=head1 DESCRIPTION
-=head2 Important Caveat
+=head2 Important Caveats
-WARNING: The implementation of Unicode support in Perl is incomplete.
+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.
-The following areas need further work.
-
-=over
+=over 4
=item Input and Output Disciplines
-There is currently no easy way to mark data read from a file or other
-external source as being utf8. This will be one of the major areas of
-focus in the near future.
+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>.
=item Regular Expressions
-The existing regular expression compiler does not produce polymorphic
-opcodes. This means that the determination on whether to match Unicode
-characters is made when the pattern is compiled, based on whether the
-pattern contains Unicode characters, and not when the matching happens
-at run time. This needs to be changed to adaptively match Unicode if
-the string to be matched is Unicode.
+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 a few features
+=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.
+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>.
-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.
+You can also use the C<encoding> pragma to change the default encoding
+of the data in your script; see L<encoding>.
=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 the UTF-8 encoding.
+represent strings internally.
-In future, Perl-level operations can be expected to work with characters
-rather than bytes, in general.
+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.
+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
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 ${^WIDE_SYSTEM_CALLS}
-global flag is set to C<1>), all system calls will use the
-corresponding wide character APIs. This is currently only implemented
-on Windows.
+On Windows platforms, if the C<-C> command line switch is used, (or the
+${^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>.
+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 in literals encountered by the parser. It may also
-be used for enabling some of the more experimental Unicode support features.
+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>.
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
+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 may be 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.
+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
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
-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 the C<use warnings> pragma of the C<-w> switch
-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.)
+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>).)
+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.
+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
+Named Unicode properties and block ranges may 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}>.
+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 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 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 replaced with underbar, and the words
+"uppercase-first-lowercase-rest". That is, "Latin-1 Supplement"
+becomes "Latin_1_Supplement".
+
+You can also negate both C<\p{}> and C<\P{}> by introducing a caret
+(^) between the first curly and the property name: C<\p{^In_Tamil}> is
+equal to C<\P{In_Tamil}>.
+
+The C<In> and C<Is> can be left out: C<\p{Greek}> is equal to
+C<\p{In_Greek}>, 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 Nonspacing_Mark
+ Mc Spacing_Mark
+ Me Enclosing_Mark
+
+ N Number
+ Nd Decimal_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 Control
+ Cf Format
+ Cs Surrogate
+ Co Private_Use
+ 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>.
+
+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, because Perl
+uses UTF-8 and not UTF-16 internally to represent Unicode.
+So you really can't use the "Cs" category.)
+
+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
+
+=back
+
+=head2 Scripts
+
+The scripts available for C<\p{In...}> and C<\P{In...}>, for example
+C<\p{InLatin}> or \p{InCyrillic>, are as follows:
+
+ Arabic
+ Armenian
+ Bengali
+ Bopomofo
+ Canadian-Aboriginal
+ Cherokee
+ Cyrillic
+ Deseret
+ Devanagari
+ Ethiopic
+ Georgian
+ Gothic
+ Greek
+ Gujarati
+ Gurmukhi
+ Han
+ Hangul
+ Hebrew
+ Hiragana
+ Inherited
+ Kannada
+ Katakana
+ Khmer
+ Lao
+ Latin
+ Malayalam
+ Mongolian
+ Myanmar
+ Ogham
+ Old-Italic
+ 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
+ Bidi_Control
+ Dash
+ Diacritic
+ Extender
+ Hex_Digit
+ Hyphen
+ Ideographic
+ Join_Control
+ Noncharacter_Code_Point
+ Other_Alphabetic
+ Other_Lowercase
+ Other_Math
+ Other_Uppercase
+ Quotation_Mark
+ White_Space
+
+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
+scripts concept is closer to natural languages, while the blocks
+concept is more an artificial grouping based on groups of 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
+
+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 only the blocks were used; in Perl 5.8.0 scripts became the
+preferential Unicode character class definition (prompted by
+recommendations from the Unicode consortium); this meant that
+the definitions of some character classes changed (the ones in
+the below list that have the C<Block> appended).
+
+ Alphabetic Presentation Forms
+ Arabic Block
+ Arabic Presentation Forms-A
+ Arabic Presentation Forms-B
+ Armenian Block
+ Arrows
+ Basic Latin
+ Bengali Block
+ Block Elements
+ Bopomofo Block
+ Bopomofo Extended
+ Box Drawing
+ Braille Patterns
+ Byzantine Musical Symbols
+ CJK Compatibility
+ CJK Compatibility Forms
+ CJK Compatibility Ideographs
+ CJK Compatibility Ideographs Supplement
+ CJK Radicals Supplement
+ CJK Symbols and Punctuation
+ CJK Unified Ideographs
+ CJK Unified Ideographs Extension A
+ CJK Unified Ideographs Extension B
+ Cherokee Block
+ Combining Diacritical Marks
+ Combining Half Marks
+ Combining Marks for Symbols
+ Control Pictures
+ Currency Symbols
+ Cyrillic Block
+ Deseret Block
+ Devanagari Block
+ Dingbats
+ Enclosed Alphanumerics
+ Enclosed CJK Letters and Months
+ Ethiopic Block
+ General Punctuation
+ Geometric Shapes
+ Georgian Block
+ Gothic Block
+ Greek Block
+ Greek Extended
+ Gujarati Block
+ Gurmukhi Block
+ Halfwidth and Fullwidth Forms
+ Hangul Compatibility Jamo
+ Hangul Jamo
+ Hangul Syllables
+ Hebrew Block
+ High Private Use Surrogates
+ High Surrogates
+ Hiragana Block
+ IPA Extensions
+ Ideographic Description Characters
+ Kanbun
+ Kangxi Radicals
+ Kannada Block
+ Katakana Block
+ Khmer Block
+ Lao Block
+ Latin 1 Supplement
+ Latin Extended Additional
+ Latin Extended-A
+ Latin Extended-B
+ Letterlike Symbols
+ Low Surrogates
+ Malayalam Block
+ Mathematical Alphanumeric Symbols
+ Mathematical Operators
+ Miscellaneous Symbols
+ Miscellaneous Technical
+ Mongolian Block
+ Musical Symbols
+ Myanmar Block
+ Number Forms
+ Ogham Block
+ Old Italic Block
+ Optical Character Recognition
+ Oriya Block
+ Private Use
+ Runic Block
+ Sinhala Block
+ Small Form Variants
+ Spacing Modifier Letters
+ Specials
+ Superscripts and Subscripts
+ Syriac Block
+ Tags
+ Tamil Block
+ Telugu Block
+ Thaana Block
+ Thai Block
+ Tibetan Block
+ Unified Canadian Aboriginal Syllabics
+ Yi Radicals
+ Yi Syllables
+
+=over 4
=item *
=item *
-The C<tr///> operator translates characters instead of bytes. It can also
-be forced to translate between 8-bit codes and UTF-8. For instance, if you
-know your input in Latin-1, you can say:
-
- 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?).
+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 *
=head2 Character encodings for input and output
-[XXX: This feature is not yet implemented.]
+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. 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
+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.
+=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 - done [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 subtraction (*)
+ [ 7] include Letters in word characters
+ [ 8] some cases of "ss"/"SS" matching U+00DF in a character
+ class are missing, but that is allowed according to the TR18.
+ [ 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{GreekBlock}
+ (?=\p{ASSIGNED})\p{GreekBlock}
+
+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 3.1.
+
+ 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+FFFF E1..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
+
+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 tells 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:
+
+=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. Remember that UTF8
+flag being on does not mean that there would be any characters
+of code points greater than 255 or 127 in the scalar, or that
+there even are any characters in the scalar. The UTF8 flag
+means that any characters added to the string will be encoded
+in UTF8 if the code points of the characters are greater than
+255. Not "if greater than 127", since 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(s, len) 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().
+
+=item *
+
+is_utf8_char(buf) returns true if the buffer points to valid UTF-8.
+
+=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.
+
+=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>.
+
+=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 (only for debug: they display B<all> characters as hexadecimal
+code points).
+
+=item *
+
+ibcmp_utf8(s1, u1, len1, s2, u2, len2) 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
+
+For more information, see L<perlapi>, and F<utf8.c> and F<utf8.h>
+in the Perl source code distribution.
+
=head1 SEE ALSO
-L<bytes>, L<utf8>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">
+L<perluniintro>, L<encoding>, L<Encode>, L<open>, L<utf8>, L<bytes>,
+L<perlretut>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">
=cut
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