perluniintro.1

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.\" ========================================================================
.\"
.IX Title "PERLUNIINTRO 1"
.TH PERLUNIINTRO 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
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.SH "NAME"
perluniintro \- Perl Unicode introduction
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This document gives a general idea of Unicode and how to use Unicode
in Perl.
.Sh "Unicode"
.IX Subsection "Unicode"
Unicode is a character set standard which plans to codify all of the
writing systems of the world, plus many other symbols.
.PP
Unicode and \s-1ISO/IEC\s0 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.
.PP
A Unicode \fIcharacter\fR is an abstract entity.  It is not bound to any
particular integer width, especially not to the C language \f(CW\*(C`char\*(C'\fR.
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.
.PP
Unicode defines characters like \f(CW\*(C`LATIN CAPITAL LETTER A\*(C'\fR or \f(CW\*(C`GREEK
SMALL LETTER ALPHA\*(C'\fR and unique numbers for the characters, in this
case 0x0041 and 0x03B1, respectively.  These unique numbers are called
\&\fIcode points\fR.
.PP
The Unicode standard prefers using hexadecimal notation for the code
points.  If numbers like \f(CW0x0041\fR are unfamiliar to you, take a peek
at a later section, \*(L"Hexadecimal Notation\*(R".  The Unicode standard
uses the notation \f(CW\*(C`U+0041 LATIN CAPITAL LETTER A\*(C'\fR, to give the
hexadecimal code point and the normative name of the character.
.PP
Unicode also defines various \fIproperties\fR for the characters, like
\&\*(L"uppercase\*(R" or \*(L"lowercase\*(R", \*(L"decimal digit\*(R", or \*(L"punctuation\*(R";
these properties are independent of the names of the characters.
Furthermore, various operations on the characters like uppercasing,
lowercasing, and collating (sorting) are defined.
.PP
A Unicode character consists either of a single code point, or a
\&\fIbase character\fR (like \f(CW\*(C`LATIN CAPITAL LETTER A\*(C'\fR), followed by one or
more \fImodifiers\fR (like \f(CW\*(C`COMBINING ACUTE ACCENT\*(C'\fR).  This sequence of
base character and modifiers is called a \fIcombining character
sequence\fR.
.PP
Whether to call these combining character sequences \*(L"characters\*(R"
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 \*(L"character\*(R".  The whole sequence could be seen as one \*(L"character\*(R",
however, from the user's point of view, since that's probably what it
looks like in the context of the user's language.
.PP
With this \*(L"whole sequence\*(R" view of characters, the total number of
characters is open-ended. But in the programmer's \*(L"one unit is one
character\*(R" point of view, the concept of \*(L"characters\*(R" is more
deterministic.  In this document, we take that second  point of view:
one \*(L"character\*(R" is one Unicode code point, be it a base character or
a combining character.
.PP
For some combinations, there are \fIprecomposed\fR characters.
\&\f(CW\*(C`LATIN CAPITAL LETTER A WITH ACUTE\*(C'\fR, 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 \s-1ISO\s0 8859).  In the general case, the composing
method is more extensible.  To support conversion between
different compositions of the characters, various \fInormalization
forms\fR to standardize representations are also defined.
.PP
Because of backward compatibility with legacy encodings, the \*(L"a unique
number for every character\*(R" idea breaks down a bit: instead, there is
\&\*(L"at least one number for every character\*(R".  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.
.PP
A common myth about Unicode is that it would be \*(L"16\-bit\*(R", that is,
Unicode is only represented as \f(CW0x10000\fR (or 65536) characters from
\&\f(CW0x0000\fR to \f(CW0xFFFF\fR.  \fBThis is untrue.\fR  Since Unicode 2.0 (July
1996), Unicode has been defined all the way up to 21 bits (\f(CW0x10FFFF\fR),
and since Unicode 3.1 (March 2001), characters have been defined
beyond \f(CW0xFFFF\fR.  The first \f(CW0x10000\fR characters are called the
\&\fIPlane 0\fR, or the \fIBasic Multilingual Plane\fR (\s-1BMP\s0).  With Unicode
3.1, 17 (yes, seventeen) planes in all were defined\*(--but they are
nowhere near full of defined characters, yet.
.PP
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.  \fBThis is also untrue.\fR
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 \fIscripts\fR,
which is more useful: there is \f(CW\*(C`Latin\*(C'\fR script, \f(CW\*(C`Greek\*(C'\fR script, and
so on.  Scripts usually span varied parts of several blocks.
For further information see Unicode::UCD.
.PP
The Unicode code points are just abstract numbers.  To input and
output these abstract numbers, the numbers must be \fIencoded\fR or
\&\fIserialised\fR somehow.  Unicode defines several \fIcharacter encoding
forms\fR, of which \fI\s-1UTF\-8\s0\fR is perhaps the most popular.  \s-1UTF\-8\s0 is a
variable length encoding that encodes Unicode characters as 1 to 6
bytes (only 4 with the currently defined characters).  Other encodings
include \s-1UTF\-16\s0 and \s-1UTF\-32\s0 and their big\- and little-endian variants
(\s-1UTF\-8\s0 is byte-order independent) The \s-1ISO/IEC\s0 10646 defines the \s-1UCS\-2\s0
and \s-1UCS\-4\s0 encoding forms.
.PP
For more information about encodings\*(--for instance, to learn what
\&\fIsurrogates\fR and \fIbyte order marks\fR (BOMs) are\*(--see perlunicode.
.Sh "Perl's Unicode Support"
.IX Subsection "Perl's Unicode Support"
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.
.PP
\&\fBStarting from Perl 5.8.0, the use of \f(CB\*(C`use utf8\*(C'\fB is no longer
necessary.\fR In earlier releases the \f(CW\*(C`utf8\*(C'\fR 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 \*(L"Unicodeness\*(R"
is now carried with the data, instead of being attached to the
operations.  Only one case remains where an explicit \f(CW\*(C`use utf8\*(C'\fR is
needed: if your Perl script itself is encoded in \s-1UTF\-8\s0, you can use
\&\s-1UTF\-8\s0 in your identifier names, and in string and regular expression
literals, by saying \f(CW\*(C`use utf8\*(C'\fR.  This is not the default because
scripts with legacy 8\-bit data in them would break.  See utf8.
.Sh "Perl's Unicode Model"
.IX Subsection "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.
.PP
Internally, Perl currently uses either whatever the native eight-bit
character set of the platform (for example Latin\-1) is, defaulting to
\&\s-1UTF\-8\s0, to encode Unicode strings. Specifically, if all code points in
the string are \f(CW0xFF\fR or less, Perl uses the native eight-bit
character set.  Otherwise, it uses \s-1UTF\-8\s0.
.PP
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 \*(L"default\*(R" encoding.  In such a case, the raw bytes used internally
(the native character set or \s-1UTF\-8\s0, as appropriate for each string)
will be used, and a \*(L"Wide character\*(R" warning will be issued if those
strings contain a character beyond 0x00FF.
.PP
For example,
.PP
.Vb 1
\&      perl \-e \*(Aqprint "\ex{DF}\en", "\ex{0100}\ex{DF}\en"\*(Aq
.Ve
.PP
produces a fairly useless mixture of native bytes and \s-1UTF\-8\s0, as well
as a warning:
.PP
.Vb 1
\&     Wide character in print at ...
.Ve
.PP
To output \s-1UTF\-8\s0, use the \f(CW\*(C`:encoding\*(C'\fR or \f(CW\*(C`:utf8\*(C'\fR output layer.  Prepending
.PP
.Vb 1
\&      binmode(STDOUT, ":utf8");
.Ve
.PP
to this sample program ensures that the output is completely \s-1UTF\-8\s0,
and removes the program's warning.
.PP
You can enable automatic UTF\-8\-ification of your standard file
handles, default \f(CW\*(C`open()\*(C'\fR layer, and \f(CW@ARGV\fR by using either
the \f(CW\*(C`\-C\*(C'\fR command line switch or the \f(CW\*(C`PERL_UNICODE\*(C'\fR environment
variable, see perlrun for the documentation of the \f(CW\*(C`\-C\*(C'\fR switch.
.PP
Note that this means that Perl expects other software to work, too:
if Perl has been led to believe that \s-1STDIN\s0 should be \s-1UTF\-8\s0, but then
\&\s-1STDIN\s0 coming in from another command is not \s-1UTF\-8\s0, Perl will complain
about the malformed \s-1UTF\-8\s0.
.PP
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 \*(L"perl \-V\*(R" and looking for
\&\f(CW\*(C`useperlio=define\*(C'\fR.
.Sh "Unicode and \s-1EBCDIC\s0"
.IX Subsection "Unicode and EBCDIC"
Perl 5.8.0 also supports Unicode on \s-1EBCDIC\s0 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.
.PP
In any case, the Unicode support on \s-1EBCDIC\s0 platforms is better than
in the 5.6 series, which didn't work much at all for \s-1EBCDIC\s0 platform.
On \s-1EBCDIC\s0 platforms, the internal Unicode encoding form is UTF-EBCDIC
instead of \s-1UTF\-8\s0.  The difference is that as \s-1UTF\-8\s0 is \*(L"ASCII-safe\*(R" in
that \s-1ASCII\s0 characters encode to \s-1UTF\-8\s0 as-is, while UTF-EBCDIC is