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 Disciplines
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>
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 discipline 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 The following cases do not yet work:
607 the "final sigma" (Greek), and
611 anything to with locales (Lithuanian, Turkish, Azeri).
615 See the Unicode Technical Report #21, Case Mappings, for more details.
619 And finally, C<scalar reverse()> reverses by character rather than by byte.
623 =head2 User-Defined Character Properties
625 You can define your own character properties by defining subroutines
626 whose names begin with "In" or "Is". The subroutines must be
627 visible in the package that uses the properties. The user-defined
628 properties can be used in the regular expression C<\p> and C<\P>
631 The subroutines must return a specially-formatted string, with one
632 or more newline-separated lines. Each line must be one of the following:
638 Two hexadecimal numbers separated by horizontal whitespace (space or
639 tabular characters) denoting a range of Unicode code points to include.
643 Something to include, prefixed by "+": a built-in character
644 property (prefixed by "utf8::"), to represent all the characters in that
645 property; two hexadecimal code points for a range; or a single
646 hexadecimal code point.
650 Something to exclude, prefixed by "-": an existing character
651 property (prefixed by "utf8::"), for all the characters in that
652 property; two hexadecimal code points for a range; or a single
653 hexadecimal code point.
657 Something to negate, prefixed "!": an existing character
658 property (prefixed by "utf8::") for all the characters except the
659 characters in the property; two hexadecimal code points for a range;
660 or a single hexadecimal code point.
664 For example, to define a property that covers both the Japanese
665 syllabaries (hiragana and katakana), you can define
674 Imagine that the here-doc end marker is at the beginning of the line.
675 Now you can use C<\p{InKana}> and C<\P{InKana}>.
677 You could also have used the existing block property names:
686 Suppose you wanted to match only the allocated characters,
687 not the raw block ranges: in other words, you want to remove
698 The negation is useful for defining (surprise!) negated classes.
708 =head2 Character Encodings for Input and Output
712 =head2 Unicode Regular Expression Support Level
714 The following list of Unicode support for regular expressions describes
715 all the features currently supported. The references to "Level N"
716 and the section numbers refer to the Unicode Technical Report 18,
717 "Unicode Regular Expression Guidelines".
723 Level 1 - Basic Unicode Support
725 2.1 Hex Notation - done [1]
726 Named Notation - done [2]
727 2.2 Categories - done [3][4]
728 2.3 Subtraction - MISSING [5][6]
729 2.4 Simple Word Boundaries - done [7]
730 2.5 Simple Loose Matches - done [8]
731 2.6 End of Line - MISSING [9][10]
735 [ 3] . \p{...} \P{...}
736 [ 4] now scripts (see UTR#24 Script Names) in addition to blocks
738 [ 6] can use regular expression look-ahead [a]
739 or user-defined character properties [b] to emulate subtraction
740 [ 7] include Letters in word characters
741 [ 8] note that Perl does Full case-folding in matching, not Simple:
742 for example U+1F88 is equivalent with U+1F000 U+03B9,
743 not with 1F80. This difference matters for certain Greek
744 capital letters with certain modifiers: the Full case-folding
745 decomposes the letter, while the Simple case-folding would map
746 it to a single character.
747 [ 9] see UTR#13 Unicode Newline Guidelines
748 [10] should do ^ and $ also on \x{85}, \x{2028} and \x{2029})
749 (should also affect <>, $., and script line numbers)
750 (the \x{85}, \x{2028} and \x{2029} do match \s)
752 [a] You can mimic class subtraction using lookahead.
753 For example, what TR18 might write as
755 [{Greek}-[{UNASSIGNED}]]
757 in Perl can be written as:
759 (?!\p{Unassigned})\p{InGreekAndCoptic}
760 (?=\p{Assigned})\p{InGreekAndCoptic}
762 But in this particular example, you probably really want
766 which will match assigned characters known to be part of the Greek script.
768 [b] See L</"User-Defined Character Properties">.
772 Level 2 - Extended Unicode Support
774 3.1 Surrogates - MISSING
775 3.2 Canonical Equivalents - MISSING [11][12]
776 3.3 Locale-Independent Graphemes - MISSING [13]
777 3.4 Locale-Independent Words - MISSING [14]
778 3.5 Locale-Independent Loose Matches - MISSING [15]
780 [11] see UTR#15 Unicode Normalization
781 [12] have Unicode::Normalize but not integrated to regexes
782 [13] have \X but at this level . should equal that
783 [14] need three classes, not just \w and \W
784 [15] see UTR#21 Case Mappings
788 Level 3 - Locale-Sensitive Support
790 4.1 Locale-Dependent Categories - MISSING
791 4.2 Locale-Dependent Graphemes - MISSING [16][17]
792 4.3 Locale-Dependent Words - MISSING
793 4.4 Locale-Dependent Loose Matches - MISSING
794 4.5 Locale-Dependent Ranges - MISSING
796 [16] see UTR#10 Unicode Collation Algorithms
797 [17] have Unicode::Collate but not integrated to regexes
801 =head2 Unicode Encodings
803 Unicode characters are assigned to I<code points>, which are abstract
804 numbers. To use these numbers, various encodings are needed.
812 UTF-8 is a variable-length (1 to 6 bytes, current character allocations
813 require 4 bytes), byte-order independent encoding. For ASCII (and we
814 really do mean 7-bit ASCII, not another 8-bit encoding), UTF-8 is
817 The following table is from Unicode 3.2.
819 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
821 U+0000..U+007F 00..7F
822 U+0080..U+07FF C2..DF 80..BF
823 U+0800..U+0FFF E0 A0..BF 80..BF
824 U+1000..U+CFFF E1..EC 80..BF 80..BF
825 U+D000..U+D7FF ED 80..9F 80..BF
826 U+D800..U+DFFF ******* ill-formed *******
827 U+E000..U+FFFF EE..EF 80..BF 80..BF
828 U+10000..U+3FFFF F0 90..BF 80..BF 80..BF
829 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
830 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
832 Note the C<A0..BF> in C<U+0800..U+0FFF>, the C<80..9F> in
833 C<U+D000...U+D7FF>, the C<90..B>F in C<U+10000..U+3FFFF>, and the
834 C<80...8F> in C<U+100000..U+10FFFF>. The "gaps" are caused by legal
835 UTF-8 avoiding non-shortest encodings: it is technically possible to
836 UTF-8-encode a single code point in different ways, but that is
837 explicitly forbidden, and the shortest possible encoding should always
838 be used. So that's what Perl does.
840 Another way to look at it is via bits:
842 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
845 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
846 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
847 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
849 As you can see, the continuation bytes all begin with C<10>, and the
850 leading bits of the start byte tell how many bytes the are in the
857 Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
861 UTF-16, UTF-16BE, UTF16-LE, Surrogates, and BOMs (Byte Order Marks)
863 The followings items are mostly for reference and general Unicode
864 knowledge, Perl doesn't use these constructs internally.
866 UTF-16 is a 2 or 4 byte encoding. The Unicode code points
867 C<U+0000..U+FFFF> are stored in a single 16-bit unit, and the code
868 points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is
869 using I<surrogates>, the first 16-bit unit being the I<high
870 surrogate>, and the second being the I<low surrogate>.
872 Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
873 range of Unicode code points in pairs of 16-bit units. The I<high
874 surrogates> are the range C<U+D800..U+DBFF>, and the I<low surrogates>
875 are the range C<U+DC00..U+DFFF>. The surrogate encoding is
877 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
878 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
882 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
884 If you try to generate surrogates (for example by using chr()), you
885 will get a warning if warnings are turned on, because those code
886 points are not valid for a Unicode character.
888 Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
889 itself can be used for in-memory computations, but if storage or
890 transfer is required either UTF-16BE (big-endian) or UTF-16LE
891 (little-endian) encodings must be chosen.
893 This introduces another problem: what if you just know that your data
894 is UTF-16, but you don't know which endianness? Byte Order Marks, or
895 BOMs, are a solution to this. A special character has been reserved
896 in Unicode to function as a byte order marker: the character with the
897 code point C<U+FEFF> is the BOM.
899 The trick is that if you read a BOM, you will know the byte order,
900 since if it was written on a big-endian platform, you will read the
901 bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
902 you will read the bytes C<0xFF 0xFE>. (And if the originating platform
903 was writing in UTF-8, you will read the bytes C<0xEF 0xBB 0xBF>.)
905 The way this trick works is that the character with the code point
906 C<U+FFFE> is guaranteed not to be a valid Unicode character, so the
907 sequence of bytes C<0xFF 0xFE> is unambiguously "BOM, represented in
908 little-endian format" and cannot be C<U+FFFE>, represented in big-endian
913 UTF-32, UTF-32BE, UTF32-LE
915 The UTF-32 family is pretty much like the UTF-16 family, expect that
916 the units are 32-bit, and therefore the surrogate scheme is not
917 needed. The BOM signatures will be C<0x00 0x00 0xFE 0xFF> for BE and
918 C<0xFF 0xFE 0x00 0x00> for LE.
924 Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
925 encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
926 because it does not use surrogates. UCS-4 is a 32-bit encoding,
927 functionally identical to UTF-32.
933 A seven-bit safe (non-eight-bit) encoding, which is useful if the
934 transport or storage is not eight-bit safe. Defined by RFC 2152.
938 =head2 Security Implications of Unicode
946 Unfortunately, the specification of UTF-8 leaves some room for
947 interpretation of how many bytes of encoded output one should generate
948 from one input Unicode character. Strictly speaking, the shortest
949 possible sequence of UTF-8 bytes should be generated,
950 because otherwise there is potential for an input buffer overflow at
951 the receiving end of a UTF-8 connection. Perl always generates the
952 shortest length UTF-8, and with warnings on Perl will warn about
953 non-shortest length UTF-8 along with other malformations, such as the
954 surrogates, which are not real Unicode code points.
958 Regular expressions behave slightly differently between byte data and
959 character (Unicode) data. For example, the "word character" character
960 class C<\w> will work differently depending on if data is eight-bit bytes
963 In the first case, the set of C<\w> characters is either small--the
964 default set of alphabetic characters, digits, and the "_"--or, if you
965 are using a locale (see L<perllocale>), the C<\w> might contain a few
966 more letters according to your language and country.
968 In the second case, the C<\w> set of characters is much, much larger.
969 Most importantly, even in the set of the first 256 characters, it will
970 probably match different characters: unlike most locales, which are
971 specific to a language and country pair, Unicode classifies all the
972 characters that are letters I<somewhere> as C<\w>. For example, your
973 locale might not think that LATIN SMALL LETTER ETH is a letter (unless
974 you happen to speak Icelandic), but Unicode does.
976 As discussed elsewhere, Perl has one foot (two hooves?) planted in
977 each of two worlds: the old world of bytes and the new world of
978 characters, upgrading from bytes to characters when necessary.
979 If your legacy code does not explicitly use Unicode, no automatic
980 switch-over to characters should happen. Characters shouldn't get
981 downgraded to bytes, either. It is possible to accidentally mix bytes
982 and characters, however (see L<perluniintro>), in which case C<\w> in
983 regular expressions might start behaving differently. Review your
984 code. Use warnings and the C<strict> pragma.
988 =head2 Unicode in Perl on EBCDIC
990 The way Unicode is handled on EBCDIC platforms is still
991 experimental. On such platforms, references to UTF-8 encoding in this
992 document and elsewhere should be read as meaning the UTF-EBCDIC
993 specified in Unicode Technical Report 16, unless ASCII vs. EBCDIC issues
994 are specifically discussed. There is no C<utfebcdic> pragma or
995 ":utfebcdic" layer; rather, "utf8" and ":utf8" are reused to mean
996 the platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic>
997 for more discussion of the issues.
1001 Usually locale settings and Unicode do not affect each other, but
1002 there are a couple of exceptions:
1008 If your locale environment variables (LANGUAGE, LC_ALL, LC_CTYPE, LANG)
1009 contain the strings 'UTF-8' or 'UTF8' (case-insensitive matching),
1010 the default encodings of your STDIN, STDOUT, and STDERR, and of
1011 B<any subsequent file open>, are considered to be UTF-8.
1015 Perl tries really hard to work both with Unicode and the old
1016 byte-oriented world. Most often this is nice, but sometimes Perl's
1017 straddling of the proverbial fence causes problems.
1021 =head2 Using Unicode in XS
1023 If you want to handle Perl Unicode in XS extensions, you may find
1024 the following C APIs useful. See L<perlapi> for details.
1030 C<DO_UTF8(sv)> returns true if the C<UTF8> flag is on and the bytes
1031 pragma is not in effect. C<SvUTF8(sv)> returns true is the C<UTF8>
1032 flag is on; the bytes pragma is ignored. The C<UTF8> flag being on
1033 does B<not> mean that there are any characters of code points greater
1034 than 255 (or 127) in the scalar or that there are even any characters
1035 in the scalar. What the C<UTF8> flag means is that the sequence of
1036 octets in the representation of the scalar is the sequence of UTF-8
1037 encoded code points of the characters of a string. The C<UTF8> flag
1038 being off means that each octet in this representation encodes a
1039 single character with code point 0..255 within the string. Perl's
1040 Unicode model is not to use UTF-8 until it is absolutely necessary.
1044 C<uvuni_to_utf8(buf, chr>) writes a Unicode character code point into
1045 a buffer encoding the code point as UTF-8, and returns a pointer
1046 pointing after the UTF-8 bytes.
1050 C<utf8_to_uvuni(buf, lenp)> reads UTF-8 encoded bytes from a buffer and
1051 returns the Unicode character code point and, optionally, the length of
1052 the UTF-8 byte sequence.
1056 C<utf8_length(start, end)> returns the length of the UTF-8 encoded buffer
1057 in characters. C<sv_len_utf8(sv)> returns the length of the UTF-8 encoded
1062 C<sv_utf8_upgrade(sv)> converts the string of the scalar to its UTF-8
1063 encoded form. C<sv_utf8_downgrade(sv)> does the opposite, if
1064 possible. C<sv_utf8_encode(sv)> is like sv_utf8_upgrade except that
1065 it does not set the C<UTF8> flag. C<sv_utf8_decode()> does the
1066 opposite of C<sv_utf8_encode()>. Note that none of these are to be
1067 used as general-purpose encoding or decoding interfaces: C<use Encode>
1068 for that. C<sv_utf8_upgrade()> is affected by the encoding pragma
1069 but C<sv_utf8_downgrade()> is not (since the encoding pragma is
1070 designed to be a one-way street).
1074 C<is_utf8_char(s)> returns true if the pointer points to a valid UTF-8
1079 C<is_utf8_string(buf, len)> returns true if C<len> bytes of the buffer
1084 C<UTF8SKIP(buf)> will return the number of bytes in the UTF-8 encoded
1085 character in the buffer. C<UNISKIP(chr)> will return the number of bytes
1086 required to UTF-8-encode the Unicode character code point. C<UTF8SKIP()>
1087 is useful for example for iterating over the characters of a UTF-8
1088 encoded buffer; C<UNISKIP()> is useful, for example, in computing
1089 the size required for a UTF-8 encoded buffer.
1093 C<utf8_distance(a, b)> will tell the distance in characters between the
1094 two pointers pointing to the same UTF-8 encoded buffer.
1098 C<utf8_hop(s, off)> will return a pointer to an UTF-8 encoded buffer
1099 that is C<off> (positive or negative) Unicode characters displaced
1100 from the UTF-8 buffer C<s>. Be careful not to overstep the buffer:
1101 C<utf8_hop()> will merrily run off the end or the beginning of the
1102 buffer if told to do so.
1106 C<pv_uni_display(dsv, spv, len, pvlim, flags)> and
1107 C<sv_uni_display(dsv, ssv, pvlim, flags)> are useful for debugging the
1108 output of Unicode strings and scalars. By default they are useful
1109 only for debugging--they display B<all> characters as hexadecimal code
1110 points--but with the flags C<UNI_DISPLAY_ISPRINT>,
1111 C<UNI_DISPLAY_BACKSLASH>, and C<UNI_DISPLAY_QQ> you can make the
1112 output more readable.
1116 C<ibcmp_utf8(s1, pe1, u1, l1, u1, s2, pe2, l2, u2)> can be used to
1117 compare two strings case-insensitively in Unicode. For case-sensitive
1118 comparisons you can just use C<memEQ()> and C<memNE()> as usual.
1122 For more information, see L<perlapi>, and F<utf8.c> and F<utf8.h>
1123 in the Perl source code distribution.
1127 =head2 Interaction with Locales
1129 Use of locales with Unicode data may lead to odd results. Currently,
1130 Perl attempts to attach 8-bit locale info to characters in the range
1131 0..255, but this technique is demonstrably incorrect for locales that
1132 use characters above that range when mapped into Unicode. Perl's
1133 Unicode support will also tend to run slower. Use of locales with
1134 Unicode is discouraged.
1136 =head2 Interaction with Extensions
1138 When Perl exchanges data with an extension, the extension should be
1139 able to understand the UTF-8 flag and act accordingly. If the
1140 extension doesn't know about the flag, it's likely that the extension
1141 will return incorrectly-flagged data.
1143 So if you're working with Unicode data, consult the documentation of
1144 every module you're using if there are any issues with Unicode data
1145 exchange. If the documentation does not talk about Unicode at all,
1146 suspect the worst and probably look at the source to learn how the
1147 module is implemented. Modules written completely in Perl shouldn't
1148 cause problems. Modules that directly or indirectly access code written
1149 in other programming languages are at risk.
1151 For affected functions, the simple strategy to avoid data corruption is
1152 to always make the encoding of the exchanged data explicit. Choose an
1153 encoding that you know the extension can handle. Convert arguments passed
1154 to the extensions to that encoding and convert results back from that
1155 encoding. Write wrapper functions that do the conversions for you, so
1156 you can later change the functions when the extension catches up.
1158 To provide an example, let's say the popular Foo::Bar::escape_html
1159 function doesn't deal with Unicode data yet. The wrapper function
1160 would convert the argument to raw UTF-8 and convert the result back to
1161 Perl's internal representation like so:
1163 sub my_escape_html ($) {
1165 return unless defined $what;
1166 Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
1169 Sometimes, when the extension does not convert data but just stores
1170 and retrieves them, you will be in a position to use the otherwise
1171 dangerous Encode::_utf8_on() function. Let's say the popular
1172 C<Foo::Bar> extension, written in C, provides a C<param> method that
1173 lets you store and retrieve data according to these prototypes:
1175 $self->param($name, $value); # set a scalar
1176 $value = $self->param($name); # retrieve a scalar
1178 If it does not yet provide support for any encoding, one could write a
1179 derived class with such a C<param> method:
1182 my($self,$name,$value) = @_;
1183 utf8::upgrade($name); # make sure it is UTF-8 encoded
1185 utf8::upgrade($value); # make sure it is UTF-8 encoded
1186 return $self->SUPER::param($name,$value);
1188 my $ret = $self->SUPER::param($name);
1189 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
1194 Some extensions provide filters on data entry/exit points, such as
1195 DB_File::filter_store_key and family. Look out for such filters in
1196 the documentation of your extensions, they can make the transition to
1197 Unicode data much easier.
1201 Some functions are slower when working on UTF-8 encoded strings than
1202 on byte encoded strings. All functions that need to hop over
1203 characters such as length(), substr() or index() can work B<much>
1204 faster when the underlying data are byte-encoded. Witness the
1205 following benchmark:
1211 our $u = our $b = "x" x $l;
1212 substr($u,0,1) = "\x{100}";
1214 LENGTH_B => q{ length($b) },
1215 LENGTH_U => q{ length($u) },
1216 SUBSTR_B => q{ substr($b, $l/4, $l/2) },
1217 SUBSTR_U => q{ substr($u, $l/4, $l/2) },
1220 Benchmark: running LENGTH_B, LENGTH_U, SUBSTR_B, SUBSTR_U for at least 2 CPU seconds...
1221 LENGTH_B: 2 wallclock secs ( 2.36 usr + 0.00 sys = 2.36 CPU) @ 5649983.05/s (n=13333960)
1222 LENGTH_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 12155.45/s (n=25648)
1223 SUBSTR_B: 3 wallclock secs ( 2.16 usr + 0.00 sys = 2.16 CPU) @ 374480.09/s (n=808877)
1224 SUBSTR_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 6791.00/s (n=14329)
1226 The numbers show an incredible slowness on long UTF-8 strings. You
1227 should carefully avoid using these functions in tight loops. If you
1228 want to iterate over characters, the superior coding technique would
1229 split the characters into an array instead of using substr, as the following
1236 our $u = our $b = "x" x $l;
1237 substr($u,0,1) = "\x{100}";
1239 SPLIT_B => q{ for my $c (split //, $b){} },
1240 SPLIT_U => q{ for my $c (split //, $u){} },
1241 SUBSTR_B => q{ for my $i (0..length($b)-1){my $c = substr($b,$i,1);} },
1242 SUBSTR_U => q{ for my $i (0..length($u)-1){my $c = substr($u,$i,1);} },
1245 Benchmark: running SPLIT_B, SPLIT_U, SUBSTR_B, SUBSTR_U for at least 5 CPU seconds...
1246 SPLIT_B: 6 wallclock secs ( 5.29 usr + 0.00 sys = 5.29 CPU) @ 56.14/s (n=297)
1247 SPLIT_U: 5 wallclock secs ( 5.17 usr + 0.01 sys = 5.18 CPU) @ 55.21/s (n=286)
1248 SUBSTR_B: 5 wallclock secs ( 5.34 usr + 0.00 sys = 5.34 CPU) @ 123.22/s (n=658)
1249 SUBSTR_U: 7 wallclock secs ( 6.20 usr + 0.00 sys = 6.20 CPU) @ 0.81/s (n=5)
1251 Even though the algorithm based on C<substr()> is faster than
1252 C<split()> for byte-encoded data, it pales in comparison to the speed
1253 of C<split()> when used with UTF-8 data.
1257 L<perluniintro>, L<encoding>, L<Encode>, L<open>, L<utf8>, L<bytes>,
1258 L<perlretut>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">
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