte_size(Bin) >= 4.
Encoding =
latin1 | utf8 | {utf16, endian()} | {utf32, endian()}
Length = integer() >= 0
endian() = big | little
Checks for a UTF Byte Order Mark (BOM) in the beginning of a bi-
nary. If the supplied binary Bin begins with a valid BOM for ei-
ther UTF-8, UTF-16, or UTF-32, the function returns the encoding
identified along with the BOM length in bytes.
If no BOM is found, the function returns {latin1,0}.
characters_to_binary(Data) -> Result
Types:
Data = latin1_chardata() | chardata() | external_chardata()
Result =
binary() |
{error, binary(), RestData} |
{incomplete, binary(), binary()}
RestData = latin1_chardata() | chardata() | external_char-
data()
Same as characters_to_binary(Data, unicode, unicode).
characters_to_binary(Data, InEncoding) -> Result
Types:
Data = latin1_chardata() | chardata() | external_chardata()
InEncoding = encoding()
Result =
binary() |
{error, binary(), RestData} |
{incomplete, binary(), binary()}
RestData = latin1_chardata() | chardata() | external_char-
data()
Same as characters_to_binary(Data, InEncoding, unicode).
characters_to_binary(Data, InEncoding, OutEncoding) -> Result
Types:
Data = latin1_chardata() | chardata() | external_chardata()
InEncoding = OutEncoding = encoding()
Result =
binary() |
{error, binary(), RestData} |
{incomplete, binary(), binary()}
RestData = latin1_chardata() | chardata() | external_char-
data()
Behaves as characters_to_list/2, but produces a binary instead
of a Unicode list.
InEncoding defines how input is to be interpreted if binaries
are present in Data
OutEncoding defines in what format output is to be generated.
Options:
unicode:
An alias for utf8, as this is the preferred encoding for
Unicode characters in binaries.
utf16:
An alias for {utf16,big}.
utf32:
An alias for {utf32,big}.
The atoms big and little denote big- or little-endian encoding.
Errors and exceptions occur as in characters_to_list/2, but the
second element in tuple error or incomplete is a binary() and
not a list().
characters_to_list(Data) -> Result
Types:
Data = latin1_chardata() | chardata() | external_chardata()
Result =
list() |
{error, list(), RestData} |
{incomplete, list(), binary()}
RestData = latin1_chardata() | chardata() | external_char-
data()
Same as characters_to_list(Data, unicode).
characters_to_list(Data, InEncoding) -> Result
Types:
Data = latin1_chardata() | chardata() | external_chardata()
InEncoding = encoding()
Result =
list() |
{error, list(), RestData} |
{incomplete, list(), binary()}
RestData = latin1_chardata() | chardata() | external_char-
data()
Converts a possibly deep list of integers and binaries into a
list of integers representing Unicode characters. The binaries
in the input can have characters encoded as one of the follow-
ing:
* ISO Latin-1 (0-255, one character per byte). Here, case pa-
rameter InEncoding is to be specified as latin1.
* One of the UTF-encodings, which is specified as parameter
InEncoding.
Note that integers in the list always represent code points re-
gardless of InEncoding passed. If InEncoding latin1 is passed,
only code points < 256 are allowed; otherwise, all valid unicode
code points are allowed.
If InEncoding is latin1, parameter Data corresponds to the io-
data() type, but for unicode, parameter Data can contain inte-
gers > 255 (Unicode characters beyond the ISO Latin-1 range),
which makes it invalid as iodata().
The purpose of the function is mainly to convert combinations of
Unicode characters into a pure Unicode string in list represen-
tation for further processing. For writing the data to an exter-
nal entity, the reverse function characters_to_binary/3 comes in
handy.
Option unicode is an alias for utf8, as this is the preferred
encoding for Unicode characters in binaries. utf16 is an alias
for {utf16,big} and utf32 is an alias for {utf32,big}. The atoms
big and little denote big- or little-endian encoding.
If the data cannot be converted, either because of illegal Uni-
code/ISO Latin-1 characters in the list, or because of invalid
UTF encoding in any binaries, an error tuple is returned. The
error tuple contains the tag error, a list representing the
characters that could be converted before the error occurred and
a representation of the characters including and after the of-
fending integer/bytes. The last part is mostly for debugging, as
it still constitutes a possibly deep or mixed list, or both, not
necessarily of the same depth as the original data. The error
occurs when traversing the list and whatever is left to decode
is returned "as is".
However, if the input Data is a pure binary, the third part of
the error tuple is guaranteed to be a binary as well.
Errors occur for the following reasons:
* Integers out of range.
If InEncoding is latin1, an error occurs whenever an integer
> 255 is found in the lists.
If InEncoding is of a Unicode type, an error occurs whenever
either of the following is found:
* An integer > 16#10FFFF (the maximum Unicode character)
* An integer in the range 16#D800 to 16#DFFF (invalid range
reserved for UTF-16 surrogate pairs)
* Incorrect UTF encoding.
If InEncoding is one of the UTF types, the bytes in any bi-
naries must be valid in that encoding.
Errors can occur for various reasons, including the follow-
ing:
* "Pure" decoding errors (like the upper bits of the bytes
being wrong).
* The bytes are decoded to a too large number.
* The bytes are decoded to a code point in the invalid Uni-
code range.
* Encoding is "overlong", meaning that a number should have
been encoded in fewer bytes.
The case of a truncated UTF is handled specially, see the
paragraph about incomplete binaries below.
If InEncoding is latin1, binaries are always valid as long
as they contain whole bytes, as each byte falls into the
valid ISO Latin-1 range.
A special type of error is when no actual invalid integers or
bytes are found, but a trailing binary() consists of too few
bytes to decode the last character. This error can occur if
bytes are read from a file in chunks or if binaries in other
ways are split on non-UTF character boundaries. An incomplete
tuple is then returned instead of the error tuple. It consists
of the same parts as the error tuple, but the tag is incomplete
instead of error and the last element is always guaranteed to be
a binary consisting of the first part of a (so far) valid UTF
character.
If one UTF character is split over two consecutive binaries in
the Data, the conversion succeeds. This means that a character
can be decoded from a range of binaries as long as the whole
range is specified as input without errors occurring.
Example:
decode_data(Data) ->
case unicode:characters_to_list(Data,unicode) of
{incomplete,Encoded, Rest} ->
More = get_some_more_data(),
Encoded ++ decode_data([Rest, More]);
{error,Encoded,Rest} ->
handle_error(Encoded,Rest);
List ->
List
end.
However, bit strings that are not whole bytes are not allowed,
so a UTF character must be split along 8-bit boundaries to ever
be decoded.
A badarg exception is thrown for the following cases:
* Any parameters are of the wrong type.
* The list structure is invalid (a number as tail).
* The binaries do not contain whole bytes (bit strings).
characters_to_nfc_list(CD :: chardata()) ->
[char()] | {error, [char()], chardata()}
Converts a possibly deep list of characters and binaries into a
Normalized Form of canonical equivalent Composed characters ac-
cording to the Unicode standard.
Any binaries in the input must be encoded with utf8 encoding.
The result is a list of characters.
3> unicode:characters_to_nfc_list([<<"abc..a">>,[778],$a,[776],$o,[776]]).
"abc..aao"
characters_to_nfc_binary(CD :: chardata()) ->
unicode_binary() |
{error, unicode_binary(), chardata()}
Converts a possibly deep list of characters and binaries into a
Normalized Form of canonical equivalent Composed characters ac-
cording to the Unicode standard.
Any binaries in the input must be encoded with utf8 encoding.
The result is an utf8 encoded binary.
4> unicode:characters_to_nfc_binary([<<"abc..a">>,[778],$a,[776],$o,[776]]).
<<"abc..aao"/utf8>>
characters_to_nfd_list(CD :: chardata()) ->
[char()] | {error, [char()], chardata()}
Converts a possibly deep list of characters and binaries into a
Normalized Form of canonical equivalent Decomposed characters
according to the Unicode standard.
Any binaries in the input must be encoded with utf8 encoding.
The result is a list of characters.
1> unicode:characters_to_nfd_list("abc..aao").
[97,98,99,46,46,97,778,97,776,111,776]
characters_to_nfd_binary(CD :: chardata()) ->
unicode_binary() |
{error, unicode_binary(), chardata()}
Converts a possibly deep list of characters and binaries into a
Normalized Form of canonical equivalent Decomposed characters
according to the Unicode standard.
Any binaries in the input must be encoded with utf8 encoding.
The result is an utf8 encoded binary.
2> unicode:characters_to_nfd_binary("abc..aao").
<<97,98,99,46,46,97,204,138,97,204,136,111,204,136>>
characters_to_nfkc_list(CD :: chardata()) ->
[char()] |
{error, [char()], chardata()}
Converts a possibly deep list of characters and binaries into a
Normalized Form of compatibly equivalent Composed characters ac-
cording to the Unicode standard.
Any binaries in the input must be encoded with utf8 encoding.
The result is a list of characters.
3> unicode:characters_to_nfkc_list([<<"abc..a">>,[778],$a,[776],$o,[776],[65299,65298]]).
"abc..aao32"
characters_to_nfkc_binary(CD :: chardata()) ->
unicode_binary() |
{error, unicode_binary(), chardata()}
Converts a possibly deep list of characters and binaries into a
Normalized Form of compatibly equivalent Composed characters ac-
cording to the Unicode standard.
Any binaries in the input must be encoded with utf8 encoding.
The result is an utf8 encoded binary.
4> unicode:characters_to_nfkc_binary([<<"abc..a">>,[778],$a,[776],$o,[776],[65299,65298]]).
<<"abc..aao32"/utf8>>
characters_to_nfkd_list(CD :: chardata()) ->
[char()] |
{error, [char()], chardata()}
Converts a possibly deep list of characters and binaries into a
Normalized Form of compatibly equivalent Decomposed characters
according to the Unicode standard.
Any binaries in the input must be encoded with utf8 encoding.
The result is a list of characters.
1> unicode:characters_to_nfkd_list(["abc..aao",[65299,65298]]).
[97,98,99,46,46,97,778,97,776,111,776,51,50]
characters_to_nfkd_binary(CD :: chardata()) ->
unicode_binary() |
{error, unicode_binary(), chardata()}
Converts a possibly deep list of characters and binaries into a
Normalized Form of compatibly equivalent Decomposed characters
according to the Unicode standard.
Any binaries in the input must be encoded with utf8 encoding.
The result is an utf8 encoded binary.
2> unicode:characters_to_nfkd_binary(["abc..aao",[65299,65298]]).
<<97,98,99,46,46,97,204,138,97,204,136,111,204,136,51,50>>
encoding_to_bom(InEncoding) -> Bin
Types:
Bin = binary()
A binary() such that byte_size(Bin) >= 4.
InEncoding = encoding()
Creates a UTF Byte Order Mark (BOM) as a binary from the sup-
plied InEncoding. The BOM is, if supported at all, expected to
be placed first in UTF encoded files or messages.
The function returns <<>> for latin1 encoding, as there is no
BOM for ISO Latin-1.
Notice that the BOM for UTF-8 is seldom used, and it is really
not a byte order mark. There are obviously no byte order issues
with UTF-8, so the BOM is only there to differentiate UTF-8 en-
coding from other UTF formats.
Ericsson AB stdlib 3.13 unicode(3erl)