=head1 NAME

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

=head1 SYNOPSIS

    use utf8;
    no utf8;

    # Convert a Perl scalar to/from UTF-8.
    $num_octets = utf8::upgrade($string);
    $success    = utf8::downgrade($string[, FAIL_OK]);

    # Change the native bytes of a Perl scalar to/from UTF-8 bytes.
    utf8::encode($string);
    utf8::decode($string);

    $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.

This pragma is primarily a compatibility device.  Perl versions
earlier than 5.6 allowed arbitrary bytes in source code, whereas
in future we would like to standardize on the UTF-8 encoding for
source text.

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
useful for their own purposes, but they are not really part of the
"pragmatic" effect.

Until UTF-8 becomes the default format for source text, either this
pragma or the L<encoding> pragma should be used to recognize UTF-8
in the source.  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-8 character.  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-8.  If you want to have such bytes and use utf8, you can disable
utf8 until the end the block (or file, if at top level) by C<no utf8;>.

If you want to automatically upgrade your 8-bit legacy bytes to UTF-8,
use the L<encoding> pragma instead of this pragma.  For example, if
you want to implicitly upgrade your ISO 8859-1 (Latin-1) bytes to UTF-8
as used in e.g. C<chr()> and C<\x{...}>, try this:

    use encoding "latin-1";
    my $c = chr(0xc4);
    my $x = "\x{c5}";

In case you are wondering: yes, C<use encoding 'utf8';> works much
the same as C<use 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 octet sequence in the native encoding
(Latin-1 or EBCDIC) to the equivalent character sequence in I<UTF-X>.
I<$string> already encoded as characters does no harm.
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 I<Encode.pm> is recommended for the general purposes.

Affected by the encoding pragma.

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

Converts in-place the character sequence in I<UTF-X>
to the equivalent octet sequence in the native encoding (Latin-1 or EBCDIC).
I<$string> already encoded as octets does no harm.
Returns true on success. On failure dies or, if the value of
C<FAIL_OK> is true, returns false.
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.

B<Note that this function does not handle arbitrary encodings.>
Therefore I<Encode.pm> is recommended for the general purposes.

B<Not> affected by the encoding pragma.

B<NOTE:> this function is experimental and may change
or be removed without notice.

=item * utf8::encode($string)

Converts in-place the character sequence to the corresponding octet sequence
in I<UTF-X>.  The UTF-8 flag is turned off.  Returns nothing.

B<Note that this function does not handle arbitrary encodings.>
Therefore I<Encode.pm> is recommended for the general purposes.

=item * utf8::decode($string)

Attempts to convert in-place the octet sequence in I<UTF-X>
to the corresponding character sequence.  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.

B<Note that this function does not handle arbitrary encodings.>
Therefore I<Encode.pm> is recommended for the general purposes.

B<NOTE:> this function is experimental and may change
or be removed without notice.

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

(Since Perl 5.8.1)  Test whether STRING is 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 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>.  Note that in the Perl 5.8.0 and 5.8.1 implementation
the functions utf8::is_utf8, utf8::valid, utf8::encode, utf8::decode,
utf8::upgrade, and utf8::downgrade are always available, without a
C<require utf8> statement-- this may change in future releases.

=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<perluniintro>, L<encoding>, L<perlrun>, L<bytes>, L<perlunicode>

=cut
