regcomp.c: Make comment more accurate

[perl5.git] / lib / charnames.pm

package charnames;
use strict;
use warnings;
our $VERSION = '1.32';
use unicore::Name;    # mktables-generated algorithmically-defined names
use _charnames ();    # The submodule for this where most of the work gets done

use bytes ();          # for $bytes::hint_bits
use re "/aa";          # Everything in here should be ASCII

# Translate between Unicode character names and their code points.
# This is a wrapper around the submodule C<_charnames>.  This design allows
# C<_charnames> to be autoloaded to enable use of \N{...}, but requires this
# module to be explicitly requested for the functions API.

$Carp::Internal{ (__PACKAGE__) } = 1;

sub import
{
  shift; ## ignore class name
  _charnames->import(@_);
}

# Cache of already looked-up values.  This is set to only contain
# official values, and user aliases can't override them, so scoping is
# not an issue.
my %viacode;

sub viacode {
  return _charnames::viacode(@_);
}

sub vianame
{
  if (@_ != 1) {
    _charnames::carp "charnames::vianame() expects one name argument";
    return ()
  }

  # Looks up the character name and returns its ordinal if
  # found, undef otherwise.

  my $arg = shift;

  if ($arg =~ /^U\+([0-9a-fA-F]+)$/) {

    # khw claims that this is poor interface design.  The function should
    # return either a an ord or a chr for all inputs; not be bipolar.  But
    # can't change it because of backward compatibility.  New code can use
    # string_vianame() instead.
    my $ord = CORE::hex $1;
    return chr $ord if $ord <= 255 || ! ((caller 0)[8] & $bytes::hint_bits);
    _charnames::carp _charnames::not_legal_use_bytes_msg($arg, chr $ord);
    return;
  }

  # The first 1 arg means wants an ord returned; the second that we are in
  # runtime, and this is the first level routine called from the user
  return _charnames::lookup_name($arg, 1, 1);
} # vianame

sub string_vianame {

  # Looks up the character name and returns its string representation if
  # found, undef otherwise.

  if (@_ != 1) {
    _charnames::carp "charnames::string_vianame() expects one name argument";
    return;
  }

  my $arg = shift;

  if ($arg =~ /^U\+([0-9a-fA-F]+)$/) {

    my $ord = CORE::hex $1;
    return chr $ord if $ord <= 255 || ! ((caller 0)[8] & $bytes::hint_bits);

    _charnames::carp _charnames::not_legal_use_bytes_msg($arg, chr $ord);
    return;
  }

  # The 0 arg means wants a string returned; the 1 arg means that we are in
  # runtime, and this is the first level routine called from the user
  return _charnames::lookup_name($arg, 0, 1);
} # string_vianame

1;
__END__

=head1 NAME

charnames - access to Unicode character names and named character sequences; also define character names

=head1 SYNOPSIS

 use charnames ':full';
 print "\N{GREEK SMALL LETTER SIGMA} is called sigma.\n";
 print "\N{LATIN CAPITAL LETTER E WITH VERTICAL LINE BELOW}",
       " is an officially named sequence of two Unicode characters\n";

 use charnames ':loose';
 print "\N{Greek small-letter  sigma}",
        "can be used to ignore case, underscores, most blanks,"
        "and when you aren't sure if the official name has hyphens\n";

 use charnames ':short';
 print "\N{greek:Sigma} is an upper-case sigma.\n";

 use charnames qw(cyrillic greek);
 print "\N{sigma} is Greek sigma, and \N{be} is Cyrillic b.\n";

 use charnames ":full", ":alias" => {
   e_ACUTE => "LATIN SMALL LETTER E WITH ACUTE",
   mychar => 0xE8000,  # Private use area
 };
 print "\N{e_ACUTE} is a small letter e with an acute.\n";
 print "\N{mychar} allows me to name private use characters.\n";

 use charnames ();
 print charnames::viacode(0x1234); # prints "ETHIOPIC SYLLABLE SEE"
 printf "%04X", charnames::vianame("GOTHIC LETTER AHSA"); # prints
                                                          # "10330"
 print charnames::vianame("LATIN CAPITAL LETTER A"); # prints 65 on
                                                     # ASCII platforms;
                                                     # 193 on EBCDIC
 print charnames::string_vianame("LATIN CAPITAL LETTER A"); # prints "A"

=head1 DESCRIPTION

Pragma C<use charnames> is used to gain access to the names of the
Unicode characters and named character sequences, and to allow you to define
your own character and character sequence names.

All forms of the pragma enable use of the following 3 functions:

=over

=item *

L</charnames::string_vianame(I<name>)> for run-time lookup of a
either a character name or a named character sequence, returning its string
representation

=item *

L</charnames::vianame(I<name>)> for run-time lookup of a
character name (but not a named character sequence) to get its ordinal value
(code point)

=item *

L</charnames::viacode(I<code>)> for run-time lookup of a code point to get its
Unicode name.

=back

Starting in Perl v5.16, any occurrence of C<\N{I<CHARNAME>}> sequences
in a double-quotish string automatically loads this module with arguments
C<:full> and C<:short> (described below) if it hasn't already been loaded with
different arguments, in order to compile the named Unicode character into
position in the string.  Prior to v5.16, an explicit S<C<use charnames>> was
required to enable this usage.  (However, prior to v5.16, the form C<S<"use
charnames ();">> did not enable C<\N{I<CHARNAME>}>.)

Note that C<\N{U+I<...>}>, where the I<...> is a hexadecimal number,
also inserts a character into a string.
The character it inserts is the one whose code point
(ordinal value) is equal to the number.  For example, C<"\N{U+263a}"> is
the Unicode (white background, black foreground) smiley face
equivalent to C<"\N{WHITE SMILING FACE}">.
Also note, C<\N{I<...>}> can mean a regex quantifier instead of a character
name, when the I<...> is a number (or comma separated pair of numbers
(see L<perlreref/QUANTIFIERS>), and is not related to this pragma.

The C<charnames> pragma supports arguments C<:full>, C<:loose>, C<:short>,
script names and L<customized aliases|/CUSTOM ALIASES>.

If C<:full> is present, for expansion of
C<\N{I<CHARNAME>}>, the string I<CHARNAME> is first looked up in the list of
standard Unicode character names.

C<:loose> is a variant of C<:full> which allows I<CHARNAME> to be less
precisely specified.  Details are in L</LOOSE MATCHES>.

If C<:short> is present, and
I<CHARNAME> has the form C<I<SCRIPT>:I<CNAME>>, then I<CNAME> is looked up
as a letter in script I<SCRIPT>, as described in the next paragraph.
Or, if C<use charnames> is used
with script name arguments, then for C<\N{I<CHARNAME>}> the name
I<CHARNAME> is looked up as a letter in the given scripts (in the
specified order). Customized aliases can override these, and are explained in
L</CUSTOM ALIASES>.

For lookup of I<CHARNAME> inside a given script I<SCRIPTNAME>,
this pragma looks in the table of standard Unicode names for the names

  SCRIPTNAME CAPITAL LETTER CHARNAME
  SCRIPTNAME SMALL LETTER CHARNAME
  SCRIPTNAME LETTER CHARNAME

If I<CHARNAME> is all lowercase,
then the C<CAPITAL> variant is ignored, otherwise the C<SMALL> variant
is ignored, and both I<CHARNAME> and I<SCRIPTNAME> are converted to all
uppercase for look-up.  Other than that, both of them follow L<loose|/LOOSE
MATCHES> rules if C<:loose> is also specified; strict otherwise.

Note that C<\N{...}> is compile-time; it's a special form of string
constant used inside double-quotish strings; this means that you cannot
use variables inside the C<\N{...}>.  If you want similar run-time
functionality, use
L<charnames::string_vianame()|/charnames::string_vianame(I<name>)>.

Since Unicode 6.0, it is deprecated to use C<BELL>.  Instead use C<ALERT> (but
C<BEL> will continue to work).

If the input name is unknown, C<\N{NAME}> raises a warning and
substitutes the Unicode REPLACEMENT CHARACTER (U+FFFD).

For C<\N{NAME}>, it is a fatal error if C<use bytes> is in effect and the
input name is that of a character that won't fit into a byte (i.e., whose
ordinal is above 255).

Otherwise, any string that includes a C<\N{I<charname>}> or
C<S<\N{U+I<code point>}>> will automatically have Unicode semantics (see
L<perlunicode/Byte and Character Semantics>).

=head1 LOOSE MATCHES

By specifying C<:loose>, Unicode's L<loose character name
matching|http://www.unicode.org/reports/tr44#Matching_Rules> rules are
selected instead of the strict exact match used otherwise.
That means that I<CHARNAME> doesn't have to be so precisely specified.
Upper/lower case doesn't matter (except with scripts as mentioned above), nor
do any underscores, and the only hyphens that matter are those at the
beginning or end of a word in the name (with one exception:  the hyphen in
U+1180 C<HANGUL JUNGSEONG O-E> does matter).
Also, blanks not adjacent to hyphens don't matter.
The official Unicode names are quite variable as to where they use hyphens
versus spaces to separate word-like units, and this option allows you to not
have to care as much.
The reason non-medial hyphens matter is because of cases like
U+0F60 C<TIBETAN LETTER -A> versus U+0F68 C<TIBETAN LETTER A>.
The hyphen here is significant, as is the space before it, and so both must be
included.

C<:loose> slows down look-ups by a factor of 2 to 3 versus
C<:full>, but the trade-off may be worth it to you.  Each individual look-up
takes very little time, and the results are cached, so the speed difference
would become a factor only in programs that do look-ups of many different
spellings, and probably only when those look-ups are through vianame() and
string_vianame(), since C<\N{...}> look-ups are done at compile time.

=head1 ALIASES

Starting in Unicode 6.1 and Perl v5.16, Unicode defines many abbreviations and
names that were formerly Perl extensions, and some additional ones that Perl
did not previously accept.  The list is getting too long to reproduce here,
but you can get the complete list from the Unicode web site:
L<http://www.unicode.org/Public/UNIDATA/NameAliases.txt>.

Earlier versions of Perl accepted almost all the 6.1 names.  These were most
extensively documented in the v5.14 version of this pod:
L<http://perldoc.perl.org/5.14.0/charnames.html#ALIASES>.

=head1 CUSTOM ALIASES

You can add customized aliases to standard (C<:full>) Unicode naming
conventions.  The aliases override any standard definitions, so, if
you're twisted enough, you can change C<"\N{LATIN CAPITAL LETTER A}"> to
mean C<"B">, etc.

Note that an alias should not be something that is a legal curly
brace-enclosed quantifier (see L<perlreref/QUANTIFIERS>).  For example
C<\N{123}> means to match 123 non-newline characters, and is not treated as a
charnames alias.  Aliases are discouraged from beginning with anything
other than an alphabetic character and from containing anything other
than alphanumerics, spaces, dashes, parentheses, and underscores.
Currently they must be ASCII.

An alias can map to either an official Unicode character name (not a loose
matched name) or to a
numeric code point (ordinal).  The latter is useful for assigning names
to code points in Unicode private use areas such as U+E800 through
U+F8FF.
A numeric code point must be a non-negative integer or a string beginning
with C<"U+"> or C<"0x"> with the remainder considered to be a
hexadecimal integer.  A literal numeric constant must be unsigned; it
will be interpreted as hex if it has a leading zero or contains
non-decimal hex digits; otherwise it will be interpreted as decimal.

Aliases are added either by the use of anonymous hashes:

    use charnames ":alias" => {
        e_ACUTE => "LATIN SMALL LETTER E WITH ACUTE",
        mychar1 => 0xE8000,
        };
    my $str = "\N{e_ACUTE}";

or by using a file containing aliases:

    use charnames ":alias" => "pro";

This will try to read C<"unicore/pro_alias.pl"> from the C<@INC> path. This
file should return a list in plain perl:

    (
    A_GRAVE         => "LATIN CAPITAL LETTER A WITH GRAVE",
    A_CIRCUM        => "LATIN CAPITAL LETTER A WITH CIRCUMFLEX",
    A_DIAERES       => "LATIN CAPITAL LETTER A WITH DIAERESIS",
    A_TILDE         => "LATIN CAPITAL LETTER A WITH TILDE",
    A_BREVE         => "LATIN CAPITAL LETTER A WITH BREVE",
    A_RING          => "LATIN CAPITAL LETTER A WITH RING ABOVE",
    A_MACRON        => "LATIN CAPITAL LETTER A WITH MACRON",
    mychar2         => "U+E8001",
    );

Both these methods insert C<":full"> automatically as the first argument (if no
other argument is given), and you can give the C<":full"> explicitly as
well, like

    use charnames ":full", ":alias" => "pro";

C<":loose"> has no effect with these.  Input names must match exactly, using
C<":full"> rules.

Also, both these methods currently allow only single characters to be named.
To name a sequence of characters, use a
L<custom translator|/CUSTOM TRANSLATORS> (described below).

=head1 charnames::string_vianame(I<name>)

This is a runtime equivalent to C<\N{...}>.  I<name> can be any expression
that evaluates to a name accepted by C<\N{...}> under the L<C<:full>
option|/DESCRIPTION> to C<charnames>.  In addition, any other options for the
controlling C<"use charnames"> in the same scope apply, like C<:loose> or any
L<script list, C<:short> option|/DESCRIPTION>, or L<custom aliases|/CUSTOM
ALIASES> you may have defined.

The only difference is that if the input name is unknown, C<string_vianame>
returns C<undef> instead of the REPLACEMENT CHARACTER and does not raise a
warning message.

=head1 charnames::vianame(I<name>)

This is similar to C<string_vianame>.  The main difference is that under most
circumstances, vianame returns an ordinal code
point, whereas C<string_vianame> returns a string.  For example,

   printf "U+%04X", charnames::vianame("FOUR TEARDROP-SPOKED ASTERISK");

prints "U+2722".

This leads to the other two differences.  Since a single code point is
returned, the function can't handle named character sequences, as these are
composed of multiple characters (it returns C<undef> for these.  And, the code
point can be that of any
character, even ones that aren't legal under the C<S<use bytes>> pragma,

See L</BUGS> for the circumstances in which the behavior differs
from  that described above.

=head1 charnames::viacode(I<code>)

Returns the full name of the character indicated by the numeric code.
For example,

    print charnames::viacode(0x2722);

prints "FOUR TEARDROP-SPOKED ASTERISK".

The name returned is the "best" (defined below) official name or alias
for the code point, if
available; otherwise your custom alias for it, if defined; otherwise C<undef>.
This means that your alias will only be returned for code points that don't
have an official Unicode name (nor alias) such as private use code points.

If you define more than one name for the code point, it is indeterminate
which one will be returned.

As mentioned, the function returns C<undef> if no name is known for the code
point.  In Unicode the proper name of these is the empty string, which
C<undef> stringifies to.  (If you ask for a code point past the legal
Unicode maximum of U+10FFFF that you haven't assigned an alias to, you
get C<undef> plus a warning.)

The input number must be a non-negative integer, or a string beginning
with C<"U+"> or C<"0x"> with the remainder considered to be a
hexadecimal integer.  A literal numeric constant must be unsigned; it
will be interpreted as hex if it has a leading zero or contains
non-decimal hex digits; otherwise it will be interpreted as decimal.

As mentioned above under L</ALIASES>, Unicode 6.1 defines extra names
(synonyms or aliases) for some code points, most of which were already
available as Perl extensions.  All these are accepted by C<\N{...}> and the
other functions in this module, but C<viacode> has to choose which one
name to return for a given input code point, so it returns the "best" name.
To understand how this works, it is helpful to know more about the Unicode
name properties.  All code points actually have only a single name, which
(starting in Unicode 2.0) can never change once a character has been assigned
to the code point.  But mistakes have been made in assigning names, for
example sometimes a clerical error was made during the publishing of the
Standard which caused words to be misspelled, and there was no way to correct
those.  The Name_Alias property was eventually created to handle these
situations.  If a name was wrong, a corrected synonym would be published for
it, using Name_Alias.  C<viacode> will return that corrected synonym as the
"best" name for a code point.  (It is even possible, though it hasn't happened
yet, that the correction itself will need to be corrected, and so another
Name_Alias can be created for that code point; C<viacode> will return the
most recent correction.)

The Unicode name for each of the control characters (such as LINE FEED) is the
empty string.  However almost all had names assigned by other standards, such
as the ASCII Standard, or were in common use.  C<viacode> returns these names
as the "best" ones available.  Unicode 6.1 has created Name_Aliases for each
of them, including alternate names, like NEW LINE.  C<viacode> uses the
original name, "LINE FEED" in preference to the alternate.  Similarly the
name returned for U+FEFF is "ZERO WIDTH NO-BREAK SPACE", not "BYTE ORDER
MARK".

Until Unicode 6.1, the 4 control characters U+0080, U+0081, U+0084, and U+0099
did not have names nor aliases.
To preserve backwards compatibility, any alias you define for these code
points will be returned by this function, in preference to the official name.

Some code points also have abbreviated names, such as "LF" or "NL".
C<viacode> never returns these.

Because a name correction may be added in future Unicode releases, the name
that C<viacode> returns may change as a result.  This is a rare event, but it
does happen.

=head1 CUSTOM TRANSLATORS

The mechanism of translation of C<\N{...}> escapes is general and not
hardwired into F<charnames.pm>.  A module can install custom
translations (inside the scope which C<use>s the module) with the
following magic incantation:

    sub import {
        shift;
        $^H{charnames} = \&translator;
    }

Here translator() is a subroutine which takes I<CHARNAME> as an
argument, and returns text to insert into the string instead of the
C<\N{I<CHARNAME>}> escape.

This is the only way you can create a custom named sequence of code points.

Since the text to insert should be different
in C<bytes> mode and out of it, the function should check the current
state of C<bytes>-flag as in:

    use bytes ();                      # for $bytes::hint_bits
    sub translator {
        if ($^H & $bytes::hint_bits) {
            return bytes_translator(@_);
        }
        else {
            return utf8_translator(@_);
        }
    }

See L</CUSTOM ALIASES> above for restrictions on I<CHARNAME>.

Of course, C<vianame>, C<viacode>, and C<string_vianame> would need to be
overridden as well.

=head1 BUGS

vianame() normally returns an ordinal code point, but when the input name is of
the form C<U+...>, it returns a chr instead.  In this case, if C<use bytes> is
in effect and the character won't fit into a byte, it returns C<undef> and
raises a warning.

Names must be ASCII characters only, which means that you are out of luck if
you want to create aliases in a language where some or all the characters of
the desired aliases are non-ASCII.

Since evaluation of the translation function (see L</CUSTOM
TRANSLATORS>) happens in the middle of compilation (of a string
literal), the translation function should not do any C<eval>s or
C<require>s.  This restriction should be lifted (but is low priority) in
a future version of Perl.

=cut

# ex: set ts=8 sts=2 sw=2 et: