perluniintro - Perl Unicode introduction
This document gives a general idea of Unicode and how to use Unicode in Perl.
Unicode is a character set standard which plans to codify all of the writing systems of the world, plus many other symbols.
Unicode and ISO/IEC 10646 are coordinated standards that provide code points for characters in almost all modern character set standards, covering more than 30 writing systems and hundreds of languages, including all commercially-important modern languages. All characters in the largest Chinese, Japanese, and Korean dictionaries are also encoded. The standards will eventually cover almost all characters in more than 250 writing systems and thousands of languages. Unicode 1.0 was released in October 1991, and 4.0 in April 2003.
A Unicode character is an abstract entity. It is not bound to any
particular integer width, especially not to the C language char
.
Unicode is language-neutral and display-neutral: it does not encode the
language of the text and it does not define fonts or other graphical
layout details. Unicode operates on characters and on text built from
those characters.
Unicode defines characters like LATIN CAPITAL LETTER A
or GREEK
SMALL LETTER ALPHA
and unique numbers for the characters, in this
case 0x0041 and 0x03B1, respectively. These unique numbers are called
code points.
The Unicode standard prefers using hexadecimal notation for the code
points. If numbers like 0x0041
are unfamiliar to you, take a peek
at a later section, Hexadecimal Notation. The Unicode standard
uses the notation U+0041 LATIN CAPITAL LETTER A, to give the
hexadecimal code point and the normative name of the character.
Unicode also defines various properties for the characters, like "uppercase" or "lowercase", "decimal digit", or "punctuation"; these properties are independent of the names of the characters. Furthermore, various operations on the characters like uppercasing, lowercasing, and collating (sorting) are defined.
A Unicode character consists either of a single code point, or a
base character (like LATIN CAPITAL LETTER A
), followed by one or
more modifiers (like COMBINING ACUTE ACCENT
). This sequence of
base character and modifiers is called a combining character
sequence.
Whether to call these combining character sequences "characters" depends on your point of view. If you are a programmer, you probably would tend towards seeing each element in the sequences as one unit, or "character". The whole sequence could be seen as one "character", however, from the user's point of view, since that's probably what it looks like in the context of the user's language.
With this "whole sequence" view of characters, the total number of characters is open-ended. But in the programmer's "one unit is one character" point of view, the concept of "characters" is more deterministic. In this document, we take that second point of view: one "character" is one Unicode code point, be it a base character or a combining character.
For some combinations, there are precomposed characters.
LATIN CAPITAL LETTER A WITH ACUTE
, for example, is defined as
a single code point. These precomposed characters are, however,
only available for some combinations, and are mainly
meant to support round-trip conversions between Unicode and legacy
standards (like the ISO 8859). In the general case, the composing
method is more extensible. To support conversion between
different compositions of the characters, various normalization
forms to standardize representations are also defined.
Because of backward compatibility with legacy encodings, the "a unique number for every character" idea breaks down a bit: instead, there is "at least one number for every character". The same character could be represented differently in several legacy encodings. The converse is also not true: some code points do not have an assigned character. Firstly, there are unallocated code points within otherwise used blocks. Secondly, there are special Unicode control characters that do not represent true characters.
A common myth about Unicode is that it would be "16-bit", that is,
Unicode is only represented as 0x10000
(or 65536) characters from
0x0000
to 0xFFFF
. This is untrue. Since Unicode 2.0 (July
1996), Unicode has been defined all the way up to 21 bits (0x10FFFF
),
and since Unicode 3.1 (March 2001), characters have been defined
beyond 0xFFFF
. The first 0x10000
characters are called the
Plane 0, or the Basic Multilingual Plane (BMP). With Unicode
3.1, 17 (yes, seventeen) planes in all were defined--but they are
nowhere near full of defined characters, yet.
Another myth is that the 256-character blocks have something to
do with languages--that each block would define the characters used
by a language or a set of languages. This is also untrue.
The division into blocks exists, but it is almost completely
accidental--an artifact of how the characters have been and
still are allocated. Instead, there is a concept called scripts,
which is more useful: there is Latin
script, Greek
script, and
so on. Scripts usually span varied parts of several blocks.
For further information see Unicode::UCD.
The Unicode code points are just abstract numbers. To input and output these abstract numbers, the numbers must be encoded or serialised somehow. Unicode defines several character encoding forms, of which UTF-8 is perhaps the most popular. UTF-8 is a variable length encoding that encodes Unicode characters as 1 to 6 bytes (only 4 with the currently defined characters). Other encodings include UTF-16 and UTF-32 and their big- and little-endian variants (UTF-8 is byte-order independent) The ISO/IEC 10646 defines the UCS-2 and UCS-4 encoding forms.
For more information about encodings--for instance, to learn what surrogates and byte order marks (BOMs) are--see perlunicode.
Starting from Perl 5.6.0, Perl has had the capacity to handle Unicode natively. Perl 5.8.0, however, is the first recommended release for serious Unicode work. The maintenance release 5.6.1 fixed many of the problems of the initial Unicode implementation, but for example regular expressions still do not work with Unicode in 5.6.1.
Starting from Perl 5.8.0, the use of use utf8
is no longer
necessary. In earlier releases the utf8
pragma was used to declare
that operations in the current block or file would be Unicode-aware.
This model was found to be wrong, or at least clumsy: the "Unicodeness"
is now carried with the data, instead of being attached to the
operations. Only one case remains where an explicit use utf8
is
needed: if your Perl script itself is encoded in UTF-8, you can use
UTF-8 in your identifier names, and in string and regular expression
literals, by saying use utf8
. This is not the default because
scripts with legacy 8-bit data in them would break. See utf8.
Perl supports both pre-5.6 strings of eight-bit native bytes, and strings of Unicode characters. The principle is that Perl tries to keep its data as eight-bit bytes for as long as possible, but as soon as Unicodeness cannot be avoided, the data is transparently upgraded to Unicode.
Internally, Perl currently uses either whatever the native eight-bit
character set of the platform (for example Latin-1) is, defaulting to
UTF-8, to encode Unicode strings. Specifically, if all code points in
the string are 0xFF
or less, Perl uses the native eight-bit
character set. Otherwise, it uses UTF-8.
A user of Perl does not normally need to know nor care how Perl happens to encode its internal strings, but it becomes relevant when outputting Unicode strings to a stream without a PerlIO layer -- one with the "default" encoding. In such a case, the raw bytes used internally (the native character set or UTF-8, as appropriate for each string) will be used, and a "Wide character" warning will be issued if those strings contain a character beyond 0x00FF.
For example,
- perl -e 'print "\x{DF}\n", "\x{0100}\x{DF}\n"'
produces a fairly useless mixture of native bytes and UTF-8, as well as a warning:
- Wide character in print at ...
To output UTF-8, use the :encoding
or :utf8
output layer. Prepending
- binmode(STDOUT, ":utf8");
to this sample program ensures that the output is completely UTF-8, and removes the program's warning.
You can enable automatic UTF-8-ification of your standard file
handles, default open() layer, and @ARGV
by using either
the -C
command line switch or the PERL_UNICODE
environment
variable, see perlrun for the documentation of the -C
switch.
Note that this means that Perl expects other software to work, too: if Perl has been led to believe that STDIN should be UTF-8, but then STDIN coming in from another command is not UTF-8, Perl will complain about the malformed UTF-8.
All features that combine Unicode and I/O also require using the new
PerlIO feature. Almost all Perl 5.8 platforms do use PerlIO, though:
you can see whether yours is by running "perl -V" and looking for
useperlio=define
.
Perl 5.8.0 also supports Unicode on EBCDIC platforms. There, Unicode support is somewhat more complex to implement since additional conversions are needed at every step. Some problems remain, see perlebcdic for details.
In any case, the Unicode support on EBCDIC platforms is better than in the 5.6 series, which didn't work much at all for EBCDIC platform. On EBCDIC platforms, the internal Unicode encoding form is UTF-EBCDIC instead of UTF-8. The difference is that as UTF-8 is "ASCII-safe" in that ASCII characters encode to UTF-8 as-is, while UTF-EBCDIC is "EBCDIC-safe".
To create Unicode characters in literals for code points above 0xFF
,
use the \x{...}
notation in double-quoted strings:
- my $smiley = "\x{263a}";
Similarly, it can be used in regular expression literals
- $smiley =~ /\x{263a}/;
At run-time you can use chr():
See Further Resources for how to find all these numeric codes.
Naturally, ord() will do the reverse: it turns a character into
a code point.
Note that \x..
(no {}
and only two hexadecimal digits), \x{...}
,
and chr(...) for arguments less than 0x100
(decimal 256)
generate an eight-bit character for backward compatibility with older
Perls. For arguments of 0x100
or more, Unicode characters are
always produced. If you want to force the production of Unicode
characters regardless of the numeric value, use pack("U", ...)
instead of \x..
, \x{...}
, or chr().
You can also use the charnames
pragma to invoke characters
by name in double-quoted strings:
And, as mentioned above, you can also pack() numbers into Unicode
characters:
Note that both \x{...}
and \N{...}
are compile-time string
constants: you cannot use variables in them. if you want similar
run-time functionality, use chr() and charnames::vianame()
.
If you want to force the result to Unicode characters, use the special
"U0"
prefix. It consumes no arguments but causes the following bytes
to be interpreted as the UTF-8 encoding of Unicode characters:
Likewise, you can stop such UTF-8 interpretation by using the special
"C0"
prefix.
Handling Unicode is for the most part transparent: just use the
strings as usual. Functions like index(), length(), and
substr() will work on the Unicode characters; regular expressions
will work on the Unicode characters (see perlunicode and perlretut).
Note that Perl considers combining character sequences to be separate characters, so for example
will print 2, not 1. The only exception is that regular expressions
have \X
for matching a combining character sequence.
Life is not quite so transparent, however, when working with legacy encodings, I/O, and certain special cases:
When you combine legacy data and Unicode the legacy data needs to be upgraded to Unicode. Normally ISO 8859-1 (or EBCDIC, if applicable) is assumed.
The Encode
module knows about many encodings and has interfaces
for doing conversions between those encodings:
- use Encode 'decode';
- $data = decode("iso-8859-3", $data); # convert from legacy to utf-8
Normally, writing out Unicode data
- print FH $some_string_with_unicode, "\n";
produces raw bytes that Perl happens to use to internally encode the
Unicode string. Perl's internal encoding depends on the system as
well as what characters happen to be in the string at the time. If
any of the characters are at code points 0x100
or above, you will get
a warning. To ensure that the output is explicitly rendered in the
encoding you desire--and to avoid the warning--open the stream with
the desired encoding. Some examples:
and on already open streams, use binmode():
The matching of encoding names is loose: case does not matter, and
many encodings have several aliases. Note that the :utf8
layer
must always be specified exactly like that; it is not subject to
the loose matching of encoding names. Also note that :utf8
is unsafe for
input, because it accepts the data without validating that it is indeed valid
UTF8.
See PerlIO for the :utf8
layer, PerlIO::encoding and
Encode::PerlIO for the :encoding()
layer, and
Encode::Supported for many encodings supported by the Encode
module.
Reading in a file that you know happens to be encoded in one of the Unicode or legacy encodings does not magically turn the data into Unicode in Perl's eyes. To do that, specify the appropriate layer when opening files
The I/O layers can also be specified more flexibly with
the open pragma. See open, or look at the following example.
With the open pragma you can use the :locale
layer
- BEGIN { $ENV{LC_ALL} = $ENV{LANG} = 'ru_RU.KOI8-R' }
- # the :locale will probe the locale environment variables like LC_ALL
- use open OUT => ':locale'; # russki parusski
- open(O, ">koi8");
- print O chr(0x430); # Unicode CYRILLIC SMALL LETTER A = KOI8-R 0xc1
- close O;
- open(I, "<koi8");
- printf "%#x\n", ord(<I>), "\n"; # this should print 0xc1
- close I;
These methods install a transparent filter on the I/O stream that converts data from the specified encoding when it is read in from the stream. The result is always Unicode.
The open pragma affects all the open() calls after the pragma by
setting default layers. If you want to affect only certain
streams, use explicit layers directly in the open() call.
You can switch encodings on an already opened stream by using
binmode(); see binmode.
The :locale
does not currently (as of Perl 5.8.0) work with
open() and binmode(), only with the open pragma. The
:utf8
and :encoding(...)
methods do work with all of open(),
binmode(), and the open pragma.
Similarly, you may use these I/O layers on output streams to automatically convert Unicode to the specified encoding when it is written to the stream. For example, the following snippet copies the contents of the file "text.jis" (encoded as ISO-2022-JP, aka JIS) to the file "text.utf8", encoded as UTF-8:
The naming of encodings, both by the open() and by the open
pragma allows for flexible names: koi8-r
and KOI8R
will both be
understood.
Common encodings recognized by ISO, MIME, IANA, and various other standardisation organisations are recognised; for a more detailed list see Encode::Supported.
read() reads characters and returns the number of characters.
seek() and tell() operate on byte counts, as do sysread()
and sysseek().
Notice that because of the default behaviour of not doing any conversion upon input if there is no default layer, it is easy to mistakenly write code that keeps on expanding a file by repeatedly encoding the data:
If you run this code twice, the contents of the file will be twice
UTF-8 encoded. A use open ':encoding(utf8)'
would have avoided the
bug, or explicitly opening also the file for input as UTF-8.
NOTE: the :utf8
and :encoding
features work only if your
Perl has been built with the new PerlIO feature (which is the default
on most systems).
Sometimes you might want to display Perl scalars containing Unicode as
simple ASCII (or EBCDIC) text. The following subroutine converts
its argument so that Unicode characters with code points greater than
255 are displayed as \x{...}
, control characters (like \n
) are
displayed as \x..
, and the rest of the characters as themselves:
For example,
- nice_string("foo\x{100}bar\n")
returns the string
- 'foo\x{0100}bar\x0A'
which is ready to be printed.
Bit Complement Operator ~ And vec()
The bit complement operator ~
may produce surprising results if
used on strings containing characters with ordinal values above
255. In such a case, the results are consistent with the internal
encoding of the characters, but not with much else. So don't do
that. Similarly for vec(): you will be operating on the
internally-encoded bit patterns of the Unicode characters, not on
the code point values, which is very probably not what you want.
Peeking At Perl's Internal Encoding
Normal users of Perl should never care how Perl encodes any particular Unicode string (because the normal ways to get at the contents of a string with Unicode--via input and output--should always be via explicitly-defined I/O layers). But if you must, there are two ways of looking behind the scenes.
One way of peeking inside the internal encoding of Unicode characters
is to use unpack("C*", ... to get the bytes of whatever the string
encoding happens to be, or unpack("U0..", ...)
to get the bytes of the
UTF-8 encoding:
Yet another way would be to use the Devel::Peek module:
- perl -MDevel::Peek -e 'Dump(chr(0x100))'
That shows the UTF8
flag in FLAGS and both the UTF-8 bytes
and Unicode characters in PV
. See also later in this document
the discussion about the utf8::is_utf8()
function.
String Equivalence
The question of string equivalence turns somewhat complicated in Unicode: what do you mean by "equal"?
(Is LATIN CAPITAL LETTER A WITH ACUTE
equal to
LATIN CAPITAL LETTER A
?)
The short answer is that by default Perl compares equivalence (eq
,
ne
) based only on code points of the characters. In the above
case, the answer is no (because 0x00C1 != 0x0041). But sometimes, any
CAPITAL LETTER As should be considered equal, or even As of any case.
The long answer is that you need to consider character normalization and casing issues: see Unicode::Normalize, Unicode Technical Reports #15 and #21, Unicode Normalization Forms and Case Mappings, http://www.unicode.org/unicode/reports/tr15/ and http://www.unicode.org/unicode/reports/tr21/
As of Perl 5.8.0, the "Full" case-folding of Case Mappings/SpecialCasing is implemented.
String Collation
People like to see their strings nicely sorted--or as Unicode parlance goes, collated. But again, what do you mean by collate?
(Does LATIN CAPITAL LETTER A WITH ACUTE
come before or after
LATIN CAPITAL LETTER A WITH GRAVE
?)
The short answer is that by default, Perl compares strings (lt
,
le
, cmp
, ge
, gt
) based only on the code points of the
characters. In the above case, the answer is "after", since
0x00C1
> 0x00C0
.
The long answer is that "it depends", and a good answer cannot be given without knowing (at the very least) the language context. See Unicode::Collate, and Unicode Collation Algorithm http://www.unicode.org/unicode/reports/tr10/
Character Ranges and Classes
Character ranges in regular expression character classes (/[a-z]/
)
and in the tr/// (also known as y///) operator are not magically
Unicode-aware. What this means that [A-Za-z]
will not magically start
to mean "all alphabetic letters"; not that it does mean that even for
8-bit characters, you should be using /[[:alpha:]]/
in that case.
For specifying character classes like that in regular expressions,
you can use the various Unicode properties--\pL
, or perhaps
\p{Alphabetic}
, in this particular case. You can use Unicode
code points as the end points of character ranges, but there is no
magic associated with specifying a certain range. For further
information--there are dozens of Unicode character classes--see
perlunicode.
String-To-Number Conversions
Unicode does define several other decimal--and numeric--characters besides the familiar 0 to 9, such as the Arabic and Indic digits. Perl does not support string-to-number conversion for digits other than ASCII 0 to 9 (and ASCII a to f for hexadecimal).
Will My Old Scripts Break?
Very probably not. Unless you are generating Unicode characters
somehow, old behaviour should be preserved. About the only behaviour
that has changed and which could start generating Unicode is the old
behaviour of chr() where supplying an argument more than 255
produced a character modulo 255. chr(300), for example, was equal
to chr(45) or "-" (in ASCII), now it is LATIN CAPITAL LETTER I WITH
BREVE.
How Do I Make My Scripts Work With Unicode?
Very little work should be needed since nothing changes until you generate Unicode data. The most important thing is getting input as Unicode; for that, see the earlier I/O discussion.
How Do I Know Whether My String Is In Unicode?
You shouldn't care. No, you really shouldn't. No, really. If you have to care--beyond the cases described above--it means that we didn't get the transparency of Unicode quite right.
Okay, if you insist:
- print utf8::is_utf8($string) ? 1 : 0, "\n";
But note that this doesn't mean that any of the characters in the
string are necessary UTF-8 encoded, or that any of the characters have
code points greater than 0xFF (255) or even 0x80 (128), or that the
string has any characters at all. All the is_utf8()
does is to
return the value of the internal "utf8ness" flag attached to the
$string
. If the flag is off, the bytes in the scalar are interpreted
as a single byte encoding. If the flag is on, the bytes in the scalar
are interpreted as the (multi-byte, variable-length) UTF-8 encoded code
points of the characters. Bytes added to an UTF-8 encoded string are
automatically upgraded to UTF-8. If mixed non-UTF-8 and UTF-8 scalars
are merged (double-quoted interpolation, explicit concatenation, and
printf/sprintf parameter substitution), the result will be UTF-8 encoded
as if copies of the byte strings were upgraded to UTF-8: for example,
- $a = "ab\x80c";
- $b = "\x{100}";
- print "$a = $b\n";
the output string will be UTF-8-encoded ab\x80c = \x{100}\n
, but
$a
will stay byte-encoded.
Sometimes you might really need to know the byte length of a string
instead of the character length. For that use either the
Encode::encode_utf8()
function or the bytes
pragma and its only
defined function length():
How Do I Detect Data That's Not Valid In a Particular Encoding?
Use the Encode
package to try converting it.
For example,
Or use unpack to try decoding it:
If invalid, a Malformed UTF-8 character
warning is produced. The "U0" means "expect strictly UTF-8 encoded
Unicode". Without that the unpack("U*", ...)
would accept also
data like chr(0xFF), similarly to the pack as we saw earlier.
How Do I Convert Binary Data Into a Particular Encoding, Or Vice Versa?
This probably isn't as useful as you might think. Normally, you shouldn't need to.
In one sense, what you are asking doesn't make much sense: encodings are for characters, and binary data are not "characters", so converting "data" into some encoding isn't meaningful unless you know in what character set and encoding the binary data is in, in which case it's not just binary data, now is it?
If you have a raw sequence of bytes that you know should be
interpreted via a particular encoding, you can use Encode
:
- use Encode 'from_to';
- from_to($data, "iso-8859-1", "utf-8"); # from latin-1 to utf-8
The call to from_to()
changes the bytes in $data
, but nothing
material about the nature of the string has changed as far as Perl is
concerned. Both before and after the call, the string $data
contains just a bunch of 8-bit bytes. As far as Perl is concerned,
the encoding of the string remains as "system-native 8-bit bytes".
You might relate this to a fictional 'Translate' module:
The contents of the string changes, but not the nature of the string. Perl doesn't know any more after the call than before that the contents of the string indicates the affirmative.
Back to converting data. If you have (or want) data in your system's native 8-bit encoding (e.g. Latin-1, EBCDIC, etc.), you can use pack/unpack to convert to/from Unicode.
If you have a sequence of bytes you know is valid UTF-8, but Perl doesn't know it yet, you can make Perl a believer, too:
- use Encode 'decode_utf8';
- $Unicode = decode_utf8($bytes);
You can convert well-formed UTF-8 to a sequence of bytes, but if
you just want to convert random binary data into UTF-8, you can't.
Any random collection of bytes isn't well-formed UTF-8. You can
use unpack("C*", $string)
for the former, and you can create
well-formed Unicode data by pack("U*", 0xff, ...)
.
How Do I Display Unicode? How Do I Input Unicode?
See http://www.alanwood.net/unicode/ and http://www.cl.cam.ac.uk/~mgk25/unicode.html
How Does Unicode Work With Traditional Locales?
In Perl, not very well. Avoid using locales through the locale
pragma. Use only one or the other. But see perlrun for the
description of the -C
switch and its environment counterpart,
$ENV{PERL_UNICODE}
to see how to enable various Unicode features,
for example by using locale settings.
The Unicode standard prefers using hexadecimal notation because
that more clearly shows the division of Unicode into blocks of 256 characters.
Hexadecimal is also simply shorter than decimal. You can use decimal
notation, too, but learning to use hexadecimal just makes life easier
with the Unicode standard. The U+HHHH
notation uses hexadecimal,
for example.
The 0x
prefix means a hexadecimal number, the digits are 0-9 and
a-f (or A-F, case doesn't matter). Each hexadecimal digit represents
four bits, or half a byte. print 0x..., "\n"
will show a
hexadecimal number in decimal, and printf "%x\n", $decimal
will
show a decimal number in hexadecimal. If you have just the
"hex digits" of a hexadecimal number, you can use the hex() function.
Unicode Consortium
Unicode FAQ
Unicode Glossary
Unicode Useful Resources
Unicode and Multilingual Support in HTML, Fonts, Web Browsers and Other Applications
UTF-8 and Unicode FAQ for Unix/Linux
Legacy Character Sets
The Unicode support files live within the Perl installation in the directory
- $Config{installprivlib}/unicore
in Perl 5.8.0 or newer, and
- $Config{installprivlib}/unicode
in the Perl 5.6 series. (The renaming to lib/unicore was done to
avoid naming conflicts with lib/Unicode in case-insensitive filesystems.)
The main Unicode data file is UnicodeData.txt (or Unicode.301 in
Perl 5.6.1.) You can find the $Config{installprivlib}
by
- perl "-V:installprivlib"
You can explore various information from the Unicode data files using
the Unicode::UCD
module.
If you cannot upgrade your Perl to 5.8.0 or later, you can still
do some Unicode processing by using the modules Unicode::String
,
Unicode::Map8
, and Unicode::Map
, available from CPAN.
If you have the GNU recode installed, you can also use the
Perl front-end Convert::Recode
for character conversions.
The following are fast conversions from ISO 8859-1 (Latin-1) bytes to UTF-8 bytes and back, the code works even with older Perl 5 versions.
- # ISO 8859-1 to UTF-8
- s/([\x80-\xFF])/chr(0xC0|ord($1)>>6).chr(0x80|ord($1)&0x3F)/eg;
- # UTF-8 to ISO 8859-1
- s/([\xC2\xC3])([\x80-\xBF])/chr(ord($1)<<6&0xC0|ord($2)&0x3F)/eg;
perlunitut, perlunicode, Encode, open, utf8, bytes, perlretut, perlrun, Unicode::Collate, Unicode::Normalize, Unicode::UCD
Thanks to the kind readers of the perl5-porters@perl.org, perl-unicode@perl.org, linux-utf8@nl.linux.org, and unicore@unicode.org mailing lists for their valuable feedback.
Copyright 2001-2002 Jarkko Hietaniemi <jhi@iki.fi>
This document may be distributed under the same terms as Perl itself.