3 perlunicode - Unicode support in Perl
7 =head2 Important Caveats
9 Unicode support is an extensive requirement. While Perl does not
10 implement the Unicode standard or the accompanying technical reports
11 from cover to cover, Perl does support many Unicode features.
15 =item Input and Output Layers
17 Perl knows when a filehandle uses Perl's internal Unicode encodings
18 (UTF-8, or UTF-EBCDIC if in EBCDIC) if the filehandle is opened with
19 the ":utf8" layer. Other encodings can be converted to Perl's
20 encoding on input or from Perl's encoding on output by use of the
21 ":encoding(...)" layer. See L<open>.
23 To indicate that Perl source itself is using a particular encoding,
26 =item Regular Expressions
28 The regular expression compiler produces polymorphic opcodes. That is,
29 the pattern adapts to the data and automatically switches to the Unicode
30 character scheme when presented with Unicode data--or instead uses
31 a traditional byte scheme when presented with byte data.
33 =item C<use utf8> still needed to enable UTF-8/UTF-EBCDIC in scripts
35 As a compatibility measure, the C<use utf8> pragma must be explicitly
36 included to enable recognition of UTF-8 in the Perl scripts themselves
37 (in string or regular expression literals, or in identifier names) on
38 ASCII-based machines or to recognize UTF-EBCDIC on EBCDIC-based
39 machines. B<These are the only times when an explicit C<use utf8>
40 is needed.> See L<utf8>.
42 You can also use the C<encoding> pragma to change the default encoding
43 of the data in your script; see L<encoding>.
47 =head2 Byte and Character Semantics
49 Beginning with version 5.6, Perl uses logically-wide characters to
50 represent strings internally.
52 In future, Perl-level operations will be expected to work with
53 characters rather than bytes.
55 However, as an interim compatibility measure, Perl aims to
56 provide a safe migration path from byte semantics to character
57 semantics for programs. For operations where Perl can unambiguously
58 decide that the input data are characters, Perl switches to
59 character semantics. For operations where this determination cannot
60 be made without additional information from the user, Perl decides in
61 favor of compatibility and chooses to use byte semantics.
63 This behavior preserves compatibility with earlier versions of Perl,
64 which allowed byte semantics in Perl operations only if
65 none of the program's inputs were marked as being as source of Unicode
66 character data. Such data may come from filehandles, from calls to
67 external programs, from information provided by the system (such as %ENV),
68 or from literals and constants in the source text.
70 On Windows platforms, if the C<-C> command line switch is used or the
71 ${^WIDE_SYSTEM_CALLS} global flag is set to C<1>, all system calls
72 will use the corresponding wide-character APIs. This feature is
73 available only on Windows to conform to the API standard already
74 established for that platform--and there are very few non-Windows
75 platforms that have Unicode-aware APIs.
77 The C<bytes> pragma will always, regardless of platform, force byte
78 semantics in a particular lexical scope. See L<bytes>.
80 The C<utf8> pragma is primarily a compatibility device that enables
81 recognition of UTF-(8|EBCDIC) in literals encountered by the parser.
82 Note that this pragma is only required while Perl defaults to byte
83 semantics; when character semantics become the default, this pragma
84 may become a no-op. See L<utf8>.
86 Unless explicitly stated, Perl operators use character semantics
87 for Unicode data and byte semantics for non-Unicode data.
88 The decision to use character semantics is made transparently. If
89 input data comes from a Unicode source--for example, if a character
90 encoding layer is added to a filehandle or a literal Unicode
91 string constant appears in a program--character semantics apply.
92 Otherwise, byte semantics are in effect. The C<bytes> pragma should
93 be used to force byte semantics on Unicode data.
95 If strings operating under byte semantics and strings with Unicode
96 character data are concatenated, the new string will be upgraded to
97 I<ISO 8859-1 (Latin-1)>, even if the old Unicode string used EBCDIC.
98 This translation is done without regard to the system's native 8-bit
99 encoding, so to change this for systems with non-Latin-1 and
100 non-EBCDIC native encodings use the C<encoding> pragma. See
103 Under character semantics, many operations that formerly operated on
104 bytes now operate on characters. A character in Perl is
105 logically just a number ranging from 0 to 2**31 or so. Larger
106 characters may encode into longer sequences of bytes internally, but
107 this internal detail is mostly hidden for Perl code.
108 See L<perluniintro> for more.
110 =head2 Effects of Character Semantics
112 Character semantics have the following effects:
118 Strings--including hash keys--and regular expression patterns may
119 contain characters that have an ordinal value larger than 255.
121 If you use a Unicode editor to edit your program, Unicode characters
122 may occur directly within the literal strings in one of the various
123 Unicode encodings (UTF-8, UTF-EBCDIC, UCS-2, etc.), but will be recognized
124 as such and converted to Perl's internal representation only if the
125 appropriate L<encoding> is specified.
127 Unicode characters can also be added to a string by using the
128 C<\x{...}> notation. The Unicode code for the desired character, in
129 hexadecimal, should be placed in the braces. For instance, a smiley
130 face is C<\x{263A}>. This encoding scheme only works for characters
131 with a code of 0x100 or above.
135 use charnames ':full';
137 you can use the C<\N{...}> notation and put the official Unicode
138 character name within the braces, such as C<\N{WHITE SMILING FACE}>.
143 If an appropriate L<encoding> is specified, identifiers within the
144 Perl script may contain Unicode alphanumeric characters, including
145 ideographs. Perl does not currently attempt to canonicalize variable
150 Regular expressions match characters instead of bytes. "." matches
151 a character instead of a byte. The C<\C> pattern is provided to force
152 a match a single byte--a C<char> in C, hence C<\C>.
156 Character classes in regular expressions match characters instead of
157 bytes and match against the character properties specified in the
158 Unicode properties database. C<\w> can be used to match a Japanese
159 ideograph, for instance.
163 Named Unicode properties, scripts, and block ranges may be used like
164 character classes via the C<\p{}> "matches property" construct and
165 the C<\P{}> negation, "doesn't match property".
167 For instance, C<\p{Lu}> matches any character with the Unicode "Lu"
168 (Letter, uppercase) property, while C<\p{M}> matches any character
169 with an "M" (mark--accents and such) property. Brackets are not
170 required for single letter properties, so C<\p{M}> is equivalent to
171 C<\pM>. Many predefined properties are available, such as
172 C<\p{Mirrored}> and C<\p{Tibetan}>.
174 The official Unicode script and block names have spaces and dashes as
175 separators, but for convenience you can use dashes, spaces, or
176 underbars, and case is unimportant. It is recommended, however, that
177 for consistency you use the following naming: the official Unicode
178 script, property, or block name (see below for the additional rules
179 that apply to block names) with whitespace and dashes removed, and the
180 words "uppercase-first-lowercase-rest". C<Latin-1 Supplement> thus
181 becomes C<Latin1Supplement>.
183 You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret
184 (^) between the first brace and the property name: C<\p{^Tamil}> is
185 equal to C<\P{Tamil}>.
187 Here are the basic Unicode General Category properties, followed by their
188 long form. You can use either; C<\p{Lu}> and C<\p{LowercaseLetter}>,
189 for instance, are identical.
211 Pc ConnectorPunctuation
215 Pi InitialPunctuation
216 (may behave like Ps or Pe depending on usage)
218 (may behave like Ps or Pe depending on usage)
230 Zp ParagraphSeparator
235 Cs Surrogate (not usable)
239 Single-letter properties match all characters in any of the
240 two-letter sub-properties starting with the same letter.
241 C<L&> is a special case, which is an alias for C<Ll>, C<Lu>, and C<Lt>.
243 Because Perl hides the need for the user to understand the internal
244 representation of Unicode characters, there is no need to implement
245 the somewhat messy concept of surrogates. C<Cs> is therefore not
248 Because scripts differ in their directionality--Hebrew is
249 written right to left, for example--Unicode supplies these properties:
254 BidiLRE Left-to-Right Embedding
255 BidiLRO Left-to-Right Override
257 BidiAL Right-to-Left Arabic
258 BidiRLE Right-to-Left Embedding
259 BidiRLO Right-to-Left Override
260 BidiPDF Pop Directional Format
261 BidiEN European Number
262 BidiES European Number Separator
263 BidiET European Number Terminator
265 BidiCS Common Number Separator
266 BidiNSM Non-Spacing Mark
267 BidiBN Boundary Neutral
268 BidiB Paragraph Separator
269 BidiS Segment Separator
271 BidiON Other Neutrals
273 For example, C<\p{BidiR}> matches characters that are normally
274 written right to left.
280 The script names which can be used by C<\p{...}> and C<\P{...}>,
281 such as in C<\p{Latin}> or C<\p{Cyrillic}>, are as follows:
328 Extended property classes can supplement the basic
329 properties, defined by the F<PropList> Unicode database:
344 LogicalOrderException
345 NoncharacterCodePoint
347 OtherDefaultIgnorableCodePoint
359 and there are further derived properties:
361 Alphabetic Lu + Ll + Lt + Lm + Lo + OtherAlphabetic
362 Lowercase Ll + OtherLowercase
363 Uppercase Lu + OtherUppercase
366 ID_Start Lu + Ll + Lt + Lm + Lo + Nl
367 ID_Continue ID_Start + Mn + Mc + Nd + Pc
370 Assigned Any non-Cn character (i.e. synonym for \P{Cn})
371 Unassigned Synonym for \p{Cn}
372 Common Any character (or unassigned code point)
373 not explicitly assigned to a script
375 For backward compatibility (with Perl 5.6), all properties mentioned
376 so far may have C<Is> prepended to their name, so C<\P{IsLu}>, for
377 example, is equal to C<\P{Lu}>.
381 In addition to B<scripts>, Unicode also defines B<blocks> of
382 characters. The difference between scripts and blocks is that the
383 concept of scripts is closer to natural languages, while the concept
384 of blocks is more of an artificial grouping based on groups of 256
385 Unicode characters. For example, the C<Latin> script contains letters
386 from many blocks but does not contain all the characters from those
387 blocks. It does not, for example, contain digits, because digits are
388 shared across many scripts. Digits and similar groups, like
389 punctuation, are in a category called C<Common>.
391 For more about scripts, see the UTR #24:
393 http://www.unicode.org/unicode/reports/tr24/
395 For more about blocks, see:
397 http://www.unicode.org/Public/UNIDATA/Blocks.txt
399 Block names are given with the C<In> prefix. For example, the
400 Katakana block is referenced via C<\p{InKatakana}>. The C<In>
401 prefix may be omitted if there is no naming conflict with a script
402 or any other property, but it is recommended that C<In> always be used
403 for block tests to avoid confusion.
405 These block names are supported:
407 InAlphabeticPresentationForms
409 InArabicPresentationFormsA
410 InArabicPresentationFormsB
421 InByzantineMusicalSymbols
423 InCJKCompatibilityForms
424 InCJKCompatibilityIdeographs
425 InCJKCompatibilityIdeographsSupplement
426 InCJKRadicalsSupplement
427 InCJKSymbolsAndPunctuation
428 InCJKUnifiedIdeographs
429 InCJKUnifiedIdeographsExtensionA
430 InCJKUnifiedIdeographsExtensionB
432 InCombiningDiacriticalMarks
433 InCombiningDiacriticalMarksforSymbols
438 InCyrillicSupplementary
442 InEnclosedAlphanumerics
443 InEnclosedCJKLettersAndMonths
453 InHalfwidthAndFullwidthForms
454 InHangulCompatibilityJamo
459 InHighPrivateUseSurrogates
463 InIdeographicDescriptionCharacters
468 InKatakanaPhoneticExtensions
473 InLatinExtendedAdditional
478 InMathematicalAlphanumericSymbols
479 InMathematicalOperators
480 InMiscellaneousMathematicalSymbolsA
481 InMiscellaneousMathematicalSymbolsB
482 InMiscellaneousSymbols
483 InMiscellaneousTechnical
490 InOpticalCharacterRecognition
496 InSpacingModifierLetters
498 InSuperscriptsAndSubscripts
499 InSupplementalArrowsA
500 InSupplementalArrowsB
501 InSupplementalMathematicalOperators
502 InSupplementaryPrivateUseAreaA
503 InSupplementaryPrivateUseAreaB
513 InUnifiedCanadianAboriginalSyllabics
522 The special pattern C<\X> matches any extended Unicode
523 sequence--"a combining character sequence" in Standardese--where the
524 first character is a base character and subsequent characters are mark
525 characters that apply to the base character. C<\X> is equivalent to
530 The C<tr///> operator translates characters instead of bytes. Note
531 that the C<tr///CU> functionality has been removed. For similar
532 functionality see pack('U0', ...) and pack('C0', ...).
536 Case translation operators use the Unicode case translation tables
537 when character input is provided. Note that C<uc()>, or C<\U> in
538 interpolated strings, translates to uppercase, while C<ucfirst>,
539 or C<\u> in interpolated strings, translates to titlecase in languages
540 that make the distinction.
544 Most operators that deal with positions or lengths in a string will
545 automatically switch to using character positions, including
546 C<chop()>, C<substr()>, C<pos()>, C<index()>, C<rindex()>,
547 C<sprintf()>, C<write()>, and C<length()>. Operators that
548 specifically do not switch include C<vec()>, C<pack()>, and
549 C<unpack()>. Operators that really don't care include C<chomp()>,
550 operators that treats strings as a bucket of bits such as C<sort()>,
551 and operators dealing with filenames.
555 The C<pack()>/C<unpack()> letters C<c> and C<C> do I<not> change,
556 since they are often used for byte-oriented formats. Again, think
557 C<char> in the C language.
559 There is a new C<U> specifier that converts between Unicode characters
564 The C<chr()> and C<ord()> functions work on characters, similar to
565 C<pack("U")> and C<unpack("U")>, I<not> C<pack("C")> and
566 C<unpack("C")>. C<pack("C")> and C<unpack("C")> are methods for
567 emulating byte-oriented C<chr()> and C<ord()> on Unicode strings.
568 While these methods reveal the internal encoding of Unicode strings,
569 that is not something one normally needs to care about at all.
573 The bit string operators, C<& | ^ ~>, can operate on character data.
574 However, for backward compatibility, such as when using bit string
575 operations when characters are all less than 256 in ordinal value, one
576 should not use C<~> (the bit complement) with characters of both
577 values less than 256 and values greater than 256. Most importantly,
578 DeMorgan's laws (C<~($x|$y) eq ~$x&~$y> and C<~($x&$y) eq ~$x|~$y>)
579 will not hold. The reason for this mathematical I<faux pas> is that
580 the complement cannot return B<both> the 8-bit (byte-wide) bit
581 complement B<and> the full character-wide bit complement.
585 lc(), uc(), lcfirst(), and ucfirst() work for the following cases:
591 the case mapping is from a single Unicode character to another
592 single Unicode character, or
596 the case mapping is from a single Unicode character to more
597 than one Unicode character.
601 Things to do with locales (Lithuanian, Turkish, Azeri) do B<not> work
602 since Perl does not understand the concept of Unicode locales.
604 See the Unicode Technical Report #21, Case Mappings, for more details.
612 And finally, C<scalar reverse()> reverses by character rather than by byte.
616 =head2 User-Defined Character Properties
618 You can define your own character properties by defining subroutines
619 whose names begin with "In" or "Is". The subroutines must be
620 visible in the package that uses the properties. The user-defined
621 properties can be used in the regular expression C<\p> and C<\P>
624 The subroutines must return a specially-formatted string, with one
625 or more newline-separated lines. Each line must be one of the following:
631 Two hexadecimal numbers separated by horizontal whitespace (space or
632 tabular characters) denoting a range of Unicode code points to include.
636 Something to include, prefixed by "+": a built-in character
637 property (prefixed by "utf8::"), to represent all the characters in that
638 property; two hexadecimal code points for a range; or a single
639 hexadecimal code point.
643 Something to exclude, prefixed by "-": an existing character
644 property (prefixed by "utf8::"), for all the characters in that
645 property; two hexadecimal code points for a range; or a single
646 hexadecimal code point.
650 Something to negate, prefixed "!": an existing character
651 property (prefixed by "utf8::") for all the characters except the
652 characters in the property; two hexadecimal code points for a range;
653 or a single hexadecimal code point.
657 For example, to define a property that covers both the Japanese
658 syllabaries (hiragana and katakana), you can define
667 Imagine that the here-doc end marker is at the beginning of the line.
668 Now you can use C<\p{InKana}> and C<\P{InKana}>.
670 You could also have used the existing block property names:
679 Suppose you wanted to match only the allocated characters,
680 not the raw block ranges: in other words, you want to remove
691 The negation is useful for defining (surprise!) negated classes.
701 =head2 Character Encodings for Input and Output
705 =head2 Unicode Regular Expression Support Level
707 The following list of Unicode support for regular expressions describes
708 all the features currently supported. The references to "Level N"
709 and the section numbers refer to the Unicode Technical Report 18,
710 "Unicode Regular Expression Guidelines".
716 Level 1 - Basic Unicode Support
718 2.1 Hex Notation - done [1]
719 Named Notation - done [2]
720 2.2 Categories - done [3][4]
721 2.3 Subtraction - MISSING [5][6]
722 2.4 Simple Word Boundaries - done [7]
723 2.5 Simple Loose Matches - done [8]
724 2.6 End of Line - MISSING [9][10]
728 [ 3] . \p{...} \P{...}
729 [ 4] now scripts (see UTR#24 Script Names) in addition to blocks
731 [ 6] can use regular expression look-ahead [a]
732 or user-defined character properties [b] to emulate subtraction
733 [ 7] include Letters in word characters
734 [ 8] note that Perl does Full case-folding in matching, not Simple:
735 for example U+1F88 is equivalent with U+1F00 U+03B9,
736 not with 1F80. This difference matters for certain Greek
737 capital letters with certain modifiers: the Full case-folding
738 decomposes the letter, while the Simple case-folding would map
739 it to a single character.
740 [ 9] see UTR#13 Unicode Newline Guidelines
741 [10] should do ^ and $ also on \x{85}, \x{2028} and \x{2029}
742 (should also affect <>, $., and script line numbers)
743 (the \x{85}, \x{2028} and \x{2029} do match \s)
745 [a] You can mimic class subtraction using lookahead.
746 For example, what TR18 might write as
748 [{Greek}-[{UNASSIGNED}]]
750 in Perl can be written as:
752 (?!\p{Unassigned})\p{InGreekAndCoptic}
753 (?=\p{Assigned})\p{InGreekAndCoptic}
755 But in this particular example, you probably really want
759 which will match assigned characters known to be part of the Greek script.
761 [b] See L</"User-Defined Character Properties">.
765 Level 2 - Extended Unicode Support
767 3.1 Surrogates - MISSING [11]
768 3.2 Canonical Equivalents - MISSING [12][13]
769 3.3 Locale-Independent Graphemes - MISSING [14]
770 3.4 Locale-Independent Words - MISSING [15]
771 3.5 Locale-Independent Loose Matches - MISSING [16]
773 [11] Surrogates are solely a UTF-16 concept and Perl's internal
774 representation is UTF-8. The Encode module does UTF-16, though.
775 [12] see UTR#15 Unicode Normalization
776 [13] have Unicode::Normalize but not integrated to regexes
777 [14] have \X but at this level . should equal that
778 [15] need three classes, not just \w and \W
779 [16] see UTR#21 Case Mappings
783 Level 3 - Locale-Sensitive Support
785 4.1 Locale-Dependent Categories - MISSING
786 4.2 Locale-Dependent Graphemes - MISSING [16][17]
787 4.3 Locale-Dependent Words - MISSING
788 4.4 Locale-Dependent Loose Matches - MISSING
789 4.5 Locale-Dependent Ranges - MISSING
791 [16] see UTR#10 Unicode Collation Algorithms
792 [17] have Unicode::Collate but not integrated to regexes
796 =head2 Unicode Encodings
798 Unicode characters are assigned to I<code points>, which are abstract
799 numbers. To use these numbers, various encodings are needed.
807 UTF-8 is a variable-length (1 to 6 bytes, current character allocations
808 require 4 bytes), byte-order independent encoding. For ASCII (and we
809 really do mean 7-bit ASCII, not another 8-bit encoding), UTF-8 is
812 The following table is from Unicode 3.2.
814 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
816 U+0000..U+007F 00..7F
817 U+0080..U+07FF C2..DF 80..BF
818 U+0800..U+0FFF E0 A0..BF 80..BF
819 U+1000..U+CFFF E1..EC 80..BF 80..BF
820 U+D000..U+D7FF ED 80..9F 80..BF
821 U+D800..U+DFFF ******* ill-formed *******
822 U+E000..U+FFFF EE..EF 80..BF 80..BF
823 U+10000..U+3FFFF F0 90..BF 80..BF 80..BF
824 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
825 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
827 Note the C<A0..BF> in C<U+0800..U+0FFF>, the C<80..9F> in
828 C<U+D000...U+D7FF>, the C<90..B>F in C<U+10000..U+3FFFF>, and the
829 C<80...8F> in C<U+100000..U+10FFFF>. The "gaps" are caused by legal
830 UTF-8 avoiding non-shortest encodings: it is technically possible to
831 UTF-8-encode a single code point in different ways, but that is
832 explicitly forbidden, and the shortest possible encoding should always
833 be used. So that's what Perl does.
835 Another way to look at it is via bits:
837 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
840 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
841 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
842 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
844 As you can see, the continuation bytes all begin with C<10>, and the
845 leading bits of the start byte tell how many bytes the are in the
852 Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
856 UTF-16, UTF-16BE, UTF16-LE, Surrogates, and BOMs (Byte Order Marks)
858 The followings items are mostly for reference and general Unicode
859 knowledge, Perl doesn't use these constructs internally.
861 UTF-16 is a 2 or 4 byte encoding. The Unicode code points
862 C<U+0000..U+FFFF> are stored in a single 16-bit unit, and the code
863 points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is
864 using I<surrogates>, the first 16-bit unit being the I<high
865 surrogate>, and the second being the I<low surrogate>.
867 Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
868 range of Unicode code points in pairs of 16-bit units. The I<high
869 surrogates> are the range C<U+D800..U+DBFF>, and the I<low surrogates>
870 are the range C<U+DC00..U+DFFF>. The surrogate encoding is
872 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
873 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
877 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
879 If you try to generate surrogates (for example by using chr()), you
880 will get a warning if warnings are turned on, because those code
881 points are not valid for a Unicode character.
883 Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
884 itself can be used for in-memory computations, but if storage or
885 transfer is required either UTF-16BE (big-endian) or UTF-16LE
886 (little-endian) encodings must be chosen.
888 This introduces another problem: what if you just know that your data
889 is UTF-16, but you don't know which endianness? Byte Order Marks, or
890 BOMs, are a solution to this. A special character has been reserved
891 in Unicode to function as a byte order marker: the character with the
892 code point C<U+FEFF> is the BOM.
894 The trick is that if you read a BOM, you will know the byte order,
895 since if it was written on a big-endian platform, you will read the
896 bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
897 you will read the bytes C<0xFF 0xFE>. (And if the originating platform
898 was writing in UTF-8, you will read the bytes C<0xEF 0xBB 0xBF>.)
900 The way this trick works is that the character with the code point
901 C<U+FFFE> is guaranteed not to be a valid Unicode character, so the
902 sequence of bytes C<0xFF 0xFE> is unambiguously "BOM, represented in
903 little-endian format" and cannot be C<U+FFFE>, represented in big-endian
908 UTF-32, UTF-32BE, UTF32-LE
910 The UTF-32 family is pretty much like the UTF-16 family, expect that
911 the units are 32-bit, and therefore the surrogate scheme is not
912 needed. The BOM signatures will be C<0x00 0x00 0xFE 0xFF> for BE and
913 C<0xFF 0xFE 0x00 0x00> for LE.
919 Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
920 encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
921 because it does not use surrogates. UCS-4 is a 32-bit encoding,
922 functionally identical to UTF-32.
928 A seven-bit safe (non-eight-bit) encoding, which is useful if the
929 transport or storage is not eight-bit safe. Defined by RFC 2152.
933 =head2 Security Implications of Unicode
941 Unfortunately, the specification of UTF-8 leaves some room for
942 interpretation of how many bytes of encoded output one should generate
943 from one input Unicode character. Strictly speaking, the shortest
944 possible sequence of UTF-8 bytes should be generated,
945 because otherwise there is potential for an input buffer overflow at
946 the receiving end of a UTF-8 connection. Perl always generates the
947 shortest length UTF-8, and with warnings on Perl will warn about
948 non-shortest length UTF-8 along with other malformations, such as the
949 surrogates, which are not real Unicode code points.
953 Regular expressions behave slightly differently between byte data and
954 character (Unicode) data. For example, the "word character" character
955 class C<\w> will work differently depending on if data is eight-bit bytes
958 In the first case, the set of C<\w> characters is either small--the
959 default set of alphabetic characters, digits, and the "_"--or, if you
960 are using a locale (see L<perllocale>), the C<\w> might contain a few
961 more letters according to your language and country.
963 In the second case, the C<\w> set of characters is much, much larger.
964 Most importantly, even in the set of the first 256 characters, it will
965 probably match different characters: unlike most locales, which are
966 specific to a language and country pair, Unicode classifies all the
967 characters that are letters I<somewhere> as C<\w>. For example, your
968 locale might not think that LATIN SMALL LETTER ETH is a letter (unless
969 you happen to speak Icelandic), but Unicode does.
971 As discussed elsewhere, Perl has one foot (two hooves?) planted in
972 each of two worlds: the old world of bytes and the new world of
973 characters, upgrading from bytes to characters when necessary.
974 If your legacy code does not explicitly use Unicode, no automatic
975 switch-over to characters should happen. Characters shouldn't get
976 downgraded to bytes, either. It is possible to accidentally mix bytes
977 and characters, however (see L<perluniintro>), in which case C<\w> in
978 regular expressions might start behaving differently. Review your
979 code. Use warnings and the C<strict> pragma.
983 =head2 Unicode in Perl on EBCDIC
985 The way Unicode is handled on EBCDIC platforms is still
986 experimental. On such platforms, references to UTF-8 encoding in this
987 document and elsewhere should be read as meaning the UTF-EBCDIC
988 specified in Unicode Technical Report 16, unless ASCII vs. EBCDIC issues
989 are specifically discussed. There is no C<utfebcdic> pragma or
990 ":utfebcdic" layer; rather, "utf8" and ":utf8" are reused to mean
991 the platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic>
992 for more discussion of the issues.
996 Usually locale settings and Unicode do not affect each other, but
997 there are a couple of exceptions:
1003 If your locale environment variables (LANGUAGE, LC_ALL, LC_CTYPE, LANG)
1004 contain the strings 'UTF-8' or 'UTF8' (case-insensitive matching),
1005 the default encodings of your STDIN, STDOUT, and STDERR, and of
1006 B<any subsequent file open>, are considered to be UTF-8.
1010 Perl tries really hard to work both with Unicode and the old
1011 byte-oriented world. Most often this is nice, but sometimes Perl's
1012 straddling of the proverbial fence causes problems.
1016 =head2 Using Unicode in XS
1018 If you want to handle Perl Unicode in XS extensions, you may find
1019 the following C APIs useful. See L<perlapi> for details.
1025 C<DO_UTF8(sv)> returns true if the C<UTF8> flag is on and the bytes
1026 pragma is not in effect. C<SvUTF8(sv)> returns true is the C<UTF8>
1027 flag is on; the bytes pragma is ignored. The C<UTF8> flag being on
1028 does B<not> mean that there are any characters of code points greater
1029 than 255 (or 127) in the scalar or that there are even any characters
1030 in the scalar. What the C<UTF8> flag means is that the sequence of
1031 octets in the representation of the scalar is the sequence of UTF-8
1032 encoded code points of the characters of a string. The C<UTF8> flag
1033 being off means that each octet in this representation encodes a
1034 single character with code point 0..255 within the string. Perl's
1035 Unicode model is not to use UTF-8 until it is absolutely necessary.
1039 C<uvuni_to_utf8(buf, chr>) writes a Unicode character code point into
1040 a buffer encoding the code point as UTF-8, and returns a pointer
1041 pointing after the UTF-8 bytes.
1045 C<utf8_to_uvuni(buf, lenp)> reads UTF-8 encoded bytes from a buffer and
1046 returns the Unicode character code point and, optionally, the length of
1047 the UTF-8 byte sequence.
1051 C<utf8_length(start, end)> returns the length of the UTF-8 encoded buffer
1052 in characters. C<sv_len_utf8(sv)> returns the length of the UTF-8 encoded
1057 C<sv_utf8_upgrade(sv)> converts the string of the scalar to its UTF-8
1058 encoded form. C<sv_utf8_downgrade(sv)> does the opposite, if
1059 possible. C<sv_utf8_encode(sv)> is like sv_utf8_upgrade except that
1060 it does not set the C<UTF8> flag. C<sv_utf8_decode()> does the
1061 opposite of C<sv_utf8_encode()>. Note that none of these are to be
1062 used as general-purpose encoding or decoding interfaces: C<use Encode>
1063 for that. C<sv_utf8_upgrade()> is affected by the encoding pragma
1064 but C<sv_utf8_downgrade()> is not (since the encoding pragma is
1065 designed to be a one-way street).
1069 C<is_utf8_char(s)> returns true if the pointer points to a valid UTF-8
1074 C<is_utf8_string(buf, len)> returns true if C<len> bytes of the buffer
1079 C<UTF8SKIP(buf)> will return the number of bytes in the UTF-8 encoded
1080 character in the buffer. C<UNISKIP(chr)> will return the number of bytes
1081 required to UTF-8-encode the Unicode character code point. C<UTF8SKIP()>
1082 is useful for example for iterating over the characters of a UTF-8
1083 encoded buffer; C<UNISKIP()> is useful, for example, in computing
1084 the size required for a UTF-8 encoded buffer.
1088 C<utf8_distance(a, b)> will tell the distance in characters between the
1089 two pointers pointing to the same UTF-8 encoded buffer.
1093 C<utf8_hop(s, off)> will return a pointer to an UTF-8 encoded buffer
1094 that is C<off> (positive or negative) Unicode characters displaced
1095 from the UTF-8 buffer C<s>. Be careful not to overstep the buffer:
1096 C<utf8_hop()> will merrily run off the end or the beginning of the
1097 buffer if told to do so.
1101 C<pv_uni_display(dsv, spv, len, pvlim, flags)> and
1102 C<sv_uni_display(dsv, ssv, pvlim, flags)> are useful for debugging the
1103 output of Unicode strings and scalars. By default they are useful
1104 only for debugging--they display B<all> characters as hexadecimal code
1105 points--but with the flags C<UNI_DISPLAY_ISPRINT>,
1106 C<UNI_DISPLAY_BACKSLASH>, and C<UNI_DISPLAY_QQ> you can make the
1107 output more readable.
1111 C<ibcmp_utf8(s1, pe1, u1, l1, u1, s2, pe2, l2, u2)> can be used to
1112 compare two strings case-insensitively in Unicode. For case-sensitive
1113 comparisons you can just use C<memEQ()> and C<memNE()> as usual.
1117 For more information, see L<perlapi>, and F<utf8.c> and F<utf8.h>
1118 in the Perl source code distribution.
1122 =head2 Interaction with Locales
1124 Use of locales with Unicode data may lead to odd results. Currently,
1125 Perl attempts to attach 8-bit locale info to characters in the range
1126 0..255, but this technique is demonstrably incorrect for locales that
1127 use characters above that range when mapped into Unicode. Perl's
1128 Unicode support will also tend to run slower. Use of locales with
1129 Unicode is discouraged.
1131 =head2 Interaction with Extensions
1133 When Perl exchanges data with an extension, the extension should be
1134 able to understand the UTF-8 flag and act accordingly. If the
1135 extension doesn't know about the flag, it's likely that the extension
1136 will return incorrectly-flagged data.
1138 So if you're working with Unicode data, consult the documentation of
1139 every module you're using if there are any issues with Unicode data
1140 exchange. If the documentation does not talk about Unicode at all,
1141 suspect the worst and probably look at the source to learn how the
1142 module is implemented. Modules written completely in Perl shouldn't
1143 cause problems. Modules that directly or indirectly access code written
1144 in other programming languages are at risk.
1146 For affected functions, the simple strategy to avoid data corruption is
1147 to always make the encoding of the exchanged data explicit. Choose an
1148 encoding that you know the extension can handle. Convert arguments passed
1149 to the extensions to that encoding and convert results back from that
1150 encoding. Write wrapper functions that do the conversions for you, so
1151 you can later change the functions when the extension catches up.
1153 To provide an example, let's say the popular Foo::Bar::escape_html
1154 function doesn't deal with Unicode data yet. The wrapper function
1155 would convert the argument to raw UTF-8 and convert the result back to
1156 Perl's internal representation like so:
1158 sub my_escape_html ($) {
1160 return unless defined $what;
1161 Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
1164 Sometimes, when the extension does not convert data but just stores
1165 and retrieves them, you will be in a position to use the otherwise
1166 dangerous Encode::_utf8_on() function. Let's say the popular
1167 C<Foo::Bar> extension, written in C, provides a C<param> method that
1168 lets you store and retrieve data according to these prototypes:
1170 $self->param($name, $value); # set a scalar
1171 $value = $self->param($name); # retrieve a scalar
1173 If it does not yet provide support for any encoding, one could write a
1174 derived class with such a C<param> method:
1177 my($self,$name,$value) = @_;
1178 utf8::upgrade($name); # make sure it is UTF-8 encoded
1180 utf8::upgrade($value); # make sure it is UTF-8 encoded
1181 return $self->SUPER::param($name,$value);
1183 my $ret = $self->SUPER::param($name);
1184 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
1189 Some extensions provide filters on data entry/exit points, such as
1190 DB_File::filter_store_key and family. Look out for such filters in
1191 the documentation of your extensions, they can make the transition to
1192 Unicode data much easier.
1196 Some functions are slower when working on UTF-8 encoded strings than
1197 on byte encoded strings. All functions that need to hop over
1198 characters such as length(), substr() or index() can work B<much>
1199 faster when the underlying data are byte-encoded. Witness the
1200 following benchmark:
1206 our $u = our $b = "x" x $l;
1207 substr($u,0,1) = "\x{100}";
1209 LENGTH_B => q{ length($b) },
1210 LENGTH_U => q{ length($u) },
1211 SUBSTR_B => q{ substr($b, $l/4, $l/2) },
1212 SUBSTR_U => q{ substr($u, $l/4, $l/2) },
1215 Benchmark: running LENGTH_B, LENGTH_U, SUBSTR_B, SUBSTR_U for at least 2 CPU seconds...
1216 LENGTH_B: 2 wallclock secs ( 2.36 usr + 0.00 sys = 2.36 CPU) @ 5649983.05/s (n=13333960)
1217 LENGTH_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 12155.45/s (n=25648)
1218 SUBSTR_B: 3 wallclock secs ( 2.16 usr + 0.00 sys = 2.16 CPU) @ 374480.09/s (n=808877)
1219 SUBSTR_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 6791.00/s (n=14329)
1221 The numbers show an incredible slowness on long UTF-8 strings. You
1222 should carefully avoid using these functions in tight loops. If you
1223 want to iterate over characters, the superior coding technique would
1224 split the characters into an array instead of using substr, as the following
1231 our $u = our $b = "x" x $l;
1232 substr($u,0,1) = "\x{100}";
1234 SPLIT_B => q{ for my $c (split //, $b){} },
1235 SPLIT_U => q{ for my $c (split //, $u){} },
1236 SUBSTR_B => q{ for my $i (0..length($b)-1){my $c = substr($b,$i,1);} },
1237 SUBSTR_U => q{ for my $i (0..length($u)-1){my $c = substr($u,$i,1);} },
1240 Benchmark: running SPLIT_B, SPLIT_U, SUBSTR_B, SUBSTR_U for at least 5 CPU seconds...
1241 SPLIT_B: 6 wallclock secs ( 5.29 usr + 0.00 sys = 5.29 CPU) @ 56.14/s (n=297)
1242 SPLIT_U: 5 wallclock secs ( 5.17 usr + 0.01 sys = 5.18 CPU) @ 55.21/s (n=286)
1243 SUBSTR_B: 5 wallclock secs ( 5.34 usr + 0.00 sys = 5.34 CPU) @ 123.22/s (n=658)
1244 SUBSTR_U: 7 wallclock secs ( 6.20 usr + 0.00 sys = 6.20 CPU) @ 0.81/s (n=5)
1246 Even though the algorithm based on C<substr()> is faster than
1247 C<split()> for byte-encoded data, it pales in comparison to the speed
1248 of C<split()> when used with UTF-8 data.
1250 =head2 Porting code from perl-5.6.X
1252 Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer
1253 was required to use the C<utf8> pragma to declare that a given scope
1254 expected to deal with Unicode data and had to make sure that only
1255 Unicode data were reaching that scope. If you have code that is
1256 working with 5.6, you will need some of the following adjustments to
1257 your code. The examples are written such that the code will continue
1258 to work under 5.6, so you should be safe to try them out.
1264 A filehandle that should read or write UTF-8
1267 binmode $fh, ":utf8";
1272 A scalar that is going to be passed to some extension
1274 Be it Compress::Zlib, Apache::Request or any extension that has no
1275 mention of Unicode in the manpage, you need to make sure that the
1276 UTF-8 flag is stripped off. Note that at the time of this writing
1277 (October 2002) the mentioned modules are not UTF-8-aware. Please
1278 check the documentation to verify if this is still true.
1282 $val = Encode::encode_utf8($val); # make octets
1287 A scalar we got back from an extension
1289 If you believe the scalar comes back as UTF-8, you will most likely
1290 want the UTF-8 flag restored:
1294 $val = Encode::decode_utf8($val);
1299 Same thing, if you are really sure it is UTF-8
1303 Encode::_utf8_on($val);
1308 A wrapper for fetchrow_array and fetchrow_hashref
1310 When the database contains only UTF-8, a wrapper function or method is
1311 a convenient way to replace all your fetchrow_array and
1312 fetchrow_hashref calls. A wrapper function will also make it easier to
1313 adapt to future enhancements in your database driver. Note that at the
1314 time of this writing (October 2002), the DBI has no standardized way
1315 to deal with UTF-8 data. Please check the documentation to verify if
1319 my($self, $sth, $what) = @_; # $what is one of fetchrow_{array,hashref}
1325 my @arr = $sth->$what;
1327 defined && /[^\000-\177]/ && Encode::_utf8_on($_);
1331 my $ret = $sth->$what;
1333 for my $k (keys %$ret) {
1334 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret->{$k};
1338 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
1348 A large scalar that you know can only contain ASCII
1350 Scalars that contain only ASCII and are marked as UTF-8 are sometimes
1351 a drag to your program. If you recognize such a situation, just remove
1354 utf8::downgrade($val) if $] > 5.007;
1360 L<perluniintro>, L<encoding>, L<Encode>, L<open>, L<utf8>, L<bytes>,
1361 L<perlretut>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">
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