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 A filehandle can be marked as containing perl's internal Unicode
18 encoding (UTF-8 or UTF-EBCDIC) by opening it with the ":utf8" layer.
19 Other encodings can be converted to perl's encoding on input, or from
20 perl's encoding on output by use of the ":encoding(...)" layer.
23 To mark the Perl source itself as being in 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 switch to the Unicode
30 character scheme when presented with Unicode data, or a traditional
31 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, this pragma must be explicitly used to
36 enable recognition of UTF-8 in the Perl scripts themselves on ASCII
37 based machines, or to recognize UTF-EBCDIC on EBCDIC based machines.
38 B<NOTE: this should be the only place where an explicit C<use utf8>
41 You can also use the C<encoding> pragma to change the default encoding
42 of the data in your script; see L<encoding>.
46 =head2 Byte and Character semantics
48 Beginning with version 5.6, Perl uses logically wide characters to
49 represent strings internally.
51 In future, Perl-level operations can be expected to work with
52 characters rather than bytes, in general.
54 However, as strictly an interim compatibility measure, Perl aims to
55 provide a safe migration path from byte semantics to character
56 semantics for programs. For operations where Perl can unambiguously
57 decide that the input data is characters, Perl now switches to
58 character semantics. For operations where this determination cannot
59 be made without additional information from the user, Perl decides in
60 favor of compatibility, and chooses to use byte semantics.
62 This behavior preserves compatibility with earlier versions of Perl,
63 which allowed byte semantics in Perl operations, but only as long as
64 none of the program's inputs are marked as being as source of Unicode
65 character data. Such data may come from filehandles, from calls to
66 external programs, from information provided by the system (such as %ENV),
67 or from literals and constants in the source text.
69 On Windows platforms, if the C<-C> command line switch is used, (or the
70 ${^WIDE_SYSTEM_CALLS} global flag is set to C<1>), all system calls
71 will use the corresponding wide character APIs. Note that this is
72 currently only implemented on Windows since other platforms lack an
73 API standard on this area.
75 Regardless of the above, the C<bytes> pragma can always be used to
76 force byte semantics in a particular lexical scope. See L<bytes>.
78 The C<utf8> pragma is primarily a compatibility device that enables
79 recognition of UTF-(8|EBCDIC) in literals encountered by the parser.
80 Note that this pragma is only required until a future version of Perl
81 in which character semantics will become the default. This pragma may
82 then become a no-op. See L<utf8>.
84 Unless mentioned otherwise, Perl operators will use character semantics
85 when they are dealing with Unicode data, and byte semantics otherwise.
86 Thus, character semantics for these operations apply transparently; if
87 the input data came from a Unicode source (for example, by adding a
88 character encoding discipline to the filehandle whence it came, or a
89 literal Unicode string constant in the program), character semantics
90 apply; otherwise, byte semantics are in effect. To force byte semantics
91 on Unicode data, the C<bytes> pragma should be used.
93 Notice that if you concatenate strings with byte semantics and strings
94 with Unicode character data, the bytes will by default be upgraded
95 I<as if they were ISO 8859-1 (Latin-1)> (or if in EBCDIC, after a
96 translation to ISO 8859-1). This is done without regard to the
97 system's native 8-bit encoding, so to change this for systems with
98 non-Latin-1 (or non-EBCDIC) native encodings, use the C<encoding>
99 pragma, see L<encoding>.
101 Under character semantics, many operations that formerly operated on
102 bytes change to operating on characters. A character in Perl is
103 logically just a number ranging from 0 to 2**31 or so. Larger
104 characters may encode to longer sequences of bytes internally, but
105 this is just an internal detail which is hidden at the Perl level.
106 See L<perluniintro> for more on this.
108 =head2 Effects of character semantics
110 Character semantics have the following effects:
116 Strings (including hash keys) and regular expression patterns may
117 contain characters that have an ordinal value larger than 255.
119 If you use a Unicode editor to edit your program, Unicode characters
120 may occur directly within the literal strings in one of the various
121 Unicode encodings (UTF-8, UTF-EBCDIC, UCS-2, etc.), but are recognized
122 as such (and converted to Perl's internal representation) only if the
123 appropriate L<encoding> is specified.
125 You can also get Unicode characters into a string by using the C<\x{...}>
126 notation, putting the Unicode code for the desired character, in
127 hexadecimal, into the curlies. For instance, a smiley face is C<\x{263A}>.
128 This works only for characters with a code 0x100 and above.
132 use charnames ':full';
134 you can use the C<\N{...}> notation, putting the official Unicode character
135 name within the curlies. For example, C<\N{WHITE SMILING FACE}>.
136 This works for all characters that have names.
140 If an appropriate L<encoding> is specified, identifiers within the
141 Perl script may contain Unicode alphanumeric characters, including
142 ideographs. (You are currently on your own when it comes to using the
143 canonical forms of characters--Perl doesn't (yet) attempt to
144 canonicalize variable names for you.)
148 Regular expressions match characters instead of bytes. For instance,
149 "." matches a character instead of a byte. (However, the C<\C> pattern
150 is provided to force a match a single byte ("C<char>" in C, hence C<\C>).)
154 Character classes in regular expressions match characters instead of
155 bytes, and match against the character properties specified in the
156 Unicode properties database. So C<\w> can be used to match an
157 ideograph, for instance.
161 Named Unicode properties, scripts, and block ranges may be used like
162 character classes via the new C<\p{}> (matches property) and C<\P{}>
163 (doesn't match property) constructs. For instance, C<\p{Lu}> matches any
164 character with the Unicode "Lu" (Letter, uppercase) property, while
165 C<\p{M}> matches any character with an "M" (mark -- accents and such)
166 property. Single letter properties may omit the brackets, so that can be
167 written C<\pM> also. Many predefined properties are available, such
168 as C<\p{Mirrored}> and C<\p{Tibetan}>.
170 The official Unicode script and block names have spaces and dashes as
171 separators, but for convenience you can have dashes, spaces, and underbars
172 at every word division, and you need not care about correct casing. It is
173 recommended, however, that for consistency you use the following naming:
174 the official Unicode script, block, or property name (see below for the
175 additional rules that apply to block names), with whitespace and dashes
176 removed, and the words "uppercase-first-lowercase-rest". That is, "Latin-1
177 Supplement" becomes "Latin1Supplement".
179 You can also negate both C<\p{}> and C<\P{}> by introducing a caret
180 (^) between the first curly and the property name: C<\p{^Tamil}> is
181 equal to C<\P{Tamil}>.
183 Here are the basic Unicode General Category properties, followed by their
184 long form (you can use either, e.g. C<\p{Lu}> and C<\p{LowercaseLetter}>
207 Pc ConnectorPunctuation
211 Pi InitialPunctuation
212 (may behave like Ps or Pe depending on usage)
214 (may behave like Ps or Pe depending on usage)
226 Zp ParagraphSeparator
231 Cs Surrogate (not usable)
235 The single-letter properties match all characters in any of the
236 two-letter sub-properties starting with the same letter.
237 There's also C<L&> which is an alias for C<Ll>, C<Lu>, and C<Lt>.
239 Because Perl hides the need for the user to understand the internal
240 representation of Unicode characters, it has no need to support the
241 somewhat messy concept of surrogates. Therefore, the C<Cs> property is not
244 Because scripts differ in their directionality (for example Hebrew is
245 written right to left), Unicode supplies these properties:
250 BidiLRE Left-to-Right Embedding
251 BidiLRO Left-to-Right Override
253 BidiAL Right-to-Left Arabic
254 BidiRLE Right-to-Left Embedding
255 BidiRLO Right-to-Left Override
256 BidiPDF Pop Directional Format
257 BidiEN European Number
258 BidiES European Number Separator
259 BidiET European Number Terminator
261 BidiCS Common Number Separator
262 BidiNSM Non-Spacing Mark
263 BidiBN Boundary Neutral
264 BidiB Paragraph Separator
265 BidiS Segment Separator
267 BidiON Other Neutrals
269 For example, C<\p{BidiR}> matches all characters that are normally
270 written right to left.
276 The scripts available via C<\p{...}> and C<\P{...}>, for example
277 C<\p{Latin}> or C<\p{Cyrillic}>, are as follows:
324 There are also extended property classes that supplement the basic
325 properties, defined by the F<PropList> Unicode database:
340 LogicalOrderException
341 NoncharacterCodePoint
343 OtherDefaultIgnorableCodePoint
355 and further derived properties:
357 Alphabetic Lu + Ll + Lt + Lm + Lo + OtherAlphabetic
358 Lowercase Ll + OtherLowercase
359 Uppercase Lu + OtherUppercase
362 ID_Start Lu + Ll + Lt + Lm + Lo + Nl
363 ID_Continue ID_Start + Mn + Mc + Nd + Pc
366 Assigned Any non-Cn character (i.e. synonym for \P{Cn})
367 Unassigned Synonym for \p{Cn}
368 Common Any character (or unassigned code point)
369 not explicitly assigned to a script
371 For backward compatibility, all properties mentioned so far may have C<Is>
372 prepended to their name (e.g. C<\P{IsLu}> is equal to C<\P{Lu}>).
376 In addition to B<scripts>, Unicode also defines B<blocks> of characters.
377 The difference between scripts and blocks is that the scripts concept is
378 closer to natural languages, while the blocks concept is more an artificial
379 grouping based on groups of mostly 256 Unicode characters. For example, the
380 C<Latin> script contains letters from many blocks. On the other hand, the
381 C<Latin> script does not contain all the characters from those blocks. It
382 does not, for example, contain digits because digits are shared across many
383 scripts. Digits and other similar groups, like punctuation, are in a
384 category called C<Common>.
386 For more about scripts, see the UTR #24:
388 http://www.unicode.org/unicode/reports/tr24/
390 For more about blocks, see:
392 http://www.unicode.org/Public/UNIDATA/Blocks.txt
394 Blocks names are given with the C<In> prefix. For example, the
395 Katakana block is referenced via C<\p{InKatakana}>. The C<In>
396 prefix may be omitted if there is no naming conflict with a script
397 or any other property, but it is recommended that C<In> always be used
400 These block names are supported:
402 InAlphabeticPresentationForms
404 InArabicPresentationFormsA
405 InArabicPresentationFormsB
416 InByzantineMusicalSymbols
418 InCJKCompatibilityForms
419 InCJKCompatibilityIdeographs
420 InCJKCompatibilityIdeographsSupplement
421 InCJKRadicalsSupplement
422 InCJKSymbolsAndPunctuation
423 InCJKUnifiedIdeographs
424 InCJKUnifiedIdeographsExtensionA
425 InCJKUnifiedIdeographsExtensionB
427 InCombiningDiacriticalMarks
428 InCombiningDiacriticalMarksforSymbols
433 InCyrillicSupplementary
437 InEnclosedAlphanumerics
438 InEnclosedCJKLettersAndMonths
448 InHalfwidthAndFullwidthForms
449 InHangulCompatibilityJamo
454 InHighPrivateUseSurrogates
458 InIdeographicDescriptionCharacters
463 InKatakanaPhoneticExtensions
468 InLatinExtendedAdditional
473 InMathematicalAlphanumericSymbols
474 InMathematicalOperators
475 InMiscellaneousMathematicalSymbolsA
476 InMiscellaneousMathematicalSymbolsB
477 InMiscellaneousSymbols
478 InMiscellaneousTechnical
485 InOpticalCharacterRecognition
491 InSpacingModifierLetters
493 InSuperscriptsAndSubscripts
494 InSupplementalArrowsA
495 InSupplementalArrowsB
496 InSupplementalMathematicalOperators
497 InSupplementaryPrivateUseAreaA
498 InSupplementaryPrivateUseAreaB
508 InUnifiedCanadianAboriginalSyllabics
517 The special pattern C<\X> matches any extended Unicode sequence
518 (a "combining character sequence" in Standardese), where the first
519 character is a base character and subsequent characters are mark
520 characters that apply to the base character. It is equivalent to
525 The C<tr///> operator translates characters instead of bytes. Note
526 that the C<tr///CU> functionality has been removed, as the interface
527 was a mistake. For similar functionality see pack('U0', ...) and
532 Case translation operators use the Unicode case translation tables
533 when provided character input. Note that C<uc()> (also known as C<\U>
534 in doublequoted strings) translates to uppercase, while C<ucfirst>
535 (also known as C<\u> in doublequoted strings) translates to titlecase
536 (for languages that make the distinction). Naturally the
537 corresponding backslash sequences have the same semantics.
541 Most operators that deal with positions or lengths in the string will
542 automatically switch to using character positions, including
543 C<chop()>, C<substr()>, C<pos()>, C<index()>, C<rindex()>,
544 C<sprintf()>, C<write()>, and C<length()>. Operators that
545 specifically don't switch include C<vec()>, C<pack()>, and
546 C<unpack()>. Operators that really don't care include C<chomp()>, as
547 well as any other operator that treats a string as a bucket of bits,
548 such as C<sort()>, and the operators dealing with filenames.
552 The C<pack()>/C<unpack()> letters "C<c>" and "C<C>" do I<not> change,
553 since they're often used for byte-oriented formats. (Again, think
554 "C<char>" in the C language.) However, there is a new "C<U>" specifier
555 that will convert between Unicode characters and integers.
559 The C<chr()> and C<ord()> functions work on characters. This is like
560 C<pack("U")> and C<unpack("U")>, not like C<pack("C")> and
561 C<unpack("C")>. In fact, the latter are how you now emulate
562 byte-oriented C<chr()> and C<ord()> for Unicode strings.
563 (Note that this reveals the internal encoding of Unicode strings,
564 which is not something one normally needs to care about at all.)
568 The bit string operators C<& | ^ ~> can operate on character data.
569 However, for backward compatibility reasons (bit string operations
570 when the characters all are less than 256 in ordinal value) one should
571 not mix C<~> (the bit complement) and characters both less than 256 and
572 equal or greater than 256. Most importantly, the DeMorgan's laws
573 (C<~($x|$y) eq ~$x&~$y>, C<~($x&$y) eq ~$x|~$y>) won't hold.
574 Another way to look at this is that the complement cannot return
575 B<both> the 8-bit (byte) wide bit complement B<and> the full character
580 lc(), uc(), lcfirst(), and ucfirst() work for the following cases:
586 the case mapping is from a single Unicode character to another
587 single Unicode character
591 the case mapping is from a single Unicode character to more
592 than one Unicode character
596 What doesn't yet work are the following cases:
602 the "final sigma" (Greek)
606 anything to with locales (Lithuanian, Turkish, Azeri)
610 See the Unicode Technical Report #21, Case Mappings, for more details.
614 And finally, C<scalar reverse()> reverses by character rather than by byte.
618 =head2 User-defined Character Properties
620 You can define your own character properties by defining subroutines
621 that have names beginning with "In" or "Is". The subroutines must be
622 visible in the package that uses the properties. The user-defined
623 properties can be used in the regular expression C<\p> and C<\P>
626 The subroutines must return a specially formatted string: one or more
627 newline-separated lines. Each line must be one of the following:
633 Two hexadecimal numbers separated by horizontal whitespace (space or
634 tabulator characters) denoting a range of Unicode codepoints to include.
638 Something to include, prefixed by "+": either an built-in character
639 property (prefixed by "utf8::"), for all the characters in that
640 property; or two hexadecimal codepoints for a range; or a single
641 hexadecimal codepoint.
645 Something to exclude, prefixed by "-": either an existing character
646 property (prefixed by "utf8::"), for all the characters in that
647 property; or two hexadecimal codepoints for a range; or a single
648 hexadecimal codepoint.
652 Something to negate, prefixed "!": either an existing character
653 property (prefixed by "utf8::") for all the characters except the
654 characters in the property; or two hexadecimal codepoints for a range;
655 or a single hexadecimal codepoint.
659 For example, to define a property that covers both the Japanese
660 syllabaries (hiragana and katakana), you can define
669 Imagine that the here-doc end marker is at the beginning of the line.
670 Now you can use C<\p{InKana}> and C<\P{InKana}>.
672 You could also have used the existing block property names:
681 Suppose you wanted to match only the allocated characters,
682 not the raw block ranges: in other words, you want to remove
693 The negation is useful for defining (surprise!) negated classes.
703 =head2 Character encodings for input and output
707 =head2 Unicode Regular Expression Support Level
709 The following list of Unicode regular expression support describes
710 feature by feature the Unicode support implemented in Perl as of Perl
711 5.8.0. The "Level N" and the section numbers refer to the Unicode
712 Technical Report 18, "Unicode Regular Expression Guidelines".
718 Level 1 - Basic Unicode Support
720 2.1 Hex Notation - done [1]
721 Named Notation - done [2]
722 2.2 Categories - done [3][4]
723 2.3 Subtraction - MISSING [5][6]
724 2.4 Simple Word Boundaries - done [7]
725 2.5 Simple Loose Matches - done [8]
726 2.6 End of Line - MISSING [9][10]
730 [ 3] . \p{...} \P{...}
731 [ 4] now scripts (see UTR#24 Script Names) in addition to blocks
733 [ 6] can use regular expression look-ahead [a]
734 or user-defined character properties [b] to emulate subtraction
735 [ 7] include Letters in word characters
736 [ 8] note that perl does Full casefolding in matching, not Simple:
737 for example U+1F88 is equivalent with U+1F000 U+03B9,
738 not with 1F80. This difference matters for certain Greek
739 capital letters with certain modifiers: the Full casefolding
740 decomposes the letter, while the Simple casefolding would map
741 it to a single character.
742 [ 9] see UTR#13 Unicode Newline Guidelines
743 [10] should do ^ and $ also on \x{85}, \x{2028} and \x{2029})
744 (should also affect <>, $., and script line numbers)
745 (the \x{85}, \x{2028} and \x{2029} do match \s)
747 [a] You can mimic class subtraction using lookahead.
748 For example, what TR18 might write as
750 [{Greek}-[{UNASSIGNED}]]
752 in Perl can be written as:
754 (?!\p{Unassigned})\p{InGreekAndCoptic}
755 (?=\p{Assigned})\p{InGreekAndCoptic}
757 But in this particular example, you probably really want
761 which will match assigned characters known to be part of the Greek script.
763 [b] See L</User-defined Character Properties>.
767 Level 2 - Extended Unicode Support
769 3.1 Surrogates - MISSING
770 3.2 Canonical Equivalents - MISSING [11][12]
771 3.3 Locale-Independent Graphemes - MISSING [13]
772 3.4 Locale-Independent Words - MISSING [14]
773 3.5 Locale-Independent Loose Matches - MISSING [15]
775 [11] see UTR#15 Unicode Normalization
776 [12] have Unicode::Normalize but not integrated to regexes
777 [13] have \X but at this level . should equal that
778 [14] need three classes, not just \w and \W
779 [15] 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), byteorder independent encoding. For ASCII, UTF-8 is
809 transparent (and we really do mean 7-bit ASCII, not another 8-bit encoding).
811 The following table is from Unicode 3.2.
813 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
815 U+0000..U+007F 00..7F
816 U+0080..U+07FF C2..DF 80..BF
817 U+0800..U+0FFF E0 A0..BF 80..BF
818 U+1000..U+CFFF E1..EC 80..BF 80..BF
819 U+D000..U+D7FF ED 80..9F 80..BF
820 U+D800..U+DFFF ******* ill-formed *******
821 U+E000..U+FFFF EE..EF 80..BF 80..BF
822 U+10000..U+3FFFF F0 90..BF 80..BF 80..BF
823 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
824 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
826 Note the A0..BF in U+0800..U+0FFF, the 80..9F in U+D000...U+D7FF,
827 the 90..BF in U+10000..U+3FFFF, and the 80...8F in U+100000..U+10FFFF.
828 The "gaps" are caused by legal UTF-8 avoiding non-shortest encodings:
829 it is technically possible to UTF-8-encode a single code point in different
830 ways, but that is explicitly forbidden, and the shortest possible encoding
831 should always be used (and that is what Perl does).
833 Or, another way to look at it, as bits:
835 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
838 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
839 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
840 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
842 As you can see, the continuation bytes all begin with C<10>, and the
843 leading bits of the start byte tell how many bytes the are in the
850 Like UTF-8, but EBCDIC-safe, as UTF-8 is ASCII-safe.
854 UTF-16, UTF-16BE, UTF16-LE, Surrogates, and BOMs (Byte Order Marks)
856 (The followings items are mostly for reference, Perl doesn't
857 use them internally.)
859 UTF-16 is a 2 or 4 byte encoding. The Unicode code points
860 U+0000..U+FFFF are stored in a single 16-bit unit, and the code points
861 U+10000..U+10FFFF in two 16-bit units. The latter case is
862 using I<surrogates>, the first 16-bit unit being the I<high
863 surrogate>, and the second being the I<low surrogate>.
865 Surrogates are code points set aside to encode the U+10000..U+10FFFF
866 range of Unicode code points in pairs of 16-bit units. The I<high
867 surrogates> are the range U+D800..U+DBFF, and the I<low surrogates>
868 are the range U+DC00..U+DFFF. The surrogate encoding is
870 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
871 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
875 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
877 If you try to generate surrogates (for example by using chr()), you
878 will get a warning if warnings are turned on (C<-w> or C<use
879 warnings;>) because those code points are not valid for a Unicode
882 Because of the 16-bitness, UTF-16 is byteorder dependent. UTF-16
883 itself can be used for in-memory computations, but if storage or
884 transfer is required, either UTF-16BE (Big Endian) or UTF-16LE
885 (Little Endian) must be chosen.
887 This introduces another problem: what if you just know that your data
888 is UTF-16, but you don't know which endianness? Byte Order Marks
889 (BOMs) are a solution to this. A special character has been reserved
890 in Unicode to function as a byte order marker: the character with the
891 code point U+FEFF is the BOM.
893 The trick is that if you read a BOM, you will know the byte order,
894 since if it was written on a big endian platform, you will read the
895 bytes 0xFE 0xFF, but if it was written on a little endian platform,
896 you will read the bytes 0xFF 0xFE. (And if the originating platform
897 was writing in UTF-8, you will read the bytes 0xEF 0xBB 0xBF.)
899 The way this trick works is that the character with the code point
900 U+FFFE is guaranteed not to be a valid Unicode character, so the
901 sequence of bytes 0xFF 0xFE is unambiguously "BOM, represented in
902 little-endian format" and cannot be "U+FFFE, represented in big-endian
907 UTF-32, UTF-32BE, UTF32-LE
909 The UTF-32 family is pretty much like the UTF-16 family, expect that
910 the units are 32-bit, and therefore the surrogate scheme is not
911 needed. The BOM signatures will be 0x00 0x00 0xFE 0xFF for BE and
912 0xFF 0xFE 0x00 0x00 for LE.
918 Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
919 encoding. Unlike UTF-16, UCS-2 is not extensible beyond U+FFFF,
920 because it does not use surrogates. UCS-4 is a 32-bit encoding,
921 functionally identical to UTF-32.
927 A seven-bit safe (non-eight-bit) encoding, useful if the
928 transport/storage is not eight-bit safe. Defined by RFC 2152.
932 =head2 Security Implications of Unicode
940 Unfortunately, the specification of UTF-8 leaves some room for
941 interpretation of how many bytes of encoded output one should generate
942 from one input Unicode character. Strictly speaking, one is supposed
943 to always generate the shortest possible sequence of UTF-8 bytes,
944 because otherwise there is potential for input buffer overflow at
945 the receiving end of a UTF-8 connection. Perl always generates the
946 shortest length UTF-8, and with warnings on (C<-w> or C<use
947 warnings;>) Perl will warn about non-shortest length UTF-8 (and other
948 malformations, too, such as the surrogates, which are not real
949 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 when the data is all eight-bit bytes
956 or when the data is Unicode.
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 and most importantly, even in the set of the first 256 characters, it
965 will most probably be different: as opposed to most locales (which are
966 specific to a language and country pair) Unicode classifies all the
967 characters that are letters as C<\w>. For example: your locale might
968 not think that LATIN SMALL LETTER ETH is a letter (unless you happen
969 to speak Icelandic), but Unicode does.
971 As discussed elsewhere, Perl tries to stand one leg (two legs, as
972 camels are quadrupeds?) in two worlds: the old world of bytes and the new
973 world of characters, upgrading from bytes to characters when necessary.
974 If your legacy code is not explicitly using Unicode, no automatic
975 switchover to characters should happen, and characters shouldn't get
976 downgraded back to bytes, either. It is possible to accidentally mix
977 bytes and characters, however (see L<perluniintro>), in which case the
978 C<\w> might start behaving differently. Review your code.
982 =head2 Unicode in Perl on EBCDIC
984 The way Unicode is handled on EBCDIC platforms is still rather
985 experimental. On such a platform, references to UTF-8 encoding in this
986 document and elsewhere should be read as meaning UTF-EBCDIC as
987 specified in Unicode Technical Report 16 unless ASCII vs EBCDIC issues
988 are specifically discussed. There is no C<utfebcdic> pragma or
989 ":utfebcdic" layer, rather, "utf8" and ":utf8" are re-used to mean
990 the platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic>
991 for more discussion of the issues.
995 Usually locale settings and Unicode do not affect each other, but
996 there are a couple of exceptions:
1002 If your locale environment variables (LANGUAGE, LC_ALL, LC_CTYPE, LANG)
1003 contain the strings 'UTF-8' or 'UTF8' (case-insensitive matching),
1004 the default encoding of your STDIN, STDOUT, and STDERR, and of
1005 B<any subsequent file open>, is UTF-8.
1009 Perl tries really hard to work both with Unicode and the old byte
1010 oriented world: most often this is nice, but sometimes this causes
1015 =head2 Using Unicode in XS
1017 If you want to handle Perl Unicode in XS extensions, you may find
1018 the following C APIs useful (see perlapi for details):
1024 DO_UTF8(sv) returns true if the UTF8 flag is on and the bytes pragma
1025 is not in effect. SvUTF8(sv) returns true is the UTF8 flag is on, the
1026 bytes pragma is ignored. The UTF8 flag being on does B<not> mean that
1027 there are any characters of code points greater than 255 (or 127) in
1028 the scalar, or that there even are any characters in the scalar.
1029 What the UTF8 flag means is that the sequence of octets in the
1030 representation of the scalar is the sequence of UTF-8 encoded
1031 code points of the characters of a string. The UTF8 flag being
1032 off means that each octet in this representation encodes a single
1033 character with codepoint 0..255 within the string. Perl's Unicode
1034 model is not to use UTF-8 until it's really necessary.
1038 uvuni_to_utf8(buf, chr) writes a Unicode character code point into a
1039 buffer encoding the code point as UTF-8, and returns a pointer
1040 pointing after the UTF-8 bytes.
1044 utf8_to_uvuni(buf, lenp) reads UTF-8 encoded bytes from a buffer and
1045 returns the Unicode character code point (and optionally the length of
1046 the UTF-8 byte sequence).
1050 utf8_length(start, end) returns the length of the UTF-8 encoded buffer
1051 in characters. sv_len_utf8(sv) returns the length of the UTF-8 encoded
1056 sv_utf8_upgrade(sv) converts the string of the scalar to its UTF-8
1057 encoded form. sv_utf8_downgrade(sv) does the opposite (if possible).
1058 sv_utf8_encode(sv) is like sv_utf8_upgrade but the UTF8 flag does not
1059 get turned on. sv_utf8_decode() does the opposite of sv_utf8_encode().
1060 Note that none of these are to be used as general purpose encoding/decoding
1061 interfaces: use Encode for that. sv_utf8_upgrade() is affected by the
1062 encoding pragma, but sv_utf8_downgrade() is not (since the encoding
1063 pragma is designed to be a one-way street).
1067 is_utf8_char(s) returns true if the pointer points to a valid UTF-8
1072 is_utf8_string(buf, len) returns true if the len bytes of the buffer
1077 UTF8SKIP(buf) will return the number of bytes in the UTF-8 encoded
1078 character in the buffer. UNISKIP(chr) will return the number of bytes
1079 required to UTF-8-encode the Unicode character code point. UTF8SKIP()
1080 is useful for example for iterating over the characters of a UTF-8
1081 encoded buffer; UNISKIP() is useful for example in computing
1082 the size required for a UTF-8 encoded buffer.
1086 utf8_distance(a, b) will tell the distance in characters between the
1087 two pointers pointing to the same UTF-8 encoded buffer.
1091 utf8_hop(s, off) will return a pointer to an UTF-8 encoded buffer that
1092 is C<off> (positive or negative) Unicode characters displaced from the
1093 UTF-8 buffer C<s>. Be careful not to overstep the buffer: utf8_hop()
1094 will merrily run off the end or the beginning if told to do so.
1098 pv_uni_display(dsv, spv, len, pvlim, flags) and sv_uni_display(dsv,
1099 ssv, pvlim, flags) are useful for debug output of Unicode strings and
1100 scalars. By default they are useful only for debug: they display
1101 B<all> characters as hexadecimal code points, but with the flags
1102 UNI_DISPLAY_ISPRINT and UNI_DISPLAY_BACKSLASH you can make the output
1107 ibcmp_utf8(s1, pe1, u1, l1, u1, s2, pe2, l2, u2) can be used to
1108 compare two strings case-insensitively in Unicode.
1109 (For case-sensitive comparisons you can just use memEQ() and memNE()
1114 For more information, see L<perlapi>, and F<utf8.c> and F<utf8.h>
1115 in the Perl source code distribution.
1119 =head2 Interaction with locales
1121 Use of locales with Unicode data may lead to odd results. Currently
1122 there is some attempt to apply 8-bit locale info to characters in the
1123 range 0..255, but this is demonstrably incorrect for locales that use
1124 characters above that range when mapped into Unicode. It will also
1125 tend to run slower. Use of locales with Unicode is discouraged.
1127 =head2 Interaction with extensions
1129 When perl exchanges data with an extension, the extension should be
1130 able to understand the UTF-8 flag and act accordingly. If the
1131 extension doesn't know about the flag, the risk is high that it will
1132 return data that are incorrectly flagged.
1134 So if you're working with Unicode data, consult the documentation of
1135 every module you're using if there are any issues with Unicode data
1136 exchange. If the documentation does not talk about Unicode at all,
1137 suspect the worst and probably look at the source to learn how the
1138 module is implemented. Modules written completely in perl shouldn't
1139 cause problems. Modules that directly or indirectly access code written
1140 in other programming languages are at risk.
1142 For affected functions the simple strategy to avoid data corruption is
1143 to always make the encoding of the exchanged data explicit. Choose an
1144 encoding you know the extension can handle. Convert arguments passed
1145 to the extensions to that encoding and convert results back from that
1146 encoding. Write wrapper functions that do the conversions for you, so
1147 you can later change the functions when the extension catches up.
1149 To provide an example let's say the popular Foo::Bar::escape_html
1150 function doesn't deal with Unicode data yet. The wrapper function
1151 would convert the argument to raw UTF-8 and convert the result back to
1152 perl's internal representation like so:
1154 sub my_escape_html ($) {
1156 return unless defined $what;
1157 Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
1160 Sometimes, when the extension does not convert data but just stores
1161 and retrieves them, you will be in a position to use the otherwise
1162 dangerous Encode::_utf8_on() function. Let's say the popular
1163 C<Foo::Bar> extension, written in C, provides a C<param> method that
1164 lets you store and retrieve data according to these prototypes:
1166 $self->param($name, $value); # set a scalar
1167 $value = $self->param($name); # retrieve a scalar
1169 If it does not yet provide support for any encoding, one could write a
1170 derived class with such a C<param> method:
1173 my($self,$name,$value) = @_;
1174 utf8::upgrade($name); # make sure it is UTF-8 encoded
1176 utf8::upgrade($value); # make sure it is UTF-8 encoded
1177 return $self->SUPER::param($name,$value);
1179 my $ret = $self->SUPER::param($name);
1180 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
1185 Some extensions provide filters on data entry/exit points, such as
1186 DB_File::filter_store_key and family. Look out for such filters in
1187 the documentation of your extensions, they can make the transition to
1188 Unicode data much easier.
1192 Some functions are slower when working on UTF-8 encoded strings than
1193 on byte encoded strings. All functions that need to hop over
1194 characters such as length(), substr() or index() can work B<much>
1195 faster when the underlying data are byte-encoded. Witness the
1196 following benchmark:
1202 our $u = our $b = "x" x $l;
1203 substr($u,0,1) = "\x{100}";
1205 LENGTH_B => q{ length($b) },
1206 LENGTH_U => q{ length($u) },
1207 SUBSTR_B => q{ substr($b, $l/4, $l/2) },
1208 SUBSTR_U => q{ substr($u, $l/4, $l/2) },
1211 Benchmark: running LENGTH_B, LENGTH_U, SUBSTR_B, SUBSTR_U for at least 2 CPU seconds...
1212 LENGTH_B: 2 wallclock secs ( 2.36 usr + 0.00 sys = 2.36 CPU) @ 5649983.05/s (n=13333960)
1213 LENGTH_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 12155.45/s (n=25648)
1214 SUBSTR_B: 3 wallclock secs ( 2.16 usr + 0.00 sys = 2.16 CPU) @ 374480.09/s (n=808877)
1215 SUBSTR_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 6791.00/s (n=14329)
1217 The numbers show an incredible slowness on long UTF-8 strings and you
1218 should carefully avoid to use these functions within tight loops. For
1219 example if you want to iterate over characters, it is infinitely
1220 better to split into an array than to use substr, as the following
1227 our $u = our $b = "x" x $l;
1228 substr($u,0,1) = "\x{100}";
1230 SPLIT_B => q{ for my $c (split //, $b){} },
1231 SPLIT_U => q{ for my $c (split //, $u){} },
1232 SUBSTR_B => q{ for my $i (0..length($b)-1){my $c = substr($b,$i,1);} },
1233 SUBSTR_U => q{ for my $i (0..length($u)-1){my $c = substr($u,$i,1);} },
1236 Benchmark: running SPLIT_B, SPLIT_U, SUBSTR_B, SUBSTR_U for at least 5 CPU seconds...
1237 SPLIT_B: 6 wallclock secs ( 5.29 usr + 0.00 sys = 5.29 CPU) @ 56.14/s (n=297)
1238 SPLIT_U: 5 wallclock secs ( 5.17 usr + 0.01 sys = 5.18 CPU) @ 55.21/s (n=286)
1239 SUBSTR_B: 5 wallclock secs ( 5.34 usr + 0.00 sys = 5.34 CPU) @ 123.22/s (n=658)
1240 SUBSTR_U: 7 wallclock secs ( 6.20 usr + 0.00 sys = 6.20 CPU) @ 0.81/s (n=5)
1242 You see, the algorithm based on substr() was faster with byte encoded
1243 data but it is pathologically slow with UTF-8 data.
1247 L<perluniintro>, L<encoding>, L<Encode>, L<open>, L<utf8>, L<bytes>,
1248 L<perlretut>, L<perlvar/"${^WIDE_SYSTEM_CALLS}">
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