Better document the difference between a block and a script.

[perl5.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 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 on ASCII based machines or recognize UTF-EBCDIC encoded literals
and identifiers on EBCDIC based machines.

=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 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 since UNIXes lack 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 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|EBCDIC)
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 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
Perl 5.8.0 (the one-letter classes):

   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

The blocks available for C<\p{InBlock}> and C<\P{InBlock}>, for
example \p{InCyrillic>, are as follows:

    BasicLatin
    Latin1Supplement
    LatinExtendedA
    LatinExtendedB
    IPAExtensions
    SpacingModifierLetters
    CombiningDiacriticalMarks
    Greek
    Cyrillic
    Armenian
    Hebrew
    Arabic
    Syriac
    Thaana
    Devanagari
    Bengali
    Gurmukhi
    Gujarati
    Oriya
    Tamil
    Telugu
    Kannada
    Malayalam
    Sinhala
    Thai
    Lao
    Tibetan
    Myanmar
    Georgian
    HangulJamo
    Ethiopic
    Cherokee
    UnifiedCanadianAboriginalSyllabics
    Ogham
    Runic
    Khmer
    Mongolian
    LatinExtendedAdditional
    GreekExtended
    GeneralPunctuation
    SuperscriptsandSubscripts
    CurrencySymbols
    CombiningMarksforSymbols
    LetterlikeSymbols
    NumberForms
    Arrows
    MathematicalOperators
    MiscellaneousTechnical
    ControlPictures
    OpticalCharacterRecognition
    EnclosedAlphanumerics
    BoxDrawing
    BlockElements
    GeometricShapes
    MiscellaneousSymbols
    Dingbats
    BraillePatterns
    CJKRadicalsSupplement
    KangxiRadicals
    IdeographicDescriptionCharacters
    CJKSymbolsandPunctuation
    Hiragana
    Katakana
    Bopomofo
    HangulCompatibilityJamo
    Kanbun
    BopomofoExtended
    EnclosedCJKLettersandMonths
    CJKCompatibility
    CJKUnifiedIdeographsExtensionA
    CJKUnifiedIdeographs
    YiSyllables
    YiRadicals
    HangulSyllables
    HighSurrogates
    HighPrivateUseSurrogates
    LowSurrogates
    PrivateUse
    CJKCompatibilityIdeographs
    AlphabeticPresentationForms
    ArabicPresentationFormsA
    CombiningHalfMarks
    CJKCompatibilityForms
    SmallFormVariants
    ArabicPresentationFormsB
    Specials
    HalfwidthandFullwidthForms
    OldItalic
    Gothic
    Deseret
    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 cannot
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, 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