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NAME
unicode - Functions for converting Unicode characters.
DESCRIPTION
This module contains functions for converting between different character 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
integers, 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 Erlang/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 canonical-equivalent Unicode characters
as equal. All characters should thus be normalized to one form once on the system borders. One of the
following functions can convert characters to their normalized forms characters_to_nfc_list/1,
characters_to_nfc_binary/1, characters_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 information 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 encoding 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 binary. If the supplied binary Bin
begins with a valid BOM for either 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_chardata()
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_chardata()
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_chardata()
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_chardata()
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_chardata()
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 following:
* ISO Latin-1 (0-255, one character per byte). Here, case parameter 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 regardless 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 iodata() type, but for unicode,
parameter Data can contain integers > 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 representation for further processing. For writing the data to an external
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 Unicode/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 offending
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 binaries must be valid in that
encoding.
Errors can occur for various reasons, including the following:
* "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 Unicode 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 according 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..åäö"
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 according 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..åäö"/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..åäö").
[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..åäö").
<<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 according 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..åäö32"
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 according 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..åäö32"/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..åäö",[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..åäö",[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 supplied 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
encoding from other UTF formats.