=head1 DESCRIPTION
-WARNING: The implementation of Unicode support in Perl is incomplete.
-Expect sudden and unannounced changes!
+=head2 Important Caveats
-Beginning with version 5.6, Perl uses logically wide characters to
-represent strings internally. This internal representation of strings
-uses the UTF-8 encoding.
+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.
+
+=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.
+See L<open>.
+
+To mark the Perl source itself as being in a particular encoding,
+see L<encoding>.
-In future, Perl-level operations will expect to work with characters
-rather than bytes, in general.
+=item Regular Expressions
+
+The regular expression compiler produces polymorphic opcodes. That is,
+the pattern adapts 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.
+
+=item C<use utf8> still needed to enable UTF-8/UTF-EBCDIC in scripts
+
+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 to recognize UTF-EBCDIC on EBCDIC based machines.
+B<NOTE: this should be the only place where an explicit C<use utf8>
+is needed>.
+
+You can also use the C<encoding> pragma to change the default encoding
+of the data in your script; see L<encoding>.
+
+=back
-However, Perl v5.6 aims to provide a safe migration path from byte
-semantics to character semantics for programs. To preserve compatibility
-with earlier versions of Perl which allowed byte semantics in Perl
-operations (owing to the fact that the internal representation for
-characters was in bytes) byte semantics will continue to be in effect
-until a the C<utf8> pragma is used in the C<main> package, or the C<$^U>
-global flag is explicitly set.
+=head2 Byte and Character semantics
+
+Beginning with version 5.6, Perl uses logically wide characters to
+represent strings internally.
+
+In future, Perl-level operations can be expected to work with
+characters rather than bytes, in general.
+
+However, as strictly an interim compatibility measure, Perl aims to
+provide a safe migration path from byte semantics to character
+semantics for programs. For operations where Perl can unambiguously
+decide that the input data is characters, Perl now switches to
+character semantics. For operations where this determination cannot
+be made without additional information from the user, Perl decides in
+favor of compatibility, and chooses to use byte semantics.
+
+This behavior preserves compatibility with earlier versions of Perl,
+which allowed byte semantics in Perl operations, but only as long as
+none of the program's inputs are marked as being as source of Unicode
+character data. Such data may come from filehandles, from calls to
+external programs, from information provided by the system (such as %ENV),
+or from literals and constants in the source text.
+
+On Windows platforms, if the C<-C> command line switch is used, (or the
+${^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 lack an
+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 Unicode 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 concatenate strings with byte semantics and strings
+with Unicode character data, the bytes will by default be upgraded
+I<as if they were ISO 8859-1 (Latin-1)> (or if in EBCDIC, after a
+translation to ISO 8859-1). This is done without regard to the
+system's native 8-bit encoding, so to change this for systems with
+non-Latin-1 (or non-EBCDIC) native encodings, use the C<encoding>
+pragma, see L<encoding>.
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 is 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.
-
-The C<byte> pragma can be used to force byte semantics in a particular
-lexical scope. See L<byte>.
-
-The C<utf8> pragma is a compatibility device to enables recognition
-of UTF-8 in literals encountered by the parser. It is also used
-for enabling some 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>.
+bytes change to operating on characters. A character in Perl is
+logically just a number ranging from 0 to 2**31 or so. Larger
+characters may encode to longer sequences of bytes internally, but
+this is just an internal detail which is hidden at the Perl level.
+See L<perluniintro> for more on this.
+
+=head2 Effects of character semantics
Character semantics have the following effects:
=item *
Strings and patterns may contain characters that have an ordinal value
-larger than 255. In Perl v5.6, this is only enabled if the lexical
-scope has a C<use utf8> declaration (due to compatibility needs) but
-future versions may enable this by default.
+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}>. A character in the Latin-1 range
-(128..255) should be written C<\x{ab}> rather than C<\xab>, since the
-former will turn into a two-byte UTF-8 code, while the latter will
-continue to be interpreted as generating a 8-bit byte rather than a
-character. In fact, if C<-w> is turned on, it will produce a warning
-that you might be generating invalid UTF-8.
+If you use a Unicode editor to edit your program, Unicode characters
+may occur directly within the literal strings in one of the various
+Unicode encodings (UTF-8, UTF-EBCDIC, UCS-2, etc.), but are recognized
+as such (and converted to Perl's internal representation) only if the
+appropriate L<encoding> is specified.
+
+You can also get Unicode characters into a string by using the C<\x{...}>
+notation, putting the Unicode code for the desired character, in
+hexadecimal, into the curlies. For instance, a smiley face is C<\x{263A}>.
+This works only for characters with a code 0x100 and above.
+
+Additionally, if you
+ use charnames ':full';
+you can use the C<\N{...}> notation, putting the official Unicode character
+name within the curlies. For example, C<\N{WHITE SMILING FACE}>.
+This works for all characters that have names.
=item *
-Identifiers within the Perl script may contain Unicode alphanumeric
+If an appropriate L<encoding> is specified,
+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.)
-
-This also needs C<use utf8> currently. [XXX: Why? High-bit chars were
-syntax errors when they occurred within identifiers in previous versions,
-so this should be enabled by default.]
+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>).)
-
-Unicode support in regular expressions needs C<use utf8> currently.
-[XXX: Because the SWASH routines need to be loaded. And the RE engine
-appears to need an overhaul to Unicode by default anyway.]
+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.
-
-C<use utf8> is needed to enable this. See above.
+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}>.
+Named Unicode properties, scripts, and block ranges may be used like
+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 "Lu" (Letter, uppercase) property, while
+C<\p{M}> matches any character with a "M" (mark -- accents and such)
+property. Single letter properties may omit the brackets, so that can be
+written C<\pM> also. 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 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, block, or property name (see below for the
+additional rules that apply to block names), with whitespace and dashes
+removed, and the words "uppercase-first-lowercase-rest". 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{^Tamil}> is
+equal to C<\P{Tamil}>.
+
+Here are the basic Unicode General Category properties, followed by their
+long form (you can use either, e.g. C<\p{Lu}> and C<\p{LowercaseLetter}>
+are identical).
+
+ Short Long
+
+ L Letter
+ 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
+
+The single-letter properties match all characters in any of the
+two-letter sub-properties starting with the same letter.
+There's also C<L&> which is an alias for C<Ll>, C<Lu>, and C<Lt>.
+
+Because Perl hides the need for the user to understand the internal
+representation of Unicode characters, it has no need to support the
+somewhat messy concept of surrogates. Therefore, the C<Cs> property is not
+supported.
+
+Because scripts differ in their directionality (for example Hebrew is
+written right to left), Unicode supplies these properties:
+
+ Property Meaning
+
+ 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
+
+For example, C<\p{BidiR}> matches all characters that are normally
+written right to left.
-C<use utf8> is needed to enable this. See above.
+=back
+
+=head2 Scripts
+
+The scripts available via C<\p{...}> and C<\P{...}>, for example
+C<\p{Latin}> or \p{Cyrillic>, are as follows:
+
+ Arabic
+ Armenian
+ Bengali
+ Bopomofo
+ CanadianAboriginal
+ Cherokee
+ Cyrillic
+ Deseret
+ Devanagari
+ Ethiopic
+ Georgian
+ Gothic
+ Greek
+ Gujarati
+ Gurmukhi
+ Han
+ Hangul
+ Hebrew
+ Hiragana
+ Inherited
+ Kannada
+ Katakana
+ Khmer
+ Lao
+ Latin
+ Malayalam
+ Mongolian
+ Myanmar
+ Ogham
+ OldItalic
+ Oriya
+ Runic
+ Sinhala
+ Syriac
+ Tamil
+ Telugu
+ Thaana
+ Thai
+ Tibetan
+ Yi
+
+There are also extended property classes that supplement the basic
+properties, defined by the F<PropList> Unicode database:
+
+ ASCII_Hex_Digit
+ BidiControl
+ Dash
+ Diacritic
+ Extender
+ HexDigit
+ Hyphen
+ Ideographic
+ JoinControl
+ NoncharacterCodePoint
+ OtherAlphabetic
+ OtherLowercase
+ OtherMath
+ OtherUppercase
+ QuotationMark
+ WhiteSpace
+
+and further derived properties:
+
+ Alphabetic Lu + Ll + Lt + Lm + Lo + OtherAlphabetic
+ Lowercase Ll + OtherLowercase
+ Uppercase Lu + OtherUppercase
+ Math Sm + OtherMath
+
+ ID_Start Lu + Ll + Lt + Lm + Lo + Nl
+ ID_Continue ID_Start + Mn + Mc + Nd + Pc
+
+ Any Any character
+ Assigned Any non-Cn character (i.e. synonym for C<\P{Cn}>)
+ Unassigned Synonym for C<\p{Cn}>
+ Common Any character (or unassigned code point)
+ not explicitly assigned to a script
+
+For backward compatability, all properties mentioned so far may have C<Is>
+prepended to their name (e.g. C<\P{IsLu}> is equal to C<\P{Lu}>).
+
+=head2 Blocks
+
+In addition to B<scripts>, Unicode also defines B<blocks> of characters.
+The difference between scripts and blocks is that the scripts concept is
+closer to natural languages, while the blocks concept is more an artificial
+grouping based on groups of mostly 256 Unicode characters. For example, the
+C<Latin> script contains letters from many blocks. On the other hand, the
+C<Latin> script does not contain all the characters from those blocks. It
+does not, for example, contain digits because digits are shared across many
+scripts. Digits and other similar groups, like punctuation, are in a
+category called C<Common>.
+
+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
+
+Blocks names are given with the C<In> prefix. For example, the
+Katakana block is referenced via C<\p{InKatakana}>. The C<In>
+prefix may be omitted if there is no nameing conflict with a script
+or any other property, but it is recommended that C<In> always be used
+to avoid confusion.
+
+These block names are supported:
+
+ InAlphabeticPresentationForms
+ InArabicBlock
+ InArabicPresentationFormsA
+ InArabicPresentationFormsB
+ InArmenianBlock
+ InArrows
+ InBasicLatin
+ InBengaliBlock
+ InBlockElements
+ InBopomofoBlock
+ InBopomofoExtended
+ InBoxDrawing
+ InBraillePatterns
+ InByzantineMusicalSymbols
+ InCJKCompatibility
+ InCJKCompatibilityForms
+ InCJKCompatibilityIdeographs
+ InCJKCompatibilityIdeographsSupplement
+ InCJKRadicalsSupplement
+ InCJKSymbolsAndPunctuation
+ InCJKUnifiedIdeographs
+ InCJKUnifiedIdeographsExtensionA
+ InCJKUnifiedIdeographsExtensionB
+ InCherokeeBlock
+ InCombiningDiacriticalMarks
+ InCombiningHalfMarks
+ InCombiningMarksForSymbols
+ InControlPictures
+ InCurrencySymbols
+ InCyrillicBlock
+ InDeseretBlock
+ InDevanagariBlock
+ InDingbats
+ InEnclosedAlphanumerics
+ InEnclosedCJKLettersAndMonths
+ InEthiopicBlock
+ InGeneralPunctuation
+ InGeometricShapes
+ InGeorgianBlock
+ InGothicBlock
+ InGreekBlock
+ InGreekExtended
+ InGujaratiBlock
+ InGurmukhiBlock
+ InHalfwidthAndFullwidthForms
+ InHangulCompatibilityJamo
+ InHangulJamo
+ InHangulSyllables
+ InHebrewBlock
+ InHighPrivateUseSurrogates
+ InHighSurrogates
+ InHiraganaBlock
+ InIPAExtensions
+ InIdeographicDescriptionCharacters
+ InKanbun
+ InKangxiRadicals
+ InKannadaBlock
+ InKatakanaBlock
+ InKhmerBlock
+ InLaoBlock
+ InLatin1Supplement
+ InLatinExtendedAdditional
+ InLatinExtended-A
+ InLatinExtended-B
+ InLetterlikeSymbols
+ InLowSurrogates
+ InMalayalamBlock
+ InMathematicalAlphanumericSymbols
+ InMathematicalOperators
+ InMiscellaneousSymbols
+ InMiscellaneousTechnical
+ InMongolianBlock
+ InMusicalSymbols
+ InMyanmarBlock
+ InNumberForms
+ InOghamBlock
+ InOldItalicBlock
+ InOpticalCharacterRecognition
+ InOriyaBlock
+ InPrivateUse
+ InRunicBlock
+ InSinhalaBlock
+ InSmallFormVariants
+ InSpacingModifierLetters
+ InSpecials
+ InSuperscriptsAndSubscripts
+ InSyriacBlock
+ InTags
+ InTamilBlock
+ InTeluguBlock
+ InThaanaBlock
+ InThaiBlock
+ InTibetanBlock
+ InUnifiedCanadianAboriginalSyllabics
+ InYiRadicals
+ InYiSyllables
+
+=over 4
=item *
-The special pattern C<\X> match matches any extended Unicode sequence
+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. It is equivalent to
C<(?:\PM\pM*)>.
-C<use utf8> is needed to enable this. See above.
-
=item *
-The C<tr///> operator translates characters instead of bytes. It can also
-be forced to translate between 8-bit codes and UTF-8 regardless of the
-surrounding utf8 state. For instance, if you know your input in Latin-1,
-you can say:
-
- use utf8;
- while (<>) {
- tr/\0-\xff//CU; # latin1 char to utf8
- ...
- }
-
-Similarly you could translate your output with
-
- tr/\0-\x{ff}//UC; # utf8 to latin1 char
-
-No, C<s///> doesn't take /U or /C (yet?).
-
-C<use utf8> is needed to enable this. See above.
+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.
+when provided character input. Note that C<uc()> (also known as C<\U>
+in doublequoted strings) translates to uppercase, while C<ucfirst>
+(also known as C<\u> in doublequoted strings) 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.
+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.)
+that will convert between Unicode characters and integers.
=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.
+byte-oriented C<chr()> and C<ord()> for Unicode strings.
+(Note that this reveals the internal encoding of Unicode strings,
+which 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 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 for the following cases:
+
+=over 8
+
+=item *
+
+the case mapping is from a single Unicode character to another
+single Unicode character
+
+=item *
+
+the case mapping is from a single Unicode character to more
+than one Unicode character
+
+=back
+
+What doesn't yet work are the following cases:
+
+=over 8
+
+=item *
+
+the "final sigma" (Greek)
+
+=item *
+
+anything to with locales (Lithuanian, Turkish, Azeri)
+
+=back
+
+See the Unicode Technical Report #21, Case Mappings, for more details.
=item *
=back
-=head1 CAVEATS
+=head2 Character encodings for input and output
+
+See L<Encode>.
+
+=head2 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 - done [8]
+ 2.6 End of Line - MISSING [9][10]
+
+ [ 1] \x{...}
+ [ 2] \N{...}
+ [ 3] . \p{...} \P{...}
+ [ 4] now scripts (see UTR#24 Script Names) in addition to blocks
+ [ 5] have negation
+ [ 6] can use look-ahead to emulate subtraction (*)
+ [ 7] include Letters in word characters
+ [ 8] note that perl does Full casefolding in matching, not Simple:
+ for example U+1F88 is equivalent with U+1F000 U+03B9,
+ not with 1F80. This difference matters for certain Greek
+ capital letters with certain modifiers: the Full casefolding
+ decomposes the letter, while the Simple casefolding would map
+ it to a single character.
+ [ 9] see UTR#13 Unicode Newline Guidelines
+ [10] should do ^ and $ also on \x{85}, \x{2028} and \x{2029})
+ (should also affect <>, $., and script line numbers)
+ (the \x{85}, \x{2028} and \x{2029} do match \s)
+
+(*) You can mimic class subtraction using lookahead.
+For example, what TR18 might write as
+
+ [{Greek}-[{UNASSIGNED}]]
+
+in Perl can be written as:
+
+ (?!\p{Unassigned})\p{InGreek}
+ (?=\p{Assigned})\p{InGreek}
+
+But in this particular example, you probably really want
+
+ \p{Greek}
+
+which will match assigned characters known to be part of the Greek script.
+
+=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
+
+=head2 Unicode Encodings
+
+Unicode characters are assigned to I<code points> which are abstract
+numbers. To use these numbers various encodings are needed.
+
+=over 4
+
+=item *
+
+UTF-8
+
+UTF-8 is a variable-length (1 to 6 bytes, current character allocations
+require 4 bytes), byteorder independent encoding. For ASCII, UTF-8 is
+transparent (and we really do mean 7-bit ASCII, not another 8-bit encoding).
+
+The following table is from Unicode 3.2.
+
+ Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
+
+ U+0000..U+007F 00..7F
+ U+0080..U+07FF C2..DF 80..BF
+ U+0800..U+0FFF E0 A0..BF 80..BF
+ U+1000..U+CFFF E1..EC 80..BF 80..BF
+ U+D000..U+D7FF ED 80..9F 80..BF
+ U+D800..U+DFFF ******* ill-formed *******
+ U+E000..U+FFFF EE..EF 80..BF 80..BF
+ U+10000..U+3FFFF F0 90..BF 80..BF 80..BF
+ U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
+ U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
+
+Note the A0..BF in U+0800..U+0FFF, the 80..9F in U+D000...U+D7FF,
+the 90..BF in U+10000..U+3FFFF, and the 80...8F in U+100000..U+10FFFF.
+Or, another way to look at it, as bits:
+
+ Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
+
+ 0aaaaaaa 0aaaaaaa
+ 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
+ ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
+ 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
+
+As you can see, the continuation bytes all begin with C<10>, and the
+leading bits of the start byte tell how many bytes the are in the
+encoded character.
+
+=item *
+
+UTF-EBCDIC
+
+Like UTF-8, but EBCDIC-safe, as UTF-8 is ASCII-safe.
+
+=item *
+
+UTF-16, UTF-16BE, UTF16-LE, Surrogates, and BOMs (Byte Order Marks)
+
+(The followings items are mostly for reference, Perl doesn't
+use them internally.)
+
+UTF-16 is a 2 or 4 byte encoding. The Unicode code points
+0x0000..0xFFFF are stored in two 16-bit units, and the code points
+0x010000..0x10FFFF in two 16-bit units. The latter case is
+using I<surrogates>, the first 16-bit unit being the I<high
+surrogate>, and the second being the I<low surrogate>.
+
+Surrogates are code points set aside to encode the 0x01000..0x10FFFF
+range of Unicode code points in pairs of 16-bit units. The I<high
+surrogates> are the range 0xD800..0xDBFF, and the I<low surrogates>
+are the range 0xDC00..0xDFFFF. The surrogate encoding is
+
+ $hi = ($uni - 0x10000) / 0x400 + 0xD800;
+ $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
+
+and the decoding is
+
+ $uni = 0x10000 + ($hi - 0xD8000) * 0x400 + ($lo - 0xDC00);
+
+If you try to generate surrogates (for example by using chr()), you
+will get a warning if warnings are turned on (C<-w> or C<use
+warnings;>) because those code points are not valid for a Unicode
+character.
+
+Because of the 16-bitness, UTF-16 is byteorder dependent. UTF-16
+itself can be used for in-memory computations, but if storage or
+transfer is required, either UTF-16BE (Big Endian) or UTF-16LE
+(Little Endian) must be chosen.
+
+This introduces another problem: what if you just know that your data
+is UTF-16, but you don't know which endianness? Byte Order Marks
+(BOMs) are a solution to this. A special character has been reserved
+in Unicode to function as a byte order marker: the character with the
+code point 0xFEFF is the BOM.
+
+The trick is that if you read a BOM, you will know the byte order,
+since if it was written on a big endian platform, you will read the
+bytes 0xFE 0xFF, but if it was written on a little endian platform,
+you will read the bytes 0xFF 0xFE. (And if the originating platform
+was writing in UTF-8, you will read the bytes 0xEF 0xBB 0xBF.)
+
+The way this trick works is that the character with the code point
+0xFFFE is guaranteed not to be a valid Unicode character, so the
+sequence of bytes 0xFF 0xFE is unambiguously "BOM, represented in
+little-endian format" and cannot be "0xFFFE, represented in big-endian
+format".
+
+=item *
+
+UTF-32, UTF-32BE, UTF32-LE
+
+The UTF-32 family is pretty much like the UTF-16 family, expect that
+the units are 32-bit, and therefore the surrogate scheme is not
+needed. The BOM signatures will be 0x00 0x00 0xFE 0xFF for BE and
+0xFF 0xFE 0x00 0x00 for LE.
+
+=item *
+
+UCS-2, UCS-4
+
+Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
+encoding, UCS-4 is a 32-bit encoding. Unlike UTF-16, UCS-2
+is not extensible beyond 0xFFFF, because it does not use surrogates.
+
+=item *
+
+UTF-7
+
+A seven-bit safe (non-eight-bit) encoding, useful if the
+transport/storage is not eight-bit safe. Defined by RFC 2152.
+
+=back
+
+=head2 Security Implications of Malformed UTF-8
+
+Unfortunately, the specification of UTF-8 leaves some room for
+interpretation of how many bytes of encoded output one should generate
+from one input Unicode character. Strictly speaking, one is supposed
+to always generate the shortest possible sequence of UTF-8 bytes,
+because otherwise there is potential for input buffer overflow at
+the receiving end of a UTF-8 connection. Perl always generates the
+shortest length UTF-8, and with warnings on (C<-w> or C<use
+warnings;>) Perl will warn about non-shortest length UTF-8 (and other
+malformations, too, such as the surrogates, which are not real
+Unicode code points.)
+
+=head2 Unicode in Perl on EBCDIC
+
+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
+the platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic>
+for more discussion of the issues.
+
+=head2 Using Unicode in XS
+
+If you want to handle Perl Unicode in XS extensions, you may find
+the following C APIs useful (see perlapi for details):
+
+=over 4
+
+=item *
+
+DO_UTF8(sv) returns true if the UTF8 flag is on and the bytes pragma
+is not in effect. SvUTF8(sv) returns true is the UTF8 flag is on, the
+bytes pragma is ignored. The UTF8 flag being on does B<not> mean that
+there are any characters of code points greater than 255 (or 127) in
+the scalar, or that there even are any characters in the scalar.
+What the UTF8 flag means is that the sequence of octets in the
+representation of the scalar is the sequence of UTF-8 encoded
+code points of the characters of a string. The UTF8 flag being
+off means that each octet in this representation encodes a single
+character with codepoint 0..255 within the string. Perl's Unicode
+model is not to use UTF-8 until it's really necessary.
+
+=item *
+
+uvuni_to_utf8(buf, chr) writes a Unicode character code point into a
+buffer encoding the code point as UTF-8, and returns a pointer
+pointing after the UTF-8 bytes.
+
+=item *
+
+utf8_to_uvuni(buf, lenp) reads UTF-8 encoded bytes from a buffer and
+returns the Unicode character code point (and optionally the length of
+the UTF-8 byte sequence).
+
+=item *
+
+utf8_length(start, end) returns the length of the UTF-8 encoded buffer
+in characters. sv_len_utf8(sv) returns the length of the UTF-8 encoded
+scalar.
+
+=item *
+
+sv_utf8_upgrade(sv) converts the string of the scalar to its UTF-8
+encoded form. sv_utf8_downgrade(sv) does the opposite (if possible).
+sv_utf8_encode(sv) is like sv_utf8_upgrade but the UTF8 flag does not
+get turned on. sv_utf8_decode() does the opposite of sv_utf8_encode().
+Note that none of these are to be used as general purpose encoding/decoding
+interfaces: use Encode for that. sv_utf8_upgrade() is affected by the
+encoding pragma, but sv_utf8_downgrade() is not (since the encoding
+pragma is designed to be a one-way street).
+
+=item *
+
+is_utf8_char(s) returns true if the pointer points to a valid UTF-8
+character.
+
+=item *
+
+is_utf8_string(buf, len) returns true if the len bytes of the buffer
+are valid UTF-8.
+
+=item *
+
+UTF8SKIP(buf) will return the number of bytes in the UTF-8 encoded
+character in the buffer. UNISKIP(chr) will return the number of bytes
+required to UTF-8-encode the Unicode character code point. UTF8SKIP()
+is useful for example for iterating over the characters of a UTF-8
+encoded buffer; UNISKIP() is useful for example in computing
+the size required for a UTF-8 encoded buffer.
+
+=item *
+
+utf8_distance(a, b) will tell the distance in characters between the
+two pointers pointing to the same UTF-8 encoded buffer.
+
+=item *
+
+utf8_hop(s, off) will return a pointer to an UTF-8 encoded buffer that
+is C<off> (positive or negative) Unicode characters displaced from the
+UTF-8 buffer C<s>. Be careful not to overstep the buffer: utf8_hop()
+will merrily run off the end or the beginning if told to do so.
+
+=item *
+
+pv_uni_display(dsv, spv, len, pvlim, flags) and sv_uni_display(dsv,
+ssv, pvlim, flags) are useful for debug output of Unicode strings and
+scalars. By default they are useful only for debug: they display
+B<all> characters as hexadecimal code points, but with the flags
+UNI_DISPLAY_ISPRINT and UNI_DISPLAY_BACKSLASH you can make the output
+more readable.
+
+=item *
+
+ibcmp_utf8(s1, pe1, u1, l1, u1, s2, pe2, l2, u2) can be used to
+compare two strings case-insensitively in Unicode.
+(For case-sensitive comparisons you can just use memEQ() and memNE()
+as usual.)
+
+=back
-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.
+For more information, see L<perlapi>, and F<utf8.c> and F<utf8.h>
+in the Perl source code distribution.
-Whether a piece of data will be treated as "characters" or "bytes"
-by internal operations cannot be divined at the current time.
+=head1 BUGS
-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
+Use of locales with Unicode data 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.
+Some functions are slower when working on UTF-8 encoded strings than
+on byte encoded strings. All functions that need to hop over
+characters such as length(), substr() or index() can work B<much>
+faster when the underlying data are byte-encoded. Witness the
+following benchmark:
+
+ % perl -e '
+ use Benchmark;
+ use strict;
+ our $l = 10000;
+ our $u = our $b = "x" x $l;
+ substr($u,0,1) = "\x{100}";
+ timethese(-2,{
+ LENGTH_B => q{ length($b) },
+ LENGTH_U => q{ length($u) },
+ SUBSTR_B => q{ substr($b, $l/4, $l/2) },
+ SUBSTR_U => q{ substr($u, $l/4, $l/2) },
+ });
+ '
+ Benchmark: running LENGTH_B, LENGTH_U, SUBSTR_B, SUBSTR_U for at least 2 CPU seconds...
+ LENGTH_B: 2 wallclock secs ( 2.36 usr + 0.00 sys = 2.36 CPU) @ 5649983.05/s (n=13333960)
+ LENGTH_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 12155.45/s (n=25648)
+ SUBSTR_B: 3 wallclock secs ( 2.16 usr + 0.00 sys = 2.16 CPU) @ 374480.09/s (n=808877)
+ SUBSTR_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 6791.00/s (n=14329)
+
+The numbers show an incredible slowness on long UTF-8 strings and you
+should carefully avoid to use these functions within tight loops. For
+example if you want to iterate over characters, it is infinitely
+better to split into an array than to use substr, as the following
+benchmark shows:
+
+ % perl -e '
+ use Benchmark;
+ use strict;
+ our $l = 10000;
+ our $u = our $b = "x" x $l;
+ substr($u,0,1) = "\x{100}";
+ timethese(-5,{
+ SPLIT_B => q{ for my $c (split //, $b){} },
+ SPLIT_U => q{ for my $c (split //, $u){} },
+ SUBSTR_B => q{ for my $i (0..length($b)-1){my $c = substr($b,$i,1);} },
+ SUBSTR_U => q{ for my $i (0..length($u)-1){my $c = substr($u,$i,1);} },
+ });
+ '
+ Benchmark: running SPLIT_B, SPLIT_U, SUBSTR_B, SUBSTR_U for at least 5 CPU seconds...
+ SPLIT_B: 6 wallclock secs ( 5.29 usr + 0.00 sys = 5.29 CPU) @ 56.14/s (n=297)
+ SPLIT_U: 5 wallclock secs ( 5.17 usr + 0.01 sys = 5.18 CPU) @ 55.21/s (n=286)
+ SUBSTR_B: 5 wallclock secs ( 5.34 usr + 0.00 sys = 5.34 CPU) @ 123.22/s (n=658)
+ SUBSTR_U: 7 wallclock secs ( 6.20 usr + 0.00 sys = 6.20 CPU) @ 0.81/s (n=5)
+
+You see, the algorithm based on substr() was faster with byte encoded
+data but it is pathologically slow with UTF-8 data.
+
=head1 SEE ALSO
-L<byte>, L<utf8>, L<perlvar/"$^U">
+L<perluniintro>, L<encoding>, L<Encode>, L<open>, L<utf8>, L<bytes>,
+L<perlretut>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">
=cut