Dispell the "use utf8" superstition.

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

=head1 NAME

perlunicode - Unicode support in Perl

=head1 DESCRIPTION

=head2 Important Caveats

WARNING: While the implementation of Unicode support in Perl is now fairly
complete it is still evolving to some extent.

In particular the way Unicode is handled on EBCDIC platforms is still
rather experimental. On such a platform references to UTF-8 encoding
in this document and elsewhere should be read as meaning UTF-EBCDIC as
specified in Unicode Technical Report 16 unless ASCII vs EBCDIC issues
are specifically discussed. There is no C<utfebcdic> pragma or
":utfebcdic" layer, rather "utf8" and ":utf8" are re-used to mean
platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic> for
more discussion of the issues.

The following areas are still under development.

=over 4

=item Input and Output Disciplines

A filehandle can be marked as containing perl's internal Unicode
encoding (UTF-8 or UTF-EBCDIC) by opening it with the ":utf8" layer.
Other encodings can be converted to perl's encoding on input, or from
perl's encoding on output by use of the ":encoding()" layer.  There is
not yet a clean way to mark the Perl source itself as being in an
particular encoding.

=item Regular Expressions

The regular expression compiler does now attempt to produce
polymorphic opcodes.  That is the pattern should now adapt to the data
and automatically switch to the Unicode character scheme when
presented with Unicode data, or a traditional byte scheme when
presented with byte data.  The implementation is still new and
(particularly on EBCDIC platforms) may need further work.

=item C<use utf8> still needed to enable UTF-8/UTF-EBCDIC in scripts

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 in the Perl scripts themselves on
ASCII based machines or recognize UTF-EBCDIC on EBCDIC based machines.
B<This should be the only place where an explicit C<use utf8> is needed>.

=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 either the UTF-8 or the UTF-EBCDIC 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 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.  Note that this is
currently only implemented on Windows since other platforms API
standard on this area.

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

The C<utf8> pragma is primarily a compatibility device that enables
recognition of UTF-(8|EBCDIC) 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 B<may> be stored in
a sequence of two or more bytes, all of which have the high bit set.

For C1 controls or Latin 1 characters on an EBCDIC platform the
character may be stored in a UTF-EBCDIC multi byte sequence.  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 (or UTF-EBCDIC on EBCDIC platforms) characters, but you can also
specify a particular character with an extension of the C<\x>
notation.  UTF-X 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}>.  The
names of the C<In> classes are the official Unicode script and block
names but with all non-alphanumeric characters removed, for example
the block name C<"Latin-1 Supplement"> becomes C<\p{InLatin1Supplement}>.

Here is the list as of Unicode 3.1.0 (the two-letter classes) and
as defined by Perl (the one-letter classes) (in Unicode materials
what Perl calls C<L> is often called C<L&>):

   L  Letter
   Lu Letter, Uppercase
   Ll Letter, Lowercase
   Lt Letter, Titlecase
   Lm Letter, Modifier
   Lo Letter, Other
   M  Mark
   Mn Mark, Non-Spacing
   Mc Mark, Spacing Combining
   Me Mark, Enclosing
   N  Number
   Nd Number, Decimal Digit
   Nl Number, Letter
   No Number, Other
   P  Punctuation
   Pc Punctuation, Connector
   Pd Punctuation, Dash
   Ps Punctuation, Open
   Pe Punctuation, Close
   Pi Punctuation, Initial quote
       (may behave like Ps or Pe depending on usage)
   Pf Punctuation, Final quote
       (may behave like Ps or Pe depending on usage)
   Po Punctuation, Other
   S  Symbol
   Sm Symbol, Math
   Sc Symbol, Currency
   Sk Symbol, Modifier
   So Symbol, Other
   Z  Separator
   Zs Separator, Space
   Zl Separator, Line
   Zp Separator, Paragraph
   C  Other
   Cc Other, Control
   Cf Other, Format
   Cs Other, Surrogate
   Co Other, Private Use
   Cn Other, Not Assigned (Unicode defines no Cn characters)

Additionally, because scripts differ in their directionality
(for example Hebrew is written right to left), all characters
have their directionality defined:

   BidiL   Left-to-Right
   BidiLRE Left-to-Right Embedding
   BidiLRO Left-to-Right Override
   BidiR   Right-to-Left
   BidiAL  Right-to-Left Arabic
   BidiRLE Right-to-Left Embedding
   BidiRLO Right-to-Left Override
   BidiPDF Pop Directional Format
   BidiEN  European Number
   BidiES  European Number Separator
   BidiET  European Number Terminator
   BidiAN  Arabic Number
   BidiCS  Common Number Separator
   BidiNSM Non-Spacing Mark
   BidiBN  Boundary Neutral
   BidiB   Paragraph Separator
   BidiS   Segment Separator
   BidiWS  Whitespace
   BidiON  Other Neutrals

=head2 Scripts

The scripts available for C<\p{In...}> and C<\P{In...}>, for example
\p{InCyrillic>, are as follows, for example C<\p{InLatin}> or C<\P{InHan}>:

   Latin
   Greek
   Cyrillic
   Armenian
   Hebrew
   Arabic
   Syriac
   Thaana
   Devanagari
   Bengali
   Gurmukhi
   Gujarati
   Oriya
   Tamil
   Telugu
   Kannada
   Malayalam
   Sinhala
   Thai
   Lao
   Tibetan
   Myanmar
   Georgian
   Hangul
   Ethiopic
   Cherokee
   CanadianAboriginal
   Ogham
   Runic
   Khmer
   Mongolian
   Hiragana
   Katakana
   Bopomofo
   Han
   Yi
   OldItalic
   Gothic
   Deseret
   Inherited

=head2 Blocks

In addition to B<scripts>, Unicode also defines B<blocks> of
characters.  The difference between scripts and blocks is that the
former concept is closer to natural languages, while the latter
concept is more an artificial grouping based on groups of 256 Unicode
characters.  For example, the C<Latin> script contains letters from
many blocks, but it does not contain all the characters from those
blocks, it does not for example contain digits.

For more about scripts see the UTR #24:
http://www.unicode.org/unicode/reports/tr24/
For more about blocks see
http://www.unicode.org/Public/UNIDATA/Blocks.txt

Because there are overlaps in naming (there are, for example, both
a script called C<Katakana> and a block called C<Katakana>, the block
version has C<Block> appended to its name, C<\p{InKatakanaBlock}>.

Notice that this definition was introduced in Perl 5.8.0: in Perl
5.6.0 only the blocks were used; in Perl 5.8.0 scripts became the
preferential character class definition; this meant that the
definitions of some character classes changed (the ones in the
below list that have the C<Block> appended).

   BasicLatin
   Latin1Supplement
   LatinExtendedA
   LatinExtendedB
   IPAExtensions
   SpacingModifierLetters
   CombiningDiacriticalMarks
   GreekBlock
   CyrillicBlock
   ArmenianBlock
   HebrewBlock
   ArabicBlock
   SyriacBlock
   ThaanaBlock
   DevanagariBlock
   BengaliBlock
   GurmukhiBlock
   GujaratiBlock
   OriyaBlock
   TamilBlock
   TeluguBlock
   KannadaBlock
   MalayalamBlock
   SinhalaBlock
   ThaiBlock
   LaoBlock
   TibetanBlock
   MyanmarBlock
   GeorgianBlock
   HangulJamo
   EthiopicBlock
   CherokeeBlock
   UnifiedCanadianAboriginalSyllabics
   OghamBlock
   RunicBlock
   KhmerBlock
   MongolianBlock
   LatinExtendedAdditional
   GreekExtended
   GeneralPunctuation
   SuperscriptsandSubscripts
   CurrencySymbols
   CombiningMarksforSymbols
   LetterlikeSymbols
   NumberForms
   Arrows
   MathematicalOperators
   MiscellaneousTechnical
   ControlPictures
   OpticalCharacterRecognition
   EnclosedAlphanumerics
   BoxDrawing
   BlockElements
   GeometricShapes
   MiscellaneousSymbols
   Dingbats
   BraillePatterns
   CJKRadicalsSupplement
   KangxiRadicals
   IdeographicDescriptionCharacters
   CJKSymbolsandPunctuation
   HiraganaBlock
   KatakanaBlock
   BopomofoBlock
   HangulCompatibilityJamo
   Kanbun
   BopomofoExtended
   EnclosedCJKLettersandMonths
   CJKCompatibility
   CJKUnifiedIdeographsExtensionA
   CJKUnifiedIdeographs
   YiSyllables
   YiRadicals
   HangulSyllables
   HighSurrogates
   HighPrivateUseSurrogates
   LowSurrogates
   PrivateUse
   CJKCompatibilityIdeographs
   AlphabeticPresentationForms
   ArabicPresentationFormsA
   CombiningHalfMarks
   CJKCompatibilityForms
   SmallFormVariants
   ArabicPresentationFormsB
   Specials
   HalfwidthandFullwidthForms
   OldItalicBlock
   GothicBlock
   DeseretBlock
   ByzantineMusicalSymbols
   MusicalSymbols
   MathematicalAlphanumericSymbols
   CJKUnifiedIdeographsExtensionB
   CJKCompatibilityIdeographsSupplement
   Tags

=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 *

The bit string operators C<& | ^ ~> can operate on character data.
However, for backward compatibility reasons (bit string operations
when the characters all are less than 256 in ordinal value) one should
not mix C<~> (the bit complement) and characters both less than 256 and
equal or greater than 256.  Most importantly, the DeMorgan's laws
(C<~($x|$y) eq ~$x&~$y>, C<~($x&$y) eq ~$x|~$y>) won't hold.
Another way to look at this is that the complement cannot return
B<both> the 8-bit (byte) wide bit complement B<and> the full character
wide bit complement.

=item *

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

=back

=head2 Character encodings for input and output

See L<Encode>.

=head1 CAVEATS

As of yet, there is no method for automatically coercing input and
output to some encoding other than UTF-8 or UTF-EBCDIC.  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<perlretut>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">

=cut