a small patch to perlintro.pod.

[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 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<NOTE: 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.
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.

Notice that if you have a string with byte semantics and you then
add character data into it, the bytes will be upgraded I<as if they
were ISO 8859-1 (Latin-1)> (or if in EBCDIC, after a translation
to ISO 8859-1).

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 C<\p{Is...}> test for "general properties" such as "letter",
"digit", while the C<\p{In...}> test for Unicode scripts and blocks.

The official Unicode script and block names have spaces and
dashes and separators, but for convenience you can have
dashes, spaces, and underbars at every word division, and
you need not care about correct casing.  It is recommended,
however, that for consistency you use the following naming:
the official Unicode script or block name (see below for
the additional rules that apply to block names), with the whitespace
and dashes removed, and the words "uppercase-first-lowercase-otherwise".
That is, "Latin-1 Supplement" becomes "Latin1Supplement".

You can also negate both C<\p{}> and C<\P{}> by introducing a caret
(^) between the first curly and the property name: C<\p{^InTamil}> is
equal to C<\P{InTamil}>.

The C<In> and C<Is> can be left out: C<\p{Greek}> is equal to
C<\p{InGreek}>, C<\P{Pd}> is equal to C<\P{Pd}>.

    Short       Long

    L           Letter
    Lu          Uppercase Letter
    Ll          Lowercase Letter
    Lt          Titlecase Letter
    Lm          Modifier Letter
    Lo          Other Letter

    M           Mark
    Mn          Non-Spacing Mark
    Mc          Spacing Combining Mark
    Me          Enclosing Mark

    N           Number
    Nd          Decimal Digit Number
    Nl          Letter Number
    No          Other Number

    P           Punctuation
    Pc          Connector Punctuation
    Pd          Dash Punctuation
    Ps          Open Punctuation
    Pe          Close Punctuation
    Pi          Initial Punctuation
                (may behave like Ps or Pe depending on usage)
    Pf          Final Punctuation
                (may behave like Ps or Pe depending on usage)
    Po          Other Punctuation

    S           Symbol
    Sm          Math Symbol
    Sc          Currency Symbol
    Sk          Modifier Symbol
    So          Other Symbol

    Z           Separator
    Zs          Space Separator
    Zl          Line Separator
    Zp          Paragraph Separator

    C           Other
    Cc          (Other) Control
    Cf          (Other) Format
    Cs          (Other) Surrogate
    Co          (Other) Private Use
    Cn          (Other) Not Assigned

There's also C<L&> which is an alias for C<Ll>, C<Lu>, and C<Lt>.

The following reserved ranges have C<In> tests:

    CJK Ideograph Extension A
    CJK Ideograph
    Hangul Syllable
    Non Private Use High Surrogate
    Private Use High Surrogate
    Low Surrogate
    Private Surrogate
    CJK Ideograph Extension B
    Plane 15 Private Use
    Plane 16 Private Use

For example C<"\x{AC00}" =~ \p{HangulSyllable}> will test true.
(Handling of surrogates is not implemented yet.)

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

There are also extended property classes that supplement the basic
properties, defined by the F<PropList> Unicode database:

    White_space
    Bidi_Control
    Join_Control
    Dash
    Hyphen
    Quotation_Mark
    Other_Math
    Hex_Digit
    ASCII_Hex_Digit
    Other_Alphabetic
    Ideographic
    Diacritic
    Extender
    Other_Lowercase
    Other_Uppercase
    Noncharacter_Code_Point

and further derived properties:

    Alphabetic      Lu + Ll + Lt + Lm + Lo + Other_Alphabetic
    Lowercase       Ll + Other_Lowercase
    Uppercase       Lu + Other_Uppercase
    Math            Sm + Other_Math

    ID_Start        Lu + Ll + Lt + Lm + Lo + Nl
    ID_Continue     ID_Start + Mn + Mc + Nd + Pc

    Any             Any character
    Assigned        Any non-Cn character
    Common          Any character (or unassigned code point)
                    not explicitly assigned to a script.

=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 Unicode 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()> for Unicode strings.
(Note that this reveals the internal UTF-8 encoding of strings and
you are not supposed to do that unless you know what you are doing.)

=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 *

lc(), uc(), lcfirst(), and ucfirst() work only for some of the
simplest cases, where the mapping goes from a single Unicode character
to another single Unicode character, and where the mapping does not
depend on surrounding characters, or on locales.  More complex cases,
where for example one character maps into several, are not yet
implemented.  See the Unicode Technical Report #21, Case Mappings,
for more details.  The Unicode::UCD module (part of Perl since 5.8.0)
casespec() and casefold() interfaces supply information about the more
complex cases.

=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 UNICODE REGULAR EXPRESSION SUPPORT LEVEL

The following list of Unicode regular expression support describes
feature by feature the Unicode support implemented in Perl as of Perl
5.8.0.  The "Level N" and the section numbers refer to the Unicode
Technical Report 18, "Unicode Regular Expression Guidelines".

=over 4

=item *

Level 1 - Basic Unicode Support

        2.1 Hex Notation                        - done          [1]
                Named Notation                  - done          [2]
        2.2 Categories                          - done          [3][4]
        2.3 Subtraction                         - MISSING       [5][6]
        2.4 Simple Word Boundaries              - done          [7]
        2.5 Simple Loose Matches                - MISSING       [8]
        2.6 End of Line                         - MISSING       [9][10]

        [ 1] \x{...}
        [ 2] \N{...}
        [ 3] . \p{Is...} \P{Is...}
        [ 4] now scripts (see UTR#24 Script Names) in  addition to blocks
        [ 5] have negation
        [ 6] can use look-ahead to emulate subtracion
        [ 7] include Letters in word characters
        [ 8] see UTR#21 Case Mappings
        [ 9] see UTR#13 Unicode Newline Guidelines
        [10] should do ^ and $ also on \x{2028} and \x{2029}

=item *

Level 2 - Extended Unicode Support

        3.1 Surrogates                          - MISSING
        3.2 Canonical Equivalents               - MISSING       [11][12]
        3.3 Locale-Independent Graphemes        - MISSING       [13]
        3.4 Locale-Independent Words            - MISSING       [14]
        3.5 Locale-Independent Loose Matches    - MISSING       [15]

        [11] see UTR#15 Unicode Normalization
        [12] have Unicode::Normalize but not integrated to regexes
        [13] have \X but at this level . should equal that
        [14] need three classes, not just \w and \W
        [15] see UTR#21 Case Mappings

=item *

Level 3 - Locale-Sensitive Support

        4.1 Locale-Dependent Categories         - MISSING
        4.2 Locale-Dependent Graphemes          - MISSING       [16][17]
        4.3 Locale-Dependent Words              - MISSING
        4.4 Locale-Dependent Loose Matches      - MISSING
        4.5 Locale-Dependent Ranges             - MISSING

        [16] see UTR#10 Unicode Collation Algorithms
        [17] have Unicode::Collate but not integrated to regexes

=back

=head1 SEE ALSO

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

=cut