perlunicode.pod

来自「视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.」· POD 代码 · 共 1,623 行 · 第 1/4 页

=head1 NAME

perlunicode - Unicode support in Perl

=head1 DESCRIPTION

=head2 Important Caveats

Unicode support is an extensive requirement. While Perl does not
implement the Unicode standard or the accompanying technical reports
from cover to cover, Perl does support many Unicode features.

People who want to learn to use Unicode in Perl, should probably read
L<the Perl Unicode tutorial|perlunitut> before reading this reference
document.

=over 4

=item Input and Output Layers

Perl knows when a filehandle uses Perl's internal Unicode encodings
(UTF-8, or UTF-EBCDIC if in EBCDIC) if the filehandle is opened 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.  See L<open>.

To indicate that Perl source itself is in UTF-8, use C<use utf8;>.

=item Regular Expressions

The regular expression compiler produces polymorphic opcodes.  That is,
the pattern adapts to the data and automatically switches to the Unicode
character scheme when presented with data that is internally encoded in
UTF-8 -- or instead uses a traditional byte scheme when presented with
byte data.

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

As a compatibility measure, the C<use utf8> pragma must be explicitly
included to enable recognition of UTF-8 in the Perl scripts themselves
(in string or regular expression literals, or in identifier names) on
ASCII-based machines or to recognize UTF-EBCDIC on EBCDIC-based
machines.  B<These are the only times when an explicit C<use utf8>
is needed.>  See L<utf8>.

=item BOM-marked scripts and UTF-16 scripts autodetected

If a Perl script begins marked with the Unicode BOM (UTF-16LE, UTF16-BE,
or UTF-8), or if the script looks like non-BOM-marked UTF-16 of either
endianness, Perl will correctly read in the script as Unicode.
(BOMless UTF-8 cannot be effectively recognized or differentiated from
ISO 8859-1 or other eight-bit encodings.)

=item C<use encoding> needed to upgrade non-Latin-1 byte strings

By default, there is a fundamental asymmetry in Perl's Unicode model:
implicit upgrading from byte strings to Unicode strings assumes that
they were encoded in I<ISO 8859-1 (Latin-1)>, but Unicode strings are
downgraded with UTF-8 encoding.  This happens because the first 256
codepoints in Unicode happens to agree with Latin-1.  

See L</"Byte and Character Semantics"> for more details.

=back

=head2 Byte and Character Semantics

Beginning with version 5.6, Perl uses logically-wide characters to
represent strings internally.

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

However, as 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 are characters, Perl 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 only if
none of the program's inputs were 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.

The C<bytes> pragma will always, regardless of platform, 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 while Perl defaults to byte
semantics; when character semantics become the default, this pragma
may become a no-op.  See L<utf8>.

Unless explicitly stated, Perl operators use character semantics
for Unicode data and byte semantics for non-Unicode data.
The decision to use character semantics is made transparently.  If
input data comes from a Unicode source--for example, if a character
encoding layer is added to a filehandle or a literal Unicode
string constant appears in a program--character semantics apply.
Otherwise, byte semantics are in effect.  The C<bytes> pragma should
be used to force byte semantics on Unicode data.

If strings operating under byte semantics and strings with Unicode
character data are concatenated, the new string will be created by
decoding the byte strings as I<ISO 8859-1 (Latin-1)>, even if the
old Unicode string used EBCDIC.  This translation is done without
regard to the system's native 8-bit encoding. 

Under character semantics, many operations that formerly operated on
bytes now operate on characters. A character in Perl is
logically just a number ranging from 0 to 2**31 or so. Larger
characters may encode into longer sequences of bytes internally, but
this internal detail is mostly hidden for Perl code.
See L<perluniintro> for more.

=head2 Effects of Character Semantics

Character semantics have the following effects:

=over 4

=item *

Strings--including hash keys--and regular expression patterns may
contain characters that have an ordinal value larger than 255.

If you use a Unicode editor to edit your program, Unicode characters may
occur directly within the literal strings in UTF-8 encoding, or UTF-16.
(The former requires a BOM or C<use utf8>, the latter requires a BOM.)

Unicode characters can also be added to a string by using the C<\x{...}>
notation.  The Unicode code for the desired character, in hexadecimal,
should be placed in the braces. For instance, a smiley face is
C<\x{263A}>.  This encoding scheme only works for all characters, but
for characters under 0x100, note that Perl may use an 8 bit encoding
internally, for optimization and/or backward compatibility.

Additionally, if you

   use charnames ':full';

you can use the C<\N{...}> notation and put the official Unicode
character name within the braces, such as C<\N{WHITE SMILING FACE}>.

=item *

If an appropriate L<encoding> is specified, identifiers within the
Perl script may contain Unicode alphanumeric characters, including
ideographs.  Perl does not currently attempt to canonicalize variable
names.

=item *

Regular expressions match characters instead of bytes.  "." matches
a character instead of a byte.

=item *

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

=item *

Named Unicode properties, scripts, and block ranges may be used like
character classes via the C<\p{}> "matches property" construct and
the C<\P{}> negation, "doesn't match property".

See L</"Unicode Character Properties"> for more details.

You can define your own character properties and use them
in the regular expression with the C<\p{}> or C<\P{}> construct.

See L</"User-Defined Character Properties"> for more details.

=item *

The special pattern C<\X> 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.  C<\X> 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.  For similar
functionality see pack('U0', ...) and pack('C0', ...).

=item *

Case translation operators use the Unicode case translation tables
when character input is provided.  Note that C<uc()>, or C<\U> in
interpolated strings, translates to uppercase, while C<ucfirst>,
or C<\u> in interpolated strings, translates to titlecase in languages
that make the distinction.

=item *

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

=item *

The C<pack()>/C<unpack()> letter C<C> does I<not> change, since it is often 
used for byte-oriented formats.  Again, think C<char> in the C language.

There is a new C<U> specifier that converts between Unicode characters
and code points. There is also a C<W> specifier that is the equivalent of
C<chr>/C<ord> and properly handles character values even if they are above 255.

=item *

The C<chr()> and C<ord()> functions work on characters, similar to
C<pack("W")> and C<unpack("W")>, I<not> C<pack("C")> and
C<unpack("C")>.  C<pack("C")> and C<unpack("C")> are methods for
emulating byte-oriented C<chr()> and C<ord()> on Unicode strings.
While these methods reveal the internal encoding of Unicode strings,
that is not something one normally needs to care about at all.

=item *

The bit string operators, C<& | ^ ~>, can operate on character data.
However, for backward compatibility, such as when using bit string
operations when characters are all less than 256 in ordinal value, one
should not use C<~> (the bit complement) with characters of both
values less than 256 and values greater than 256.  Most importantly,
DeMorgan's laws (C<~($x|$y) eq ~$x&~$y> and C<~($x&$y) eq ~$x|~$y>)
will not hold.  The reason for this mathematical I<faux pas> 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 for the following cases:

=over 8

=item *

the case mapping is from a single Unicode character to another
single Unicode character, or

=item *

the case mapping is from a single Unicode character to more
than one Unicode character.

=back

Things to do with locales (Lithuanian, Turkish, Azeri) do B<not> work
since Perl does not understand the concept of Unicode locales.

See the Unicode Technical Report #21, Case Mappings, for more details.

But you can also define your own mappings to be used in the lc(),
lcfirst(), uc(), and ucfirst() (or their string-inlined versions).

See L</"User-Defined Case Mappings"> for more details.

=back

=over 4

=item *

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

=back

=head2 Unicode Character Properties

Named Unicode properties, scripts, and block ranges may be used like
character classes via the C<\p{}> "matches property" construct and
the C<\P{}> negation, "doesn't match property".

For instance, C<\p{Lu}> matches any character with the Unicode "Lu"
(Letter, uppercase) property, while C<\p{M}> matches any character
with an "M" (mark--accents and such) property.  Brackets are not
required for single letter properties, so C<\p{M}> is equivalent to
C<\pM>. Many predefined properties are available, such as
C<\p{Mirrored}> and C<\p{Tibetan}>.

The official Unicode script and block names have spaces and dashes as
separators, but for convenience you can use dashes, spaces, or
underbars, and case is unimportant. It is recommended, however, that
for consistency you use the following naming: the official Unicode
script, property, or block name (see below for the additional rules
that apply to block names) with whitespace and dashes removed, and the
words "uppercase-first-lowercase-rest". C<Latin-1 Supplement> thus
becomes C<Latin1Supplement>.

You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret
(^) between the first brace and the property name: C<\p{^Tamil}> is
equal to C<\P{Tamil}>.

B<NOTE: the properties, scripts, and blocks listed here are as of
Unicode 5.0.0 in July 2006.>

=over 4

=item General Category

Here are the basic Unicode General Category properties, followed by their
long form.  You can use either; C<\p{Lu}> and C<\p{UppercaseLetter}>,
for instance, are identical.

    Short       Long

    L           Letter
    LC          CasedLetter
    Lu          UppercaseLetter
    Ll          LowercaseLetter
    Lt          TitlecaseLetter
    Lm          ModifierLetter
    Lo          OtherLetter

    M           Mark
    Mn          NonspacingMark
    Mc          SpacingMark
    Me          EnclosingMark

    N           Number
    Nd          DecimalNumber
    Nl          LetterNumber
    No          OtherNumber

    P           Punctuation
    Pc          ConnectorPunctuation
    Pd          DashPunctuation
    Ps          OpenPunctuation
    Pe          ClosePunctuation
    Pi          InitialPunctuation
                (may behave like Ps or Pe depending on usage)
    Pf          FinalPunctuation
                (may behave like Ps or Pe depending on usage)
    Po          OtherPunctuation

    S           Symbol
    Sm          MathSymbol
    Sc          CurrencySymbol
    Sk          ModifierSymbol
    So          OtherSymbol

    Z           Separator
    Zs          SpaceSeparator
    Zl          LineSeparator
    Zp          ParagraphSeparator

    C           Other
    Cc          Control
    Cf          Format
    Cs          Surrogate   (not usable)
    Co          PrivateUse
    Cn          Unassigned

Single-letter properties match all characters in any of the
two-letter sub-properties starting with the same letter.
C<LC> and C<L&> are special cases, which are aliases for the set of
C<Ll>, C<Lu>, and C<Lt>.

Because Perl hides the need for the user to understand the internal
representation of Unicode characters, there is no need to implement
the somewhat messy concept of surrogates. C<Cs> is therefore not
supported.

=item Bidirectional Character Types

Because scripts differ in their directionality--Hebrew is
written right to left, for example--Unicode supplies these properties in
the BidiClass class:

    Property    Meaning

    L           Left-to-Right
    LRE         Left-to-Right Embedding
    LRO         Left-to-Right Override
    R           Right-to-Left
    AL          Right-to-Left Arabic
    RLE         Right-to-Left Embedding
    RLO         Right-to-Left Override
    PDF         Pop Directional Format
    EN          European Number
    ES          European Number Separator
    ET          European Number Terminator
    AN          Arabic Number
    CS          Common Number Separator
    NSM         Non-Spacing Mark
    BN          Boundary Neutral
    B           Paragraph Separator
    S           Segment Separator
    WS          Whitespace
    ON          Other Neutrals

For example, C<\p{BidiClass:R}> matches characters that are normally
written right to left.