perluniintro
DESCRIPTION
This document gives a general idea of Unicode and how to
use Unicode in Perl.
Unicode
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 commer
cially-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 thou
sands 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 lay
out 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 unfamil
iar 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 charac
ters, 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 col
lating (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 modi
ment, 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 charac
ters 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 extensi
ble. To support conversion between different compositions
of the characters, various normalization forms to stan
dardize 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 dif
ferently 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 some
thing 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
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 regu
lar 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's Unicode Model
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 ...
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 mal
formed UTF-8.
All features that combine Unicode and I/O also require
using the new PerlIO feature. Almost all Perl 5.8 plat
forms do use PerlIO, though: you can see whether yours is
by running "perl -V" and looking for "useperlio=define".
Unicode and EBCDIC
Perl 5.8.0 also supports Unicode on EBCDIC platforms.
There, Unicode support is somewhat more complex to imple
ment 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 inter
nal 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".
Creating Unicode
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()":
my $hebrew_alef = chr(0x05d0);
See "Further Resources" for how to find all these numeric
codes.
Naturally, "ord()" will do the reverse: it turns a charac
ter into a code point.
Note that "\x.." (no "{}" and only two hexadecimal dig
its), "\x{...}", and "chr(...)" for arguments less than
0x100 (decimal 256) generate an eight-bit character for
backward compatibility with older Perls. For arguments of
my $georgian_an = pack("U", 0x10a0);
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()".
Also note that if all the code points for pack "U" are
below 0x100, bytes will be generated, just like if you
were using "chr()".
my $bytes = pack("U*", 0x80, 0xFF);
If you want to force the result to Unicode characters, use
the special "U0" prefix. It consumes no arguments but
forces the result to be in Unicode characters, instead of
bytes.
my $chars = pack("U0U*", 0x80, 0xFF);
Handling Unicode
Handling Unicode is for the most part transparent: just
use the strings as usual. Functions like "index()",
"length()", and "substr()" will work on the Unicode char
acters; regular expressions will work on the Unicode char
acters (see perlunicode and perlretut).
Note that Perl considers combining character sequences to
be characters, so for example
use charnames ':full';
print length("\N{LATIN CAPITAL LETTER A}\N{COMBINING ACUTE ACCENT}"), "\n";
will print 2, not 1. The only exception is that regular
expressions have "\X" for matching a combining character
sequence.
Life is not quite so transparent, however, when working
with legacy encodings, I/O, and certain special cases:
Legacy Encodings
When you combine legacy data and Unicode the legacy data
needs to be upgraded to Unicode. Normally ISO 8859-1 (or
EBCDIC, if applicable) is assumed. You can override this
assumption by using the "encoding" pragma, for example
use encoding 'latin2'; # ISO 8859-2
in which case literals (string or regular expressions),
"chr()", and "ord()" in your whole script are assumed to
use Encode 'from_to';
from_to($data, "iso-8859-3", "utf-8"); # from legacy to utf-8
Unicode I/O
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:
open FH, ">:utf8", "file";
open FH, ">:encoding(ucs2)", "file";
open FH, ">:encoding(UTF-8)", "file";
open FH, ">:encoding(shift_jis)", "file";
and on already open streams, use "binmode()":
binmode(STDOUT, ":utf8");
binmode(STDOUT, ":encoding(ucs2)");
binmode(STDOUT, ":encoding(UTF-8)");
binmode(STDOUT, ":encoding(shift_jis)");
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.
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
open(my $fh,'<:utf8', 'anything');
my $line_of_unicode = <$fh>;
open(my $fh,'<:encoding(Big5)', 'anything');
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;
or you can also use the ':encoding(...)' layer
open(my $epic,'<:encoding(iso-8859-7)','iliad.greek');
my $line_of_unicode = <$epic>;
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 Uni
code.
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" in perlfunc.
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 follow
ing 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:
open(my $nihongo, '<:encoding(iso-2022-jp)', 'text.jis');
open(my $unicode, '>:utf8', 'text.utf8');
while (<$nihongo>) { print $unicode $_ }
The naming of encodings, both by the "open()" and by the
"open" pragma, is similar to the "encoding" pragma in that
it allows for flexible names: "koi8-r" and "KOI8R" will
both be understood.
open F, "file";
local $/; ## read in the whole file of 8-bit characters
$t = <F>;
close F;
open F, ">:utf8", "file";
print F $t; ## convert to UTF-8 on output
close F;
If you run this code twice, the contents of the file will
be twice UTF-8 encoded. A "use open ':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).
Displaying Unicode As Text
Sometimes you might want to display Perl scalars contain
ing Unicode as simple ASCII (or EBCDIC) text. The follow
ing subroutine converts its argument so that Unicode char
acters 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:
sub nice_string {
join("",
map { $_ > 255 ? # if wide character...
sprintf("\\x{%04X}", $_) : # \x{...}
chr($_) =~ /[[:cntrl:]]/ ? # else if control character ...
sprintf("\\x%02X", $_) : # \x..
quotemeta(chr($_)) # else quoted or as themselves
} unpack("U*", $_[0])); # unpack Unicode characters
}
For example,
nice_string("foo\x{100}bar\n")
returns the string
'foo\x{0100}bar\x0A'
which is ready to be printed.
Special Cases
· Bit Complement Operator ~ And vec()
The bit complement operator "~" may produce surprising
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 or "unpack("H*", ...)" to display the
bytes:
# this prints c4 80 for the UTF-8 bytes 0xc4 0x80
print join(" ", unpack("H*", pack("U", 0x100))), "\n";
Yet another way would be to use the Devel::Peek mod
ule:
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.
Advanced Topics
· String Equivalence
The question of string equivalence turns somewhat com
plicated 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 CAPI
TAL 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::Normal
ize, Unicode Technical Reports #15 and #21, Unicode
Normalization Forms and Case Mappings, http://www.uni
code.org/unicode/reports/tr15/ and http://www.uni
code.org/unicode/reports/tr21/
As of Perl 5.8.0, the "Full" case-folding of Case Map
pings/SpecialCasing is implemented.
· String Collation
least) the language context. See Unicode::Collate,
and Unicode Collation Algorithm http://www.uni
code.org/unicode/reports/tr10/
Miscellaneous
· 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 proper
ties--"\pL", or perhaps "\p{Alphabetic}", in this par
ticular 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 fur
ther information--there are dozens of Unicode charac
ter 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).
Questions With Answers
· 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 pro
duced a character modulo 255. "chr(300)", for exam
ple, 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
But note that this doesn't mean that any of the char
acters 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()" func
tion or the "bytes" pragma and its only defined func
tion "length()":
my $unicode = chr(0x100);
print length($unicode), "\n"; # will print 1
require Encode;
print length(Encode::encode_utf8($unicode)), "\n"; # will print 2
use bytes;
print length($unicode), "\n"; # will also print 2
# (the 0xC4 0x80 of the UTF-8)
· How Do I Detect Data That's Not Valid In a Particular
Encoding?
Use the "Encode" package to try converting it. For
example,
use Encode 'encode_utf8';
if (encode_utf8($string_of_bytes_that_I_think_is_utf8)) {
# valid
} else {
# invalid
}
ing, Or Vice Versa?
This probably isn't as useful as you might think.
Normally, you shouldn't need to.
In one sense, what you are asking doesn't make much
sense: encodings are for characters, and binary data
are not "characters", so converting "data" into some
encoding isn't meaningful unless you know in what
character set and encoding the binary data is in, in
which case it's not just binary data, now is it?
If you have a raw sequence of bytes that you know
should be interpreted via a particular encoding, you
can use "Encode":
use Encode 'from_to';
from_to($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' mod
ule:
use Translate;
my $phrase = "Yes";
Translate::from_to($phrase, 'english', 'deutsch');
## phrase now contains "Ja"
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 indi
cates 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.
$native_string = pack("C*", unpack("U*", $Unicode_string));
$Unicode_string = pack("U*", unpack("C*", $native_string));
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:
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.
Hexadecimal Notation
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 hexadeci
mal 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.
print 0x0009, "\n"; # 9
print 0x000a, "\n"; # 10
print 0x000f, "\n"; # 15
print 0x0010, "\n"; # 16
print 0x0011, "\n"; # 17
print 0x0100, "\n"; # 256
print 0x0041, "\n"; # 65
printf "%x\n", 65; # 41
printf "%#x\n", 65; # 0x41
print hex("41"), "\n"; # 65
Further Resources
· Unicode Consortium
http://www.unicode.org/
· Unicode FAQ
· UTF-8 and Unicode FAQ for Unix/Linux
http://www.cl.cam.ac.uk/~mgk25/unicode.html
· Legacy Character Sets
http://www.czyborra.com/
http://www.eki.ee/letter/
· The Unicode support files live within the Perl instal
lation 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.
UNICODE IN OLDER PERLS
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 "Con
vert::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;
SEE ALSO
perlunicode, Encode, encoding, open, utf8, bytes, perlre
tut, perlrun, Unicode::Collate, Unicode::Normalize, Uni
code::UCD
perl v5.8.1 2003-09-02 PERLUNIINTRO(1)
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