perl-doc 5.18.2-2ubuntu1.7 / usr / share / perl / 5.18.2 / utf8.pod

=head1 NAME

utf8 - Perl pragma to enable/disable UTF-8 (or UTF-EBCDIC) in source code

=head1 SYNOPSIS

    use utf8;
    no utf8;

    # Convert the internal representation of a Perl scalar to/from UTF-8.

    $num_octets = utf8::upgrade($string);
    $success    = utf8::downgrade($string[, FAIL_OK]);

    # Change each character of a Perl scalar to/from a series of
    # characters that represent the UTF-8 bytes of each original character.

    utf8::encode($string);  # "\x{100}"  becomes "\xc4\x80"
    utf8::decode($string);  # "\xc4\x80" becomes "\x{100}"

    $flag = utf8::is_utf8(STRING); # since Perl 5.8.1
    $flag = utf8::valid(STRING);

=head1 DESCRIPTION

The C<use utf8> pragma tells the Perl parser to allow UTF-8 in the
program text in the current lexical scope (allow UTF-EBCDIC on EBCDIC based
platforms).  The C<no utf8> pragma tells Perl to switch back to treating
the source text as literal bytes in the current lexical scope.

B<Do not use this pragma for anything else than telling Perl that your
script is written in UTF-8.> The utility functions described below are
directly usable without C<use utf8;>.

Because it is not possible to reliably tell UTF-8 from native 8 bit
encodings, you need either a Byte Order Mark at the beginning of your
source code, or C<use utf8;>, to instruct perl.

When UTF-8 becomes the standard source format, this pragma will
effectively become a no-op.  For convenience in what follows the term
I<UTF-X> is used to refer to UTF-8 on ASCII and ISO Latin based
platforms and UTF-EBCDIC on EBCDIC based platforms.

See also the effects of the C<-C> switch and its cousin, the
C<$ENV{PERL_UNICODE}>, in L<perlrun>.

Enabling the C<utf8> pragma has the following effect:

=over 4

=item *

Bytes in the source text that have their high-bit set will be treated
as being part of a literal UTF-X sequence.  This includes most
literals such as identifier names, string constants, and constant
regular expression patterns.

On EBCDIC platforms characters in the Latin 1 character set are
treated as being part of a literal UTF-EBCDIC character.

=back

Note that if you have bytes with the eighth bit on in your script
(for example embedded Latin-1 in your string literals), C<use utf8>
will be unhappy since the bytes are most probably not well-formed
UTF-X.  If you want to have such bytes under C<use utf8>, you can disable
this pragma until the end the block (or file, if at top level) by
C<no utf8;>.

=head2 Utility functions

The following functions are defined in the C<utf8::> package by the
Perl core.  You do not need to say C<use utf8> to use these and in fact
you should not say that  unless you really want to have UTF-8 source code.

=over 4

=item * $num_octets = utf8::upgrade($string)

Converts in-place the internal representation of the string from an octet
sequence in the native encoding (Latin-1 or EBCDIC) to I<UTF-X>. The
logical character sequence itself is unchanged.  If I<$string> is already
stored as I<UTF-X>, then this is a no-op. Returns the
number of octets necessary to represent the string as I<UTF-X>.  Can be
used to make sure that the UTF-8 flag is on, so that C<\w> or C<lc()>
work as Unicode on strings containing characters in the range 0x80-0xFF
(on ASCII and derivatives).

B<Note that this function does not handle arbitrary encodings.>
Therefore Encode is recommended for the general purposes; see also
L<Encode>.

=item * $success = utf8::downgrade($string[, FAIL_OK])

Converts in-place the internal representation of the string from
I<UTF-X> to the equivalent octet sequence in the native encoding (Latin-1
or EBCDIC). The logical character sequence itself is unchanged. If
I<$string> is already stored as native 8 bit, then this is a no-op.  Can
be used to
make sure that the UTF-8 flag is off, e.g. when you want to make sure
that the substr() or length() function works with the usually faster
byte algorithm.

Fails if the original I<UTF-X> sequence cannot be represented in the
native 8 bit encoding. On failure dies or, if the value of C<FAIL_OK> is
true, returns false. 

Returns true on success.

B<Note that this function does not handle arbitrary encodings.>
Therefore Encode is recommended for the general purposes; see also
L<Encode>.

=item * utf8::encode($string)

Converts in-place the character sequence to the corresponding octet
sequence in I<UTF-X>. That is, every (possibly wide) character gets
replaced with a sequence of one or more characters that represent the
individual I<UTF-X> bytes of the character.  The UTF8 flag is turned off.
Returns nothing.

    my $a = "\x{100}"; # $a contains one character, with ord 0x100
    utf8::encode($a);  # $a contains two characters, with ords 0xc4 and 0x80

B<Note that this function does not handle arbitrary encodings.>
Therefore Encode is recommended for the general purposes; see also
L<Encode>.

=item * $success = utf8::decode($string)

Attempts to convert in-place the octet sequence in I<UTF-X> to the
corresponding character sequence. That is, it replaces each sequence of
characters in the string whose ords represent a valid UTF-X byte
sequence, with the corresponding single character.  The UTF-8 flag is
turned on only if the source string contains multiple-byte I<UTF-X>
characters.  If I<$string> is invalid as I<UTF-X>, returns false;
otherwise returns true.

    my $a = "\xc4\x80"; # $a contains two characters, with ords 0xc4 and 0x80
    utf8::decode($a);   # $a contains one character, with ord 0x100

B<Note that this function does not handle arbitrary encodings.>
Therefore Encode is recommended for the general purposes; see also
L<Encode>.

=item * $flag = utf8::is_utf8(STRING)

(Since Perl 5.8.1)  Test whether STRING is encoded internally in UTF-8.
Functionally the same as Encode::is_utf8().

=item * $flag = utf8::valid(STRING)

[INTERNAL] Test whether STRING is in a consistent state regarding
UTF-8.  Will return true if it is well-formed UTF-8 and has the UTF-8 flag
on B<or> if STRING is held as bytes (both these states are 'consistent').
Main reason for this routine is to allow Perl's testsuite to check
that operations have left strings in a consistent state.  You most
probably want to use utf8::is_utf8() instead.

=back

C<utf8::encode> is like C<utf8::upgrade>, but the UTF8 flag is
cleared.  See L<perlunicode> for more on the UTF8 flag and the C API
functions C<sv_utf8_upgrade>, C<sv_utf8_downgrade>, C<sv_utf8_encode>,
and C<sv_utf8_decode>, which are wrapped by the Perl functions
C<utf8::upgrade>, C<utf8::downgrade>, C<utf8::encode> and
C<utf8::decode>.  Also, the functions utf8::is_utf8, utf8::valid,
utf8::encode, utf8::decode, utf8::upgrade, and utf8::downgrade are
actually internal, and thus always available, without a C<require utf8>
statement.

=head1 BUGS

One can have Unicode in identifier names, but not in package/class or
subroutine names.  While some limited functionality towards this does
exist as of Perl 5.8.0, that is more accidental than designed; use of
Unicode for the said purposes is unsupported.

One reason of this unfinishedness is its (currently) inherent
unportability: since both package names and subroutine names may need
to be mapped to file and directory names, the Unicode capability of
the filesystem becomes important-- and there unfortunately aren't
portable answers.

=head1 SEE ALSO

L<perlunitut>, L<perluniintro>, L<perlrun>, L<bytes>, L<perlunicode>

=cut