Make the UTF-8 decoding stricter and more verbose when

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

=head1 NAME

perlunicode - Unicode support in Perl

=head1 DESCRIPTION

=head2 Important Caveat

WARNING: The implementation of Unicode support in Perl is incomplete.

The following areas need further work.

=over

=item Input and Output Disciplines

There is currently no easy way to mark data read from a file or other
external source as being utf8.  This will be one of the major areas of
focus in the near future.

=item Regular Expressions

The existing regular expression compiler does not produce polymorphic
opcodes.  This means that the determination on whether to match Unicode
characters is made when the pattern is compiled, based on whether the
pattern contains Unicode characters, and not when the matching happens
at run time.  This needs to be changed to adaptively match Unicode if
the string to be matched is Unicode.

=item C<use utf8> still needed to enable a few features

The C<utf8> pragma implements the tables used for Unicode support.  These
tables are automatically loaded on demand, so the C<utf8> pragma need not
normally be used.

However, as a compatibility measure, this pragma must be explicitly used
to enable recognition of UTF-8 encoded literals and identifiers in the
source text.

=back

=head2 Byte and Character semantics

Beginning with version 5.6, Perl uses logically wide characters to
represent strings internally.  This internal representation of strings
uses the UTF-8 encoding.

In future, Perl-level operations can be expected to work with characters
rather than bytes, in general.

However, as strictly an interim compatibility measure, Perl v5.6 aims to
provide a safe migration path from byte semantics to character semantics
for programs.  For operations where Perl can unambiguously decide that the
input data is characters, Perl now switches to character semantics.
For operations where this determination cannot be made without additional
information from the user, Perl decides in favor of compatibility, and
chooses to use byte semantics.

This behavior preserves compatibility with earlier versions of Perl,
which allowed byte semantics in Perl operations, but only as long as
none of the program's inputs are marked as being as source of Unicode
character data.  Such data may come from filehandles, from calls to
external programs, from information provided by the system (such as %ENV),
or from literals and constants in the source text.

If the C<-C> command line switch is used, (or the ${^WIDE_SYSTEM_CALLS}
global flag is set to C<1>), all system calls will use the
corresponding wide character APIs.  This is currently only implemented
on Windows.

Regardless of the above, the C<bytes> pragma can always be used to force
byte semantics in a particular lexical scope.  See L<bytes>.

One effect of the C<utf8> pragma is that the internal UTF-8 decoding
becomes stricter so that the character 0xFFFF (UTF-8 bytes 0xEF 0xBF
0xBF), and the bytes 0xFE and 0xFF, start to cause warnings if they
appear in the data.

The C<utf8> pragma is primarily a compatibility device that enables
recognition of UTF-8 in literals encountered by the parser.  It may also
be used for enabling some of the more experimental Unicode support features.
Note that this pragma is only required until a future version of Perl
in which character semantics will become the default.  This pragma may
then become a no-op.  See L<utf8>.

Unless mentioned otherwise, Perl operators will use character semantics
when they are dealing with Unicode data, and byte semantics otherwise.
Thus, character semantics for these operations apply transparently; if
the input data came from a Unicode source (for example, by adding a
character encoding discipline to the filehandle whence it came, or a
literal UTF-8 string constant in the program), character semantics
apply; otherwise, byte semantics are in effect.  To force byte semantics
on Unicode data, the C<bytes> pragma should be used.

Under character semantics, many operations that formerly operated on
bytes change to operating on characters.  For ASCII data this makes
no difference, because UTF-8 stores ASCII in single bytes, but for
any character greater than C<chr(127)>, the character may be stored in
a sequence of two or more bytes, all of which have the high bit set.
But by and large, the user need not worry about this, because Perl
hides it from the user.  A character in Perl is logically just a number
ranging from 0 to 2**32 or so.  Larger characters encode to longer
sequences of bytes internally, but again, this is just an internal
detail which is hidden at the Perl level.

=head2 Effects of character semantics

Character semantics have the following effects:

=over 4

=item *

Strings and patterns may contain characters that have an ordinal value
larger than 255.

Presuming you use a Unicode editor to edit your program, such characters
will typically occur directly within the literal strings as UTF-8
characters, but you can also specify a particular character with an
extension of the C<\x> notation.  UTF-8 characters are specified by
putting the hexadecimal code within curlies after the C<\x>.  For instance,
a Unicode smiley face is C<\x{263A}>.

=item *

Identifiers within the Perl script may contain Unicode alphanumeric
characters, including ideographs.  (You are currently on your own when
it comes to using the canonical forms of characters--Perl doesn't (yet)
attempt to canonicalize variable names for you.)

=item *

Regular expressions match characters instead of bytes.  For instance,
"." matches a character instead of a byte.  (However, the C<\C> pattern
is provided to force a match a single byte ("C<char>" in C, hence
C<\C>).)

=item *

Character classes in regular expressions match characters instead of
bytes, and match against the character properties specified in the
Unicode properties database.  So C<\w> can be used to match an ideograph,
for instance.

=item *

Named Unicode properties and block ranges make be used as character
classes via the new C<\p{}> (matches property) and C<\P{}> (doesn't
match property) constructs.  For instance, C<\p{Lu}> matches any
character with the Unicode uppercase property, while C<\p{M}> matches
any mark character.  Single letter properties may omit the brackets, so
that can be written C<\pM> also.  Many predefined character classes are
available, such as C<\p{IsMirrored}> and  C<\p{InTibetan}>.

=item *

The special pattern C<\X> match matches any extended Unicode sequence
(a "combining character sequence" in Standardese), where the first
character is a base character and subsequent characters are mark
characters that apply to the base character.  It is equivalent to
C<(?:\PM\pM*)>.

=item *

The C<tr///> operator translates characters instead of bytes.  Note
that the C<tr///CU> functionality has been removed, as the interface
was a mistake.  For similar functionality see pack('U0', ...) and
pack('C0', ...).

=item *

Case translation operators use the Unicode case translation tables
when provided character input.  Note that C<uc()> translates to
uppercase, while C<ucfirst> translates to titlecase (for languages
that make the distinction).  Naturally the corresponding backslash
sequences have the same semantics.

=item *

Most operators that deal with positions or lengths in the string will
automatically switch to using character positions, including C<chop()>,
C<substr()>, C<pos()>, C<index()>, C<rindex()>, C<sprintf()>,
C<write()>, and C<length()>.  Operators that specifically don't switch
include C<vec()>, C<pack()>, and C<unpack()>.  Operators that really
don't care include C<chomp()>, as well as any other operator that
treats a string as a bucket of bits, such as C<sort()>, and the
operators dealing with filenames.

=item *

The C<pack()>/C<unpack()> letters "C<c>" and "C<C>" do I<not> change,
since they're often used for byte-oriented formats.  (Again, think
"C<char>" in the C language.)  However, there is a new "C<U>" specifier
that will convert between UTF-8 characters and integers.  (It works
outside of the utf8 pragma too.)

=item *

The C<chr()> and C<ord()> functions work on characters.  This is like
C<pack("U")> and C<unpack("U")>, not like C<pack("C")> and
C<unpack("C")>.  In fact, the latter are how you now emulate
byte-oriented C<chr()> and C<ord()> under utf8.

=item *

And finally, C<scalar reverse()> reverses by character rather than by byte.

=back

=head2 Character encodings for input and output

[XXX: This feature is not yet implemented.]

=head1 CAVEATS

As of yet, there is no method for automatically coercing input and
output to some encoding other than UTF-8.  This is planned in the near
future, however.

Whether an arbitrary piece of data will be treated as "characters" or
"bytes" by internal operations cannot be divined at the current time.

Use of locales with utf8 may lead to odd results.  Currently there is
some attempt to apply 8-bit locale info to characters in the range
0..255, but this is demonstrably incorrect for locales that use
characters above that range (when mapped into Unicode).  It will also
tend to run slower.  Avoidance of locales is strongly encouraged.

=head1 SEE ALSO

L<bytes>, L<utf8>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">

=cut