VMS pre7 default signal handling

[p5sagit/p5-mst-13.2.git] / pod / perluniintro.pod

=head1 NAME

perluniintro - Perl Unicode introduction

=head1 DESCRIPTION

This document gives a general idea of Unicode and how to use Unicode
in Perl.

=head2 Unicode

Unicode is a character set standard with plans to cover 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 the 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.

A Unicode I<character> is an abstract entity.  It is not bound to any
particular integer width, and especially not to the C language C<char>.
Unicode is language neutral and display neutral: it doesn't encode the
language of the text, and it doesn't define fonts or other graphical
layout details.  Unicode operates on characters and on text built from
those characters.

Unicode defines characters like C<LATIN CAPITAL LETTER A> or C<GREEK
SMALL LETTER ALPHA>, and then unique numbers for those, hexadecimal
0x0041 or 0x03B1 for those particular characters.  Such unique
numbers are called I<code points>.

The Unicode standard prefers using hexadecimal notation for the code
points.  (In case this notation, numbers like 0x0041, is unfamiliar to
you, take a peek at a later section, L</"Hexadecimal Notation">.)
The Unicode standard uses the notation C<U+0041 LATIN CAPITAL LETTER A>,
which gives the hexadecimal code point, and the normative name of
the character.

Unicode also defines various I<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
I<base character> (like C<LATIN CAPITAL LETTER A>), followed by one or
more I<modifiers> (like C<COMBINING ACUTE ACCENT>).  This sequence of
a base character and modifiers is called a I<combining character
sequence>.

Whether to call these combining character sequences, as a whole,            
"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, one "character", but from the user viewpoint, the sequence as a
whole is probably considered one "character", since that's probably what   
it looks like in the context of the user's language.

With this "as a whole" view of characters, the 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, and so we take
that point of view in this document: one "character" is one Unicode
code point, be it a base character or a combining character.

For some of the combinations there are I<precomposed> characters,
for example C<LATIN CAPITAL LETTER A WITH ACUTE> is defined as
a single code point.  These precomposed characters are, however,
often available only for some combinations, and mainly they are
meant to support round-trip conversions between Unicode and legacy
standards (like the ISO 8859), and in general case the composing
method is more extensible.  To support conversion between the
different compositions of the characters, various I<normalization
forms> are also defined.

Because of backward compatibility with legacy encodings, the "a unique
number for every character" breaks down a bit: "at least one number
for every character" is closer to truth.  (This happens when the same
character has been encoded in several legacy encodings.)  The converse
is also not true: not every code point has 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,
0x10000 (or 65536) characters from 0x0000 to 0xFFFF.  B<This is untrue.>
Since Unicode 2.0 Unicode has been defined all the way up to 21 bits
(0x10FFFF), and since 3.1 characters have been defined beyond 0xFFFF.
The first 0x10000 characters are called the I<Plane 0>, or the I<Basic
Multilingual Plane> (BMP).  With the Unicode 3.1, 17 planes in all are
defined (but nowhere near full of defined characters yet).

Another myth is that the 256-character blocks have something to do
with languages: a block per language.  B<Also this is untrue.>
The division into the blocks exists but it is almost completely
accidental, an artifact of how the characters have been historically
allocated.  Instead, there is a concept called I<scripts>, which may
be more useful: there is C<Latin> script, C<Greek> script, and so on.
Scripts usually span several parts of several blocks.  For further
information see L<Unicode::UCD>.

The Unicode code points are just abstract numbers.  To input and
output these abstract numbers, the numbers must be I<encoded> somehow.
Unicode defines several I<character encoding forms>, of which I<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 are UTF-16 and UTF-32 and their
big and little endian variants (UTF-8 is byteorder independent).
The ISO/IEC 10646 defines the UCS-2 and UCS-4 encoding forms.

For more information about encodings, for example to learn what
I<surrogates> and I<byte order marks> (BOMs) are, see L<perlunicode>.

=head2 Perl's Unicode Support

Starting from Perl 5.6.0, Perl has had the capability of handling
Unicode natively.  The first recommended release for serious Unicode
work is Perl 5.8.0, however.  The maintenance release 5.6.1 fixed many
of the problems of the initial implementation of Unicode, but for
example regular expressions didn't really work with Unicode.

B<Starting from Perl 5.8.0, the use of C<use utf8> is no longer
necessary.> In earlier releases the C<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, not attached to the operations.  (There
is one remaining case where an explicit C<use utf8> is needed: if your
Perl script is in UTF-8, you can use UTF-8 in your variable and
subroutine names, and in your string and regular expression literals,
by saying C<use utf8>.  This is not the default because that would
break existing scripts having legacy 8-bit data in them.)

=head2 Perl's Unicode Model

Perl supports both the old, pre-5.6, model of 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.

The internal encoding of Unicode in Perl is UTF-8.  The internal
encoding is normally hidden, however, and one need not and should not
worry about the internal encoding at all: it is all just characters.

Perl 5.8.0 will also support Unicode on EBCDIC platforms.  There the
support is somewhat harder to implement since additional conversions
are needed at every step.  Because of these difficulties the Unicode
support won't be quite as full as in other, mainly ASCII-based,
platforms (the Unicode support will be better than in the 5.6 series,
which didn't work much at all for EBCDIC platform).  On EBCDIC
platforms the internal encoding form used is UTF-EBCDIC.

=head2 Creating Unicode

To create Unicode literals, use the C<\x{...}> notation in
doublequoted strings:

    my $smiley = "\x{263a}";

Similarly for regular expression literals

    $smiley =~ /\x{263a}/;

At run-time you can use C<chr()>:

    my $hebrew_alef = chr(0x05d0);

(See L</"Further Resources"> for how to find all these numeric codes.)

Naturally, C<ord()> will do the reverse: turn a character to a code point.

Note that C<\x..> (no C<{}> and only two hexadecimal digits), C<\x{...}>
and C<chr(...)> for arguments less than 0x100 (decimal 256) will
generate an eight-bit character for backward compatibility with older
Perls.  For arguments of 0x100 or more, Unicode will always be
produced.  If you want UTF-8 always, use C<pack("U", ...)> instead of
C<\x..>, C<\x{...}>, or C<chr()>.

You can also use the C<charnames> pragma to invoke characters
by name in doublequoted strings:

    use charnames ':full';
    my $arabic_alef = "\N{ARABIC LETTER ALEF}";

And, as mentioned above, you can also C<pack()> numbers into Unicode
characters:

   my $georgian_an  = pack("U", 0x10a0);

Note that both C<\x{...}> and C<\N{...}> are compile-time string
constants: you cannot use variables in them.  if you want similar
run-time functionality, use C<chr()> and C<charnames::vianame()>.

=head2 Handling Unicode

Handling Unicode is for the most part transparent: just use the
strings as usual.  Functions like C<index()>, C<length()>, and
C<substr()> will work on the Unicode characters; regular expressions
will work on the Unicode characters (see L<perlunicode> and L<perlretut>).

Note that Perl does B<not> consider combining character sequences
to be characters, such 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 C<\X> for matching a combining character sequence.

When life is not quite so transparent is working with legacy
encodings, and I/O, and certain special cases.

=head2 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 C<encoding> pragma, for example

    use encoding 'latin2'; # ISO 8859-2

in which case literals (string or regular expression) and chr/ord
in your whole script are assumed to produce Unicode characters from
ISO 8859-2 code points.  Note that the matching for the encoding 
names is forgiving: instead of C<latin2> you could have said 
C<Latin 2>, or C<iso8859-2>, and so forth.  With just

    use encoding;

first the environment variable C<PERL_ENCODING> will be consulted,
and if that doesn't exist, ISO 8859-1 (Latin 1) will be assumed.

The C<Encode> module knows about many encodings and it has interfaces
for doing conversions between those encodings:

    use Encode 'from_to';
    from_to($data, "iso-8859-3", "utf-8"); # from legacy to utf-8

=head2 Unicode I/O

Normally writing out Unicode data

    print FH chr(0x100), "\n";

will print out the raw UTF-8 bytes, but you will get a warning
out of that if you use C<-w> or C<use warnings>.  To avoid the
warning open the stream explicitly in UTF-8:

    open FH, ">:utf8", "file";

and on already open streams use C<binmode()>:

    binmode(STDOUT, ":utf8");

Reading in correctly formed UTF-8 data will not magically turn
the data into Unicode in Perl's eyes.

You can use either the C<':utf8'> I/O discipline when opening files

    open(my $fh,'<:utf8', 'anything');
    my $line_of_utf8 = <$fh>;

The I/O disciplines can also be specified more flexibly with
the C<open> pragma; see L<open>:

    use open ':utf8'; # input and output default discipline will be UTF-8
    open X, ">file";
    print X chr(0x100), "\n";
    close X;
    open Y, "<file";
    printf "%#x\n", ord(<Y>); # this should print 0x100
    close Y;

With the C<open> pragma you can use the C<:locale> discipline

    $ENV{LANG} = 'ru_RU.KOI8-R';
    # the :locale will probe the locale environment variables like LANG
    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 C<':encoding(...)'> discipline

    open(my $epic,'<:encoding(iso-8859-7)','iliad.greek');
    my $line_of_iliad = <$epic>;

Both of these methods install a transparent filter on the I/O stream that
will convert data from the specified encoding when it is read in from the
stream.  In the first example the F<anything> file is assumed to be UTF-8
encoded Unicode, in the second example the F<iliad.greek> file is assumed
to be ISO-8858-7 encoded Greek, but the lines read in will be in both
cases Unicode.

The L<open> pragma affects all the C<open()> calls after the pragma by
setting default disciplines.  If you want to affect only certain
streams, use explicit disciplines directly in the C<open()> call.

You can switch encodings on an already opened stream by using
C<binmode()>, see L<perlfunc/binmode>.

The C<:locale> does not currently work with C<open()> and
C<binmode()>, only with the C<open> pragma.  The C<:utf8> and
C<:encoding(...)> do work with all of C<open()>, C<binmode()>,
and the C<open> pragma.

Similarly, you may use these I/O disciplines on input streams to
automatically convert data from the specified encoding when it is
written to the stream.

    open(my $unicode, '<:utf8',                 'japanese.uni');
    open(my $nihongo, '>:encoding(iso2022-jp)', 'japanese.jp');
    while (<$unicode>) { print $nihongo }

The naming of encodings, both by the C<open()> and by the C<open>
pragma, is similarly understanding as with the C<encoding> pragma:
C<koi8-r> and C<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 L<Encode>.

C<read()> reads characters and returns the number of characters.
C<seek()> and C<tell()> operate on byte counts, as do C<sysread()>
and C<sysseek()>.

Notice that because of the default behaviour "input is not UTF-8"
it is easy to mistakenly write code that keeps on expanding a file
by repeatedly encoding it in UTF-8:

    # BAD CODE WARNING
    open F, "file";
    local $/; # read in the whole file
    $t = <F>;
    close F;
    open F, ">:utf8", "file";
    print F $t;
    close F;

If you run this code twice, the contents of the F<file> will be twice
UTF-8 encoded.  A C<use open ':utf8'> would have avoided the bug, or
explicitly opening also the F<file> for input as UTF-8.

=head2 Special Cases

=over 4

=item *

Bit Complement Operator ~ And vec()

The bit complement operator C<~> will produce surprising results if
used on strings containing Unicode characters.  The results are
consistent with the internal UTF-8 encoding of the characters, but not
with much else.  So don't do that.  Similarly for vec(): you will be
operating on the UTF-8 bit patterns of the Unicode characters, not on
the bytes, which is very probably not what you want.

=item *

Peeking At UTF-8

One way of peeking inside the internal encoding of Unicode characters
is to use C<unpack("C*", ...> to get the bytes, or C<unpack("H*", ...)>
to display the bytes:

    # this will print 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 module:

    perl -MDevel::Peek -e 'Dump(chr(0x100))'

That will show 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 C<is_utf8> function of the C<Encode> module.

=back

=head2 Advanced Topics

=over 4

=item *

String Equivalence

The question of string equivalence turns somewhat complicated
in Unicode: what do you mean by equal?

(Is C<LATIN CAPITAL LETTER A WITH ACUTE> equal to
C<LATIN CAPITAL LETTER A>?)

The short answer is that by default Perl compares equivalence
(C<eq>, C<ne>) based only on code points of the characters.
In the above case, no (because 0x00C1 != 0x0041).  But sometimes any
CAPITAL LETTER As being considered equal, or even any As of any case,
would be desirable.

The long answer is that you need to consider character normalization
and casing issues: see L<Unicode::Normalize>, and Unicode Technical
Reports #15 and #21, I<Unicode Normalization Forms> and I<Case
Mappings>, http://www.unicode.org/unicode/reports/tr15/
http://www.unicode.org/unicode/reports/tr21/

As of Perl 5.8.0, the's regular expression case-ignoring matching
implements only 1:1 semantics: one character matches one character.
In I<Case Mappings> both 1:N and N:1 matches are defined.

=item *

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 C<LATIN CAPITAL LETTER A WITH ACUTE> come before or after
C<LATIN CAPITAL LETTER A WITH GRAVE>?)

The short answer is that by default Perl compares strings (C<lt>,
C<le>, C<cmp>, C<ge>, C<gt>) based only on the code points of the
characters.  In the above case, 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 L<Unicode::Collate>, and I<Unicode Collation Algorithm>
http://www.unicode.org/unicode/reports/tr10/

=back

=head2 Miscellaneous

=over 4

=item *

Character Ranges

Character ranges in regular expression character classes (C</[a-z]/>)
and in the C<tr///> (also known as C<y///>) operator are not magically
Unicode-aware.  What this means that C<[a-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 C</[[:alpha]]/> for that).

For specifying things like that in regular expressions you can use the
various Unicode properties, C<\pL> in this particular case.  You can
use Unicode code points as the end points of character ranges, but
that means that particular code point range, nothing more.  For
further information, see L<perlunicode>.

=item *

String-To-Number Conversions

Unicode does define several other decimal (and numeric) characters
than just the familiar 0 to 9, such as the Arabic and Indic digits.
Perl does not support string-to-number conversion for digits other
than the 0 to 9 (and a to f for hexadecimal).

=back

=head2 Questions With Answers

=over 4

=item Will My Old Scripts Break?

Very probably not.  Unless you are generating Unicode characters
somehow, any old behaviour should be preserved.  About the only
behaviour that has changed and which could start generating Unicode
is the old behaviour of C<chr()> where supplying an argument more
than 255 produced a character modulo 255 (for example, C<chr(300)>
was equal to C<chr(45)>).

=item How Do I Make My Scripts Work With Unicode?

Very little work should be needed since nothing changes until you
somehow generate Unicode data.  The greatest trick will be getting
input as Unicode, and for that see the earlier I/O discussion.

=item How Do I Know Whether My String Is In Unicode?

You shouldn't care.  No, you really shouldn't.  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:

    use Encode 'is_utf8';
    print 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 C<is_utf8()> does is to
return the value of the internal "utf8ness" flag attached to the
$string.  If the flag is on, characters added to that string will be
automatically upgraded to UTF-8 (and even then only if they really
need to be upgraded, that is, if their code point is greater than 0xFF).

Sometimes you might really need to know the byte length of a string
instead of the character length.  For that use the C<bytes> pragma
and its only defined function C<length()>:

    my $unicode = chr(0x100);
    print length($unicode), "\n"; # will print 1
    use bytes;
    print length($unicode), "\n"; # will print 2 (the 0xC4 0x80 of the UTF-8)

=item How Do I Detect Invalid UTF-8?

Either

    use Encode 'encode_utf8';
    if (encode_utf8($string)) {
        # valid
    } else {
        # invalid
    }

or

    use warnings;
    @chars = unpack("U0U*", "\xFF"); # will warn

The warning will be C<Malformed UTF-8 character (byte 0xff) in
unpack>.  The "U0" means "expect strictly UTF-8 encoded Unicode".
Without that the C<unpack("U*", ...)> would accept also data like
C<chr(0xFF>).

=item How Do I Convert Data Into UTF-8?  Or Vice Versa?

This probably isn't as useful (or simple) as you might think.
Also, normally you shouldn't need to.

In one sense what you are asking doesn't make much sense: UTF-8 is
(intended as an) Unicode encoding, so converting "data" into UTF-8
isn't meaningful unless you know in what character set and encoding
the binary data is in, and in this case you can use C<Encode>.

    use Encode 'from_to';
    from_to($data, "iso-8859-1", "utf-8"); # from latin-1 to utf-8

If you have ASCII (really 7-bit US-ASCII), you already have valid
UTF-8, the lowest 128 characters of UTF-8 encoded Unicode and US-ASCII
are equivalent.

If you have Latin-1 (or want Latin-1), you can just use pack/unpack:

    $latin1 = pack("C*", unpack("U*", $utf8));
    $utf8   = pack("U*", unpack("C*", $latin1));

(The same works for EBCDIC.)

If you have a sequence of bytes you B<know> is valid UTF-8,
but Perl doesn't know it yet, you can make Perl a believer, too:

    use Encode 'decode_utf8';
    $utf8 = decode_utf8($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 C<unpack("C*", $string)> for the former, and you can create
well-formed Unicode/UTF-8 data by C<pack("U*", 0xff, ...)>.

=item How Do I Display Unicode?  How Do I Input Unicode?

See http://www.hclrss.demon.co.uk/unicode/ and
http://www.cl.cam.ac.uk/~mgk25/unicode.html

=item How Does Unicode Work With Traditional Locales?

In Perl, not very well.  Avoid using locales through the C<locale>
pragma.  Use only one or the other.

=back

=head2 Hexadecimal Notation

The Unicode standard prefers using hexadecimal notation because that
shows better 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 C<0x> prefix means a hexadecimal number, the digits are 0-9 I<and>
a-f (or A-F, case doesn't matter).  Each hexadecimal digit represents
four bits, or half a byte.  C<print 0x..., "\n"> will show a
hexadecimal number in decimal, and C<printf "%x\n", $decimal> will
show a decimal number in hexadecimal.  If you have just the
"hexdigits" of a hexadecimal number, you can use the C<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

=head2 Further Resources

=over 4

=item *

Unicode Consortium

    http://www.unicode.org/

=item *

Unicode FAQ

    http://www.unicode.org/unicode/faq/

=item *

Unicode Glossary

    http://www.unicode.org/glossary/

=item *

Unicode Useful Resources

    http://www.unicode.org/unicode/onlinedat/resources.html

=item *

Unicode and Multilingual Support in HTML, Fonts, Web Browsers and Other Applications

    http://www.hclrss.demon.co.uk/unicode/

=item *

UTF-8 and Unicode FAQ for Unix/Linux

    http://www.cl.cam.ac.uk/~mgk25/unicode.html

=item *

Legacy Character Sets

    http://www.czyborra.com/
    http://www.eki.ee/letter/

=item *

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 F<lib/unicore> was done to
avoid naming conflicts with lib/Unicode in case-insensitive filesystems.)
The main Unicode data file is F<Unicode.txt> (or F<Unicode.301> in
Perl 5.6.1.)  You can find the C<$Config{installprivlib}> by

    perl "-V:installprivlib"

Note that some of the files have been renamed from the Unicode
standard since the Perl installation tries to live by the "8.3"
filenaming restrictions.  The renamings are shown in the
accompanying F<rename> file.

You can explore various information from the Unicode data files using
the C<Unicode::UCD> module.

=back

=head1 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 C<Unicode::String>,
C<Unicode::Map8>, and C<Unicode::Map>, available from CPAN.
If you have the GNU recode installed, you can also use the
Perl frontend C<Convert::Recode> for character conversions.

=head1 SEE ALSO

L<perlunicode>, L<Encode>, L<encoding>, L<open>, L<utf8>, L<bytes>,
L<perlretut>, L<Unicode::Collate>, L<Unicode::Normalize>, L<Unicode::UCD>

=head1 ACKNOWLEDGEMENTS

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.

=head1 AUTHOR, COPYRIGHT, AND LICENSE

Copyright 2001 Jarkko Hietaniemi <jhi@iki.fi>

This document may be distributed under the same terms as Perl itself.