3 perlunicode - Unicode support in Perl
7 If you haven't already, before reading this document, you should become
8 familiar with both L<perlunitut> and L<perluniintro>.
10 Unicode aims to B<UNI>-fy the en-B<CODE>-ings of all the world's
11 character sets into a single Standard. For quite a few of the various
12 coding standards that existed when Unicode was first created, converting
13 from each to Unicode essentially meant adding a constant to each code
14 point in the original standard, and converting back meant just
15 subtracting that same constant. For ASCII and ISO-8859-1, the constant
16 is 0. For ISO-8859-5, (Cyrillic) the constant is 864; for Hebrew
17 (ISO-8859-8), it's 1488; Thai (ISO-8859-11), 3424; and so forth. This
18 made it easy to do the conversions, and facilitated the adoption of
21 And it worked; nowadays, those legacy standards are rarely used. Most
22 everyone uses Unicode.
24 Unicode is a comprehensive standard. It specifies many things outside
25 the scope of Perl, such as how to display sequences of characters. For
26 a full discussion of all aspects of Unicode, see
27 L<http://www.unicode.org>.
29 =head2 Important Caveats
31 Even though some of this section may not be understandable to you on
32 first reading, we think it's important enough to highlight some of the
33 gotchas before delving further, so here goes:
35 Unicode support is an extensive requirement. While Perl does not
36 implement the Unicode standard or the accompanying technical reports
37 from cover to cover, Perl does support many Unicode features.
39 Also, the use of Unicode may present security issues that aren't obvious.
40 Read L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
44 =item Safest if you C<use feature 'unicode_strings'>
46 In order to preserve backward compatibility, Perl does not turn
47 on full internal Unicode support unless the pragma
48 L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
49 is specified. (This is automatically
50 selected if you S<C<use 5.012>> or higher.) Failure to do this can
51 trigger unexpected surprises. See L</The "Unicode Bug"> below.
53 This pragma doesn't affect I/O. Nor does it change the internal
54 representation of strings, only their interpretation. There are still
55 several places where Unicode isn't fully supported, such as in
58 =item Input and Output Layers
60 Use the C<:encoding(...)> layer to read from and write to
61 filehandles using the specified encoding. (See L<open>.)
63 =item You should convert your non-ASCII, non-UTF-8 Perl scripts to be
68 =item C<use utf8> still needed to enable L<UTF-8|/Unicode Encodings> in scripts
70 If your Perl script is itself encoded in L<UTF-8|/Unicode Encodings>,
71 the S<C<use utf8>> pragma must be explicitly included to enable
72 recognition of that (in string or regular expression literals, or in
73 identifier names). B<This is the only time when an explicit S<C<use
74 utf8>> is needed.> (See L<utf8>).
76 =item C<BOM>-marked scripts and L<UTF-16|/Unicode Encodings> scripts autodetected
78 However, if a Perl script begins with the Unicode C<BOM> (UTF-16LE,
79 UTF16-BE, or UTF-8), or if the script looks like non-C<BOM>-marked
80 UTF-16 of either endianness, Perl will correctly read in the script as
81 the appropriate Unicode encoding. (C<BOM>-less UTF-8 cannot be
82 effectively recognized or differentiated from ISO 8859-1 or other
87 =head2 Byte and Character Semantics
89 Before Unicode, most encodings used 8 bits (a single byte) to encode
90 each character. Thus a character was a byte, and a byte was a
91 character, and there could be only 256 or fewer possible characters.
92 "Byte Semantics" in the title of this section refers to
93 this behavior. There was no need to distinguish between "Byte" and
96 Then along comes Unicode which has room for over a million characters
97 (and Perl allows for even more). This means that a character may
98 require more than a single byte to represent it, and so the two terms
99 are no longer equivalent. What matter are the characters as whole
100 entities, and not usually the bytes that comprise them. That's what the
101 term "Character Semantics" in the title of this section refers to.
103 Perl had to change internally to decouple "bytes" from "characters".
104 It is important that you too change your ideas, if you haven't already,
105 so that "byte" and "character" no longer mean the same thing in your
108 The basic building block of Perl strings has always been a "character".
109 The changes basically come down to that the implementation no longer
110 thinks that a character is always just a single byte.
112 There are various things to note:
118 String handling functions, for the most part, continue to operate in
119 terms of characters. C<length()>, for example, returns the number of
120 characters in a string, just as before. But that number no longer is
121 necessarily the same as the number of bytes in the string (there may be
122 more bytes than characters). The other such functions include
123 C<chop()>, C<chomp()>, C<substr()>, C<pos()>, C<index()>, C<rindex()>,
124 C<sort()>, C<sprintf()>, and C<write()>.
132 the bit-oriented C<vec>
138 the byte-oriented C<pack>/C<unpack> C<"C"> format
140 However, the C<W> specifier does operate on whole characters, as does the
145 some operators that interact with the platform's operating system
147 Operators dealing with filenames are examples.
151 when the functions are called from within the scope of the
152 S<C<L<use bytes|bytes>>> pragma
154 Likely, you should use this only for debugging anyway.
160 Strings--including hash keys--and regular expression patterns may
161 contain characters that have ordinal values larger than 255.
163 If you use a Unicode editor to edit your program, Unicode characters may
164 occur directly within the literal strings in UTF-8 encoding, or UTF-16.
165 (The former requires a C<BOM> or C<use utf8>, the latter requires a C<BOM>.)
167 L<perluniintro/Creating Unicode> gives other ways to place non-ASCII
168 characters in your strings.
172 The C<chr()> and C<ord()> functions work on whole characters.
176 Regular expressions match whole characters. For example, C<"."> matches
177 a whole character instead of only a single byte.
181 The C<tr///> operator translates whole characters. (Note that the
182 C<tr///CU> functionality has been removed. For similar functionality to
183 that, see C<pack('U0', ...)> and C<pack('C0', ...)>).
187 C<scalar reverse()> reverses by character rather than by byte.
191 The bit string operators, C<& | ^ ~> and (starting in v5.22)
192 C<&. |. ^. ~.> can operate on characters that don't fit into a byte.
193 However, the current behavior is likely to change. You should not use
194 these operators on strings that are encoded in UTF-8. If you're not
195 sure about the encoding of a string, downgrade it before using any of
196 these operators; you can use
197 L<C<utf8::utf8_downgrade()>|utf8/Utility functions>.
201 The bottom line is that Perl has always practiced "Character Semantics",
202 but with the advent of Unicode, that is now different than "Byte
205 =head2 ASCII Rules versus Unicode Rules
207 Before Unicode, when a character was a byte was a character,
208 Perl knew only about the 128 characters defined by ASCII, code points 0
209 through 127 (except for under S<C<use locale>>). That left the code
210 points 128 to 255 as unassigned, and available for whatever use a
211 program might want. The only semantics they have is their ordinal
212 numbers, and that they are members of none of the non-negative character
213 classes. None are considered to match C<\w> for example, but all match
216 Unicode, of course, assigns each of those code points a particular
217 meaning (along with ones above 255). To preserve backward
218 compatibility, Perl only uses the Unicode meanings when there is some
219 indication that Unicode is what is intended; otherwise the non-ASCII
220 code points remain treated as if they are unassigned.
222 Here are the ways that Perl knows that a string should be treated as
229 Within the scope of S<C<use utf8>>
231 If the whole program is Unicode (signified by using 8-bit B<U>nicode
232 B<T>ransformation B<F>ormat), then all strings within it must be
238 L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
240 This pragma was created so you can explicitly tell Perl that operations
241 executed within its scope are to use Unicode rules. More operations are
242 affected with newer perls. See L</The "Unicode Bug">.
246 Within the scope of S<C<use 5.012>> or higher
248 This implicitly turns on S<C<use feature 'unicode_strings'>>.
253 L<S<C<use locale 'not_characters'>>|perllocale/Unicode and UTF-8>,
254 or L<S<C<use locale>>|perllocale> and the current
255 locale is a UTF-8 locale.
257 The former is defined to imply Unicode handling; and the latter
258 indicates a Unicode locale, hence a Unicode interpretation of all
263 When the string contains a Unicode-only code point
265 Perl has never accepted code points above 255 without them being
266 Unicode, so their use implies Unicode for the whole string.
270 When the string contains a Unicode named code point C<\N{...}>
272 The C<\N{...}> construct explicitly refers to a Unicode code point,
273 even if it is one that is also in ASCII. Therefore the string
274 containing it must be Unicode.
278 When the string has come from an external source marked as
281 The L<C<-C>|perlrun/-C [numberE<sol>list]> command line option can
282 specify that certain inputs to the program are Unicode, and the values
283 of this can be read by your Perl code, see L<perlvar/"${^UNICODE}">.
285 =item * When the string has been upgraded to UTF-8
287 The function L<C<utf8::utf8_upgrade()>|utf8/Utility functions>
288 can be explicitly used to permanently (unless a subsequent
289 C<utf8::utf8_downgrade()> is called) cause a string to be treated as
292 =item * There are additional methods for regular expression patterns
294 A pattern that is compiled with the C<< /u >> or C<< /a >> modifiers is
295 treated as Unicode (though there are some restrictions with C<< /a >>).
296 Under the C<< /d >> and C<< /l >> modifiers, there are several other
297 indications for Unicode; see L<perlre/Character set modifiers>.
301 Note that all of the above are overridden within the scope of
302 C<L<use bytes|bytes>>; but you should be using this pragma only for
305 Note also that some interactions with the platform's operating system
306 never use Unicode rules.
308 When Unicode rules are in effect:
314 Case translation operators use the Unicode case translation tables.
316 Note that C<uc()>, or C<\U> in interpolated strings, translates to
317 uppercase, while C<ucfirst>, or C<\u> in interpolated strings,
318 translates to titlecase in languages that make the distinction (which is
319 equivalent to uppercase in languages without the distinction).
321 There is a CPAN module, C<L<Unicode::Casing>>, which allows you to
322 define your own mappings to be used in C<lc()>, C<lcfirst()>, C<uc()>,
323 C<ucfirst()>, and C<fc> (or their double-quoted string inlined versions
324 such as C<\U>). (Prior to Perl 5.16, this functionality was partially
325 provided in the Perl core, but suffered from a number of insurmountable
326 drawbacks, so the CPAN module was written instead.)
330 Character classes in regular expressions match based on the character
331 properties specified in the Unicode properties database.
333 C<\w> can be used to match a Japanese ideograph, for instance; and
334 C<[[:digit:]]> a Bengali number.
338 Named Unicode properties, scripts, and block ranges may be used (like
339 bracketed character classes) by using the C<\p{}> "matches property"
340 construct and the C<\P{}> negation, "doesn't match property".
342 See L</"Unicode Character Properties"> for more details.
344 You can define your own character properties and use them
345 in the regular expression with the C<\p{}> or C<\P{}> construct.
346 See L</"User-Defined Character Properties"> for more details.
350 =head2 Extended Grapheme Clusters (Logical characters)
352 Consider a character, say C<H>. It could appear with various marks around it,
353 such as an acute accent, or a circumflex, or various hooks, circles, arrows,
354 I<etc.>, above, below, to one side or the other, I<etc>. There are many
355 possibilities among the world's languages. The number of combinations is
356 astronomical, and if there were a character for each combination, it would
357 soon exhaust Unicode's more than a million possible characters. So Unicode
358 took a different approach: there is a character for the base C<H>, and a
359 character for each of the possible marks, and these can be variously combined
360 to get a final logical character. So a logical character--what appears to be a
361 single character--can be a sequence of more than one individual characters.
362 The Unicode standard calls these "extended grapheme clusters" (which
363 is an improved version of the no-longer much used "grapheme cluster");
364 Perl furnishes the C<\X> regular expression construct to match such
365 sequences in their entirety.
367 But Unicode's intent is to unify the existing character set standards and
368 practices, and several pre-existing standards have single characters that
369 mean the same thing as some of these combinations, like ISO-8859-1,
370 which has quite a few of them. For example, C<"LATIN CAPITAL LETTER E
371 WITH ACUTE"> was already in this standard when Unicode came along.
372 Unicode therefore added it to its repertoire as that single character.
373 But this character is considered by Unicode to be equivalent to the
374 sequence consisting of the character C<"LATIN CAPITAL LETTER E">
375 followed by the character C<"COMBINING ACUTE ACCENT">.
377 C<"LATIN CAPITAL LETTER E WITH ACUTE"> is called a "pre-composed"
378 character, and its equivalence with the "E" and the "COMBINING ACCENT"
379 sequence is called canonical equivalence. All pre-composed characters
380 are said to have a decomposition (into the equivalent sequence), and the
381 decomposition type is also called canonical. A string may be comprised
382 as much as possible of precomposed characters, or it may be comprised of
383 entirely decomposed characters. Unicode calls these respectively,
384 "Normalization Form Composed" (NFC) and "Normalization Form Decomposed".
385 The C<L<Unicode::Normalize>> module contains functions that convert
386 between the two. A string may also have both composed characters and
387 decomposed characters; this module can be used to make it all one or the
390 You may be presented with strings in any of these equivalent forms.
391 There is currently nothing in Perl 5 that ignores the differences. So
392 you'll have to specially hanlde it. The usual advice is to convert your
393 inputs to C<NFD> before processing further.
395 For more detailed information, see L<http://unicode.org/reports/tr15/>.
397 =head2 Unicode Character Properties
399 (The only time that Perl considers a sequence of individual code
400 points as a single logical character is in the C<\X> construct, already
401 mentioned above. Therefore "character" in this discussion means a single
404 Very nearly all Unicode character properties are accessible through
405 regular expressions by using the C<\p{}> "matches property" construct
406 and the C<\P{}> "doesn't match property" for its negation.
408 For instance, C<\p{Uppercase}> matches any single character with the Unicode
409 C<"Uppercase"> property, while C<\p{L}> matches any character with a
410 C<General_Category> of C<"L"> (letter) property (see
411 L</General_Category> below). Brackets are not
412 required for single letter property names, so C<\p{L}> is equivalent to C<\pL>.
414 More formally, C<\p{Uppercase}> matches any single character whose Unicode
415 C<Uppercase> property value is C<True>, and C<\P{Uppercase}> matches any character
416 whose C<Uppercase> property value is C<False>, and they could have been written as
417 C<\p{Uppercase=True}> and C<\p{Uppercase=False}>, respectively.
419 This formality is needed when properties are not binary; that is, if they can
420 take on more values than just C<True> and C<False>. For example, the
421 C<Bidi_Class> property (see L</"Bidirectional Character Types"> below),
422 can take on several different
423 values, such as C<Left>, C<Right>, C<Whitespace>, and others. To match these, one needs
424 to specify both the property name (C<Bidi_Class>), AND the value being
426 (C<Left>, C<Right>, I<etc.>). This is done, as in the examples above, by having the
427 two components separated by an equal sign (or interchangeably, a colon), like
428 C<\p{Bidi_Class: Left}>.
430 All Unicode-defined character properties may be written in these compound forms
431 of C<\p{I<property>=I<value>}> or C<\p{I<property>:I<value>}>, but Perl provides some
432 additional properties that are written only in the single form, as well as
433 single-form short-cuts for all binary properties and certain others described
434 below, in which you may omit the property name and the equals or colon
437 Most Unicode character properties have at least two synonyms (or aliases if you
438 prefer): a short one that is easier to type and a longer one that is more
439 descriptive and hence easier to understand. Thus the C<"L"> and
440 C<"Letter"> properties above are equivalent and can be used
441 interchangeably. Likewise, C<"Upper"> is a synonym for C<"Uppercase">,
442 and we could have written C<\p{Uppercase}> equivalently as C<\p{Upper}>.
443 Also, there are typically various synonyms for the values the property
444 can be. For binary properties, C<"True"> has 3 synonyms: C<"T">,
445 C<"Yes">, and C<"Y">; and C<"False"> has correspondingly C<"F">,
446 C<"No">, and C<"N">. But be careful. A short form of a value for one
447 property may not mean the same thing as the same short form for another.
448 Thus, for the C<L</General_Category>> property, C<"L"> means
449 C<"Letter">, but for the L<C<Bidi_Class>|/Bidirectional Character Types>
450 property, C<"L"> means C<"Left">. A complete list of properties and
451 synonyms is in L<perluniprops>.
453 Upper/lower case differences in property names and values are irrelevant;
454 thus C<\p{Upper}> means the same thing as C<\p{upper}> or even C<\p{UpPeR}>.
455 Similarly, you can add or subtract underscores anywhere in the middle of a
456 word, so that these are also equivalent to C<\p{U_p_p_e_r}>. And white space
457 is irrelevant adjacent to non-word characters, such as the braces and the equals
458 or colon separators, so C<\p{ Upper }> and C<\p{ Upper_case : Y }> are
459 equivalent to these as well. In fact, white space and even
460 hyphens can usually be added or deleted anywhere. So even C<\p{ Up-per case = Yes}> is
461 equivalent. All this is called "loose-matching" by Unicode. The few places
462 where stricter matching is used is in the middle of numbers, and in the Perl
463 extension properties that begin or end with an underscore. Stricter matching
464 cares about white space (except adjacent to non-word characters),
465 hyphens, and non-interior underscores.
467 You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret
468 (C<^>) between the first brace and the property name: C<\p{^Tamil}> is
469 equal to C<\P{Tamil}>.
471 Almost all properties are immune to case-insensitive matching. That is,
472 adding a C</i> regular expression modifier does not change what they
473 match. There are two sets that are affected.
477 and C<Titlecase_Letter>,
478 all of which match C<Cased_Letter> under C</i> matching.
479 And the second set is
483 all of which match C<Cased> under C</i> matching.
484 This set also includes its subsets C<PosixUpper> and C<PosixLower> both
485 of which under C</i> match C<PosixAlpha>.
486 (The difference between these sets is that some things, such as Roman
487 numerals, come in both upper and lower case so they are C<Cased>, but
488 aren't considered letters, so they aren't C<Cased_Letter>'s.)
490 See L</Beyond Unicode code points> for special considerations when
491 matching Unicode properties against non-Unicode code points.
493 =head3 B<General_Category>
495 Every Unicode character is assigned a general category, which is the "most
496 usual categorization of a character" (from
497 L<http://www.unicode.org/reports/tr44>).
499 The compound way of writing these is like C<\p{General_Category=Number}>
500 (short: C<\p{gc:n}>). But Perl furnishes shortcuts in which everything up
501 through the equal or colon separator is omitted. So you can instead just write
504 Here are the short and long forms of the values the C<General Category> property
510 LC, L& Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
523 Nd Decimal_Number (also Digit)
527 P Punctuation (also Punct)
528 Pc Connector_Punctuation
532 Pi Initial_Punctuation
533 (may behave like Ps or Pe depending on usage)
535 (may behave like Ps or Pe depending on usage)
547 Zp Paragraph_Separator
550 Cc Control (also Cntrl)
556 Single-letter properties match all characters in any of the
557 two-letter sub-properties starting with the same letter.
558 C<LC> and C<L&> are special: both are aliases for the set consisting of everything matched by C<Ll>, C<Lu>, and C<Lt>.
560 =head3 B<Bidirectional Character Types>
562 Because scripts differ in their directionality (Hebrew and Arabic are
563 written right to left, for example) Unicode supplies a C<Bidi_Class> property.
564 Some of the values this property can have are:
569 LRE Left-to-Right Embedding
570 LRO Left-to-Right Override
573 RLE Right-to-Left Embedding
574 RLO Right-to-Left Override
575 PDF Pop Directional Format
577 ES European Separator
578 ET European Terminator
583 B Paragraph Separator
588 This property is always written in the compound form.
589 For example, C<\p{Bidi_Class:R}> matches characters that are normally
590 written right to left. Unlike the
591 C<L</General_Category>> property, this
592 property can have more values added in a future Unicode release. Those
593 listed above comprised the complete set for many Unicode releases, but
594 others were added in Unicode 6.3; you can always find what the
595 current ones are in in L<perluniprops>. And
596 L<http://www.unicode.org/reports/tr9/> describes how to use them.
600 The world's languages are written in many different scripts. This sentence
601 (unless you're reading it in translation) is written in Latin, while Russian is
602 written in Cyrillic, and Greek is written in, well, Greek; Japanese mainly in
603 Hiragana or Katakana. There are many more.
605 The Unicode C<Script> and C<Script_Extensions> properties give what script a
606 given character is in. Either property can be specified with the
608 C<\p{Script=Hebrew}> (short: C<\p{sc=hebr}>), or
609 C<\p{Script_Extensions=Javanese}> (short: C<\p{scx=java}>).
610 In addition, Perl furnishes shortcuts for all
611 C<Script> property names. You can omit everything up through the equals
612 (or colon), and simply write C<\p{Latin}> or C<\P{Cyrillic}>.
613 (This is not true for C<Script_Extensions>, which is required to be
614 written in the compound form.)
616 The difference between these two properties involves characters that are
617 used in multiple scripts. For example the digits '0' through '9' are
618 used in many parts of the world. These are placed in a script named
619 C<Common>. Other characters are used in just a few scripts. For
620 example, the C<"KATAKANA-HIRAGANA DOUBLE HYPHEN"> is used in both Japanese
621 scripts, Katakana and Hiragana, but nowhere else. The C<Script>
622 property places all characters that are used in multiple scripts in the
623 C<Common> script, while the C<Script_Extensions> property places those
624 that are used in only a few scripts into each of those scripts; while
625 still using C<Common> for those used in many scripts. Thus both these
628 "0" =~ /\p{sc=Common}/ # Matches
629 "0" =~ /\p{scx=Common}/ # Matches
631 and only the first of these match:
633 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common} # Matches
634 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match
636 And only the last two of these match:
638 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana} # No match
639 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana} # No match
640 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
641 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches
643 C<Script_Extensions> is thus an improved C<Script>, in which there are
644 fewer characters in the C<Common> script, and correspondingly more in
645 other scripts. It is new in Unicode version 6.0, and its data are likely
646 to change significantly in later releases, as things get sorted out.
647 New code should probably be using C<Script_Extensions> and not plain
650 (Actually, besides C<Common>, the C<Inherited> script, contains
651 characters that are used in multiple scripts. These are modifier
652 characters which inherit the script value
653 of the controlling character. Some of these are used in many scripts,
654 and so go into C<Inherited> in both C<Script> and C<Script_Extensions>.
655 Others are used in just a few scripts, so are in C<Inherited> in
656 C<Script>, but not in C<Script_Extensions>.)
658 It is worth stressing that there are several different sets of digits in
659 Unicode that are equivalent to 0-9 and are matchable by C<\d> in a
660 regular expression. If they are used in a single language only, they
661 are in that language's C<Script> and C<Script_Extension>. If they are
662 used in more than one script, they will be in C<sc=Common>, but only
663 if they are used in many scripts should they be in C<scx=Common>.
665 A complete list of scripts and their shortcuts is in L<perluniprops>.
667 =head3 B<Use of the C<"Is"> Prefix>
669 For backward compatibility (with Perl 5.6), all properties writable
670 without using the compound form mentioned
671 so far may have C<Is> or C<Is_> prepended to their name, so C<\P{Is_Lu}>, for
672 example, is equal to C<\P{Lu}>, and C<\p{IsScript:Arabic}> is equal to
677 In addition to B<scripts>, Unicode also defines B<blocks> of
678 characters. The difference between scripts and blocks is that the
679 concept of scripts is closer to natural languages, while the concept
680 of blocks is more of an artificial grouping based on groups of Unicode
681 characters with consecutive ordinal values. For example, the C<"Basic Latin">
682 block is all the characters whose ordinals are between 0 and 127, inclusive; in
683 other words, the ASCII characters. The C<"Latin"> script contains some letters
684 from this as well as several other blocks, like C<"Latin-1 Supplement">,
685 C<"Latin Extended-A">, I<etc.>, but it does not contain all the characters from
686 those blocks. It does not, for example, contain the digits 0-9, because
687 those digits are shared across many scripts, and hence are in the
690 For more about scripts versus blocks, see UAX#24 "Unicode Script Property":
691 L<http://www.unicode.org/reports/tr24>
693 The C<Script> or C<Script_Extensions> properties are likely to be the
694 ones you want to use when processing
695 natural language; the C<Block> property may occasionally be useful in working
696 with the nuts and bolts of Unicode.
698 Block names are matched in the compound form, like C<\p{Block: Arrows}> or
699 C<\p{Blk=Hebrew}>. Unlike most other properties, only a few block names have a
700 Unicode-defined short name. But Perl does provide a (slight, no longer
701 recommended) shortcut: You can say, for example C<\p{In_Arrows}> or
704 For backwards compatibility, the C<In> prefix may be
705 omitted if there is no naming conflict with a script or any other
706 property, and you can even use an C<Is> prefix instead in those cases.
707 But don't do this for new code because your code could break in new
708 releases, and this has already happened: There was a time in very
709 early Unicode releases when C<\p{Hebrew}> would have matched the
710 I<block> Hebrew; now it doesn't.
712 Using the C<In> prefix avoids this ambiguity, so far. But new versions
713 of Unicode continue to add new properties whose names begin with C<In>.
714 There is a possibility that one of them someday will conflict with your
715 usage. Since this is just a Perl extension, Unicode's name will take
716 precedence and your code will become broken. Also, Unicode is free to
717 add a script whose name begins with C<In>; that would cause problems.
719 So it's clearer and best to use the compound form when specifying
720 blocks. And be sure that is what you really really want to do. In most
721 cases scripts are what you want instead.
723 A complete list of blocks and their shortcuts is in L<perluniprops>.
725 =head3 B<Other Properties>
727 There are many more properties than the very basic ones described here.
728 A complete list is in L<perluniprops>.
730 Unicode defines all its properties in the compound form, so all single-form
731 properties are Perl extensions. Most of these are just synonyms for the
732 Unicode ones, but some are genuine extensions, including several that are in
733 the compound form. And quite a few of these are actually recommended by Unicode
734 (in L<http://www.unicode.org/reports/tr18>).
736 This section gives some details on all extensions that aren't just
737 synonyms for compound-form Unicode properties
738 (for those properties, you'll have to refer to the
739 L<Unicode Standard|http://www.unicode.org/reports/tr44>.
745 This matches every possible code point. It is equivalent to C<qr/./s>.
746 Unlike all the other non-user-defined C<\p{}> property matches, no
747 warning is ever generated if this is property is matched against a
748 non-Unicode code point (see L</Beyond Unicode code points> below).
750 =item B<C<\p{Alnum}>>
752 This matches any C<\p{Alphabetic}> or C<\p{Decimal_Number}> character.
756 This matches any of the 1_114_112 Unicode code points. It is a synonym
759 =item B<C<\p{ASCII}>>
761 This matches any of the 128 characters in the US-ASCII character set,
762 which is a subset of Unicode.
764 =item B<C<\p{Assigned}>>
766 This matches any assigned code point; that is, any code point whose L<general
767 category|/General_Category> is not C<Unassigned> (or equivalently, not C<Cn>).
769 =item B<C<\p{Blank}>>
771 This is the same as C<\h> and C<\p{HorizSpace}>: A character that changes the
772 spacing horizontally.
774 =item B<C<\p{Decomposition_Type: Non_Canonical}>> (Short: C<\p{Dt=NonCanon}>)
776 Matches a character that has a non-canonical decomposition.
778 The L</Extended Grapheme Clusters (Logical characters)> section above
779 talked about canonical decompositions. However, many more characters
780 have a different type of decomposition, a "compatible" or
781 "non-canonical" decomposition. The sequences that form these
782 decompositions are not considered canonically equivalent to the
783 pre-composed character. An example is the C<"SUPERSCRIPT ONE">. It is
784 somewhat like a regular digit 1, but not exactly; its decomposition into
785 the digit 1 is called a "compatible" decomposition, specifically a
786 "super" decomposition. There are several such compatibility
787 decompositions (see L<http://www.unicode.org/reports/tr44>), including
788 one called "compat", which means some miscellaneous type of
789 decomposition that doesn't fit into the other decomposition categories
790 that Unicode has chosen.
792 Note that most Unicode characters don't have a decomposition, so their
793 decomposition type is C<"None">.
795 For your convenience, Perl has added the C<Non_Canonical> decomposition
796 type to mean any of the several compatibility decompositions.
798 =item B<C<\p{Graph}>>
800 Matches any character that is graphic. Theoretically, this means a character
801 that on a printer would cause ink to be used.
803 =item B<C<\p{HorizSpace}>>
805 This is the same as C<\h> and C<\p{Blank}>: a character that changes the
806 spacing horizontally.
810 This is a synonym for C<\p{Present_In=*}>
812 =item B<C<\p{PerlSpace}>>
814 This is the same as C<\s>, restricted to ASCII, namely C<S<[ \f\n\r\t]>>
815 and starting in Perl v5.18, a vertical tab.
817 Mnemonic: Perl's (original) space
819 =item B<C<\p{PerlWord}>>
821 This is the same as C<\w>, restricted to ASCII, namely C<[A-Za-z0-9_]>
823 Mnemonic: Perl's (original) word.
825 =item B<C<\p{Posix...}>>
827 There are several of these, which are equivalents, using the C<\p{}>
828 notation, for Posix classes and are described in
829 L<perlrecharclass/POSIX Character Classes>.
831 =item B<C<\p{Present_In: *}>> (Short: C<\p{In=*}>)
833 This property is used when you need to know in what Unicode version(s) a
836 The "*" above stands for some two digit Unicode version number, such as
837 C<1.1> or C<4.0>; or the "*" can also be C<Unassigned>. This property will
838 match the code points whose final disposition has been settled as of the
839 Unicode release given by the version number; C<\p{Present_In: Unassigned}>
840 will match those code points whose meaning has yet to be assigned.
842 For example, C<U+0041> C<"LATIN CAPITAL LETTER A"> was present in the very first
843 Unicode release available, which is C<1.1>, so this property is true for all
844 valid "*" versions. On the other hand, C<U+1EFF> was not assigned until version
845 5.1 when it became C<"LATIN SMALL LETTER Y WITH LOOP">, so the only "*" that
846 would match it are 5.1, 5.2, and later.
848 Unicode furnishes the C<Age> property from which this is derived. The problem
849 with Age is that a strict interpretation of it (which Perl takes) has it
850 matching the precise release a code point's meaning is introduced in. Thus
851 C<U+0041> would match only 1.1; and C<U+1EFF> only 5.1. This is not usually what
854 Some non-Perl implementations of the Age property may change its meaning to be
855 the same as the Perl C<Present_In> property; just be aware of that.
857 Another confusion with both these properties is that the definition is not
858 that the code point has been I<assigned>, but that the meaning of the code point
859 has been I<determined>. This is because 66 code points will always be
860 unassigned, and so the C<Age> for them is the Unicode version in which the decision
861 to make them so was made. For example, C<U+FDD0> is to be permanently
862 unassigned to a character, and the decision to do that was made in version 3.1,
863 so C<\p{Age=3.1}> matches this character, as also does C<\p{Present_In: 3.1}> and up.
865 =item B<C<\p{Print}>>
867 This matches any character that is graphical or blank, except controls.
869 =item B<C<\p{SpacePerl}>>
871 This is the same as C<\s>, including beyond ASCII.
873 Mnemonic: Space, as modified by Perl. (It doesn't include the vertical tab
874 until v5.18, which both the Posix standard and Unicode consider white space.)
876 =item B<C<\p{Title}>> and B<C<\p{Titlecase}>>
878 Under case-sensitive matching, these both match the same code points as
879 C<\p{General Category=Titlecase_Letter}> (C<\p{gc=lt}>). The difference
880 is that under C</i> caseless matching, these match the same as
881 C<\p{Cased}>, whereas C<\p{gc=lt}> matches C<\p{Cased_Letter>).
883 =item B<C<\p{Unicode}>>
885 This matches any of the 1_114_112 Unicode code points.
888 =item B<C<\p{VertSpace}>>
890 This is the same as C<\v>: A character that changes the spacing vertically.
894 This is the same as C<\w>, including over 100_000 characters beyond ASCII.
896 =item B<C<\p{XPosix...}>>
898 There are several of these, which are the standard Posix classes
899 extended to the full Unicode range. They are described in
900 L<perlrecharclass/POSIX Character Classes>.
905 =head2 User-Defined Character Properties
907 You can define your own binary character properties by defining subroutines
908 whose names begin with C<"In"> or C<"Is">. (The experimental feature
909 L<perlre/(?[ ])> provides an alternative which allows more complex
910 definitions.) The subroutines can be defined in any
911 package. The user-defined properties can be used in the regular expression
912 C<\p{}> and C<\P{}> constructs; if you are using a user-defined property from a
913 package other than the one you are in, you must specify its package in the
914 C<\p{}> or C<\P{}> construct.
916 # assuming property Is_Foreign defined in Lang::
917 package main; # property package name required
918 if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
920 package Lang; # property package name not required
921 if ($txt =~ /\p{IsForeign}+/) { ... }
924 Note that the effect is compile-time and immutable once defined.
925 However, the subroutines are passed a single parameter, which is 0 if
926 case-sensitive matching is in effect and non-zero if caseless matching
927 is in effect. The subroutine may return different values depending on
928 the value of the flag, and one set of values will immutably be in effect
929 for all case-sensitive matches, and the other set for all case-insensitive
932 Note that if the regular expression is tainted, then Perl will die rather
933 than calling the subroutine when the name of the subroutine is
934 determined by the tainted data.
936 The subroutines must return a specially-formatted string, with one
937 or more newline-separated lines. Each line must be one of the following:
943 A single hexadecimal number denoting a code point to include.
947 Two hexadecimal numbers separated by horizontal whitespace (space or
948 tabular characters) denoting a range of code points to include.
952 Something to include, prefixed by C<"+">: a built-in character
953 property (prefixed by C<"utf8::">) or a fully qualified (including package
954 name) user-defined character property,
955 to represent all the characters in that property; two hexadecimal code
956 points for a range; or a single hexadecimal code point.
960 Something to exclude, prefixed by C<"-">: an existing character
961 property (prefixed by C<"utf8::">) or a fully qualified (including package
962 name) user-defined character property,
963 to represent all the characters in that property; two hexadecimal code
964 points for a range; or a single hexadecimal code point.
968 Something to negate, prefixed C<"!">: an existing character
969 property (prefixed by C<"utf8::">) or a fully qualified (including package
970 name) user-defined character property,
971 to represent all the characters in that property; two hexadecimal code
972 points for a range; or a single hexadecimal code point.
976 Something to intersect with, prefixed by C<"&">: an existing character
977 property (prefixed by C<"utf8::">) or a fully qualified (including package
978 name) user-defined character property,
979 for all the characters except the characters in the property; two
980 hexadecimal code points for a range; or a single hexadecimal code point.
984 For example, to define a property that covers both the Japanese
985 syllabaries (hiragana and katakana), you can define
994 Imagine that the here-doc end marker is at the beginning of the line.
995 Now you can use C<\p{InKana}> and C<\P{InKana}>.
997 You could also have used the existing block property names:
1006 Suppose you wanted to match only the allocated characters,
1007 not the raw block ranges: in other words, you want to remove
1008 the unassigned characters:
1018 The negation is useful for defining (surprise!) negated classes.
1028 This will match all non-Unicode code points, since every one of them is
1029 not in Kana. You can use intersection to exclude these, if desired, as
1030 this modified example shows:
1041 C<&utf8::Any> must be the last line in the definition.
1043 Intersection is used generally for getting the common characters matched
1044 by two (or more) classes. It's important to remember not to use C<"&"> for
1045 the first set; that would be intersecting with nothing, resulting in an
1048 Unlike non-user-defined C<\p{}> property matches, no warning is ever
1049 generated if these properties are matched against a non-Unicode code
1050 point (see L</Beyond Unicode code points> below).
1052 =head2 User-Defined Case Mappings (for serious hackers only)
1054 B<This feature has been removed as of Perl 5.16.>
1055 The CPAN module C<L<Unicode::Casing>> provides better functionality without
1056 the drawbacks that this feature had. If you are using a Perl earlier
1057 than 5.16, this feature was most fully documented in the 5.14 version of
1059 L<http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>
1061 =head2 Character Encodings for Input and Output
1065 =head2 Unicode Regular Expression Support Level
1067 The following list of Unicode supported features for regular expressions describes
1068 all features currently directly supported by core Perl. The references to "Level N"
1069 and the section numbers refer to the Unicode Technical Standard #18,
1070 "Unicode Regular Expressions", version 13, from August 2008.
1076 Level 1 - Basic Unicode Support
1078 RL1.1 Hex Notation - done [1]
1079 RL1.2 Properties - done [2][3]
1080 RL1.2a Compatibility Properties - done [4]
1081 RL1.3 Subtraction and Intersection - experimental [5]
1082 RL1.4 Simple Word Boundaries - done [6]
1083 RL1.5 Simple Loose Matches - done [7]
1084 RL1.6 Line Boundaries - MISSING [8][9]
1085 RL1.7 Supplementary Code Points - done [10]
1089 =item [1] C<\N{U+...}> and C<\x{...}>
1091 =item [2] C<\p{...}> C<\P{...}>
1093 =item [3] supports not only minimal list, but all Unicode character
1094 properties (see Unicode Character Properties above)
1096 =item [4] C<\d> C<\D> C<\s> C<\S> C<\w> C<\W> C<\X> C<[:I<prop>:]>
1099 =item [5] The experimental feature starting in v5.18 C<"(?[...])"> accomplishes
1102 See L<perlre/(?[ ])>. If you don't want to use an experimental
1103 feature, you can use one of the following:
1109 Regular expression look-ahead
1111 You can mimic class subtraction using lookahead.
1112 For example, what UTS#18 might write as
1114 [{Block=Greek}-[{UNASSIGNED}]]
1116 in Perl can be written as:
1118 (?!\p{Unassigned})\p{Block=Greek}
1119 (?=\p{Assigned})\p{Block=Greek}
1121 But in this particular example, you probably really want
1125 which will match assigned characters known to be part of the Greek script.
1129 CPAN module C<L<Unicode::Regex::Set>>
1131 It does implement the full UTS#18 grouping, intersection, union, and
1132 removal (subtraction) syntax.
1136 L</"User-Defined Character Properties">
1138 C<"+"> for union, C<"-"> for removal (set-difference), C<"&"> for intersection
1142 =item [6] C<\b> C<\B>
1145 Note that Perl does Full case-folding in matching, not Simple:
1147 For example C<U+1F88> is equivalent to C<U+1F00 U+03B9>, instead of just
1148 C<U+1F80>. This difference matters mainly for certain Greek capital
1149 letters with certain modifiers: the Full case-folding decomposes the
1150 letter, while the Simple case-folding would map it to a single
1154 Perl treats C<\n> as the start- and end-line delimiter. Unicode
1155 specifies more characters that should be so-interpreted.
1166 C<^> and C<$> in regular expression patterns are supposed to match all
1168 These characters also don't, but should, affect C<< <> >> C<$.>, and
1169 script line numbers.
1171 Also, lines should not be split within C<CRLF> (i.e. there is no
1172 empty line between C<\r> and C<\n>). For C<CRLF>, try the C<:crlf>
1173 layer (see L<PerlIO>).
1175 =item [9] But C<L<Unicode::LineBreak>> is available.
1177 This module supplies line breaking conformant with
1178 L<UAX#14 "Unicode Line Breaking Algorithm"|http://www.unicode.org/reports/tr14>.
1181 UTF-8/UTF-EBDDIC used in Perl allows not only C<U+10000> to
1182 C<U+10FFFF> but also beyond C<U+10FFFF>
1188 Level 2 - Extended Unicode Support
1190 RL2.1 Canonical Equivalents - MISSING [10][11]
1191 RL2.2 Default Grapheme Clusters - MISSING [12]
1192 RL2.3 Default Word Boundaries - DONE [14]
1193 RL2.4 Default Loose Matches - MISSING [15]
1194 RL2.5 Name Properties - DONE
1195 RL2.6 Wildcard Properties - MISSING
1197 [10] see UAX#15 "Unicode Normalization Forms"
1198 [11] have Unicode::Normalize but not integrated to regexes
1199 [12] have \X and \b{gcb} but we don't have a "Grapheme Cluster
1201 [14] see UAX#29, Word Boundaries
1202 [15] This is covered in Chapter 3.13 (in Unicode 6.0)
1206 Level 3 - Tailored Support
1208 RL3.1 Tailored Punctuation - MISSING
1209 RL3.2 Tailored Grapheme Clusters - MISSING [17][18]
1210 RL3.3 Tailored Word Boundaries - MISSING
1211 RL3.4 Tailored Loose Matches - MISSING
1212 RL3.5 Tailored Ranges - MISSING
1213 RL3.6 Context Matching - MISSING [19]
1214 RL3.7 Incremental Matches - MISSING
1215 ( RL3.8 Unicode Set Sharing )
1216 RL3.9 Possible Match Sets - MISSING
1217 RL3.10 Folded Matching - MISSING [20]
1218 RL3.11 Submatchers - MISSING
1220 [17] see UAX#10 "Unicode Collation Algorithms"
1221 [18] have Unicode::Collate but not integrated to regexes
1222 [19] have (?<=x) and (?=x), but look-aheads or look-behinds
1223 should see outside of the target substring
1224 [20] need insensitive matching for linguistic features other
1225 than case; for example, hiragana to katakana, wide and
1226 narrow, simplified Han to traditional Han (see UTR#30
1227 "Character Foldings")
1231 =head2 Unicode Encodings
1233 Unicode characters are assigned to I<code points>, which are abstract
1234 numbers. To use these numbers, various encodings are needed.
1242 UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
1243 encoding. In most of Perl's documentation, including elsewhere in this
1244 document, the term "UTF-8" means also "UTF-EBCDIC". But in this section,
1245 "UTF-8" refers only to the encoding used on ASCII platforms. It is a
1246 superset of 7-bit US-ASCII, so anything encoded in ASCII has the
1247 identical representation when encoded in UTF-8.
1249 The following table is from Unicode 3.2.
1251 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
1253 U+0000..U+007F 00..7F
1254 U+0080..U+07FF * C2..DF 80..BF
1255 U+0800..U+0FFF E0 * A0..BF 80..BF
1256 U+1000..U+CFFF E1..EC 80..BF 80..BF
1257 U+D000..U+D7FF ED 80..9F 80..BF
1258 U+D800..U+DFFF +++++ utf16 surrogates, not legal utf8 +++++
1259 U+E000..U+FFFF EE..EF 80..BF 80..BF
1260 U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF
1261 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
1262 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
1264 Note the gaps marked by "*" before several of the byte entries above. These are
1265 caused by legal UTF-8 avoiding non-shortest encodings: it is technically
1266 possible to UTF-8-encode a single code point in different ways, but that is
1267 explicitly forbidden, and the shortest possible encoding should always be used
1268 (and that is what Perl does).
1270 Another way to look at it is via bits:
1272 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
1275 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
1276 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
1277 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
1279 As you can see, the continuation bytes all begin with C<"10">, and the
1280 leading bits of the start byte tell how many bytes there are in the
1283 The original UTF-8 specification allowed up to 6 bytes, to allow
1284 encoding of numbers up to C<0x7FFF_FFFF>. Perl continues to allow those,
1285 and has extended that up to 13 bytes to encode code points up to what
1286 can fit in a 64-bit word. However, Perl will warn if you output any of
1287 these as being non-portable; and under strict UTF-8 input protocols,
1294 Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
1295 This means that all the basic characters (which includes all
1296 those that have ASCII equivalents (like C<"A">, C<"0">, C<"%">, I<etc.>)
1297 are the same in both EBCDIC and UTF-EBCDIC.)
1299 UTF-EBCDIC is used on EBCDIC platforms. The largest Unicode code points
1300 take 5 bytes to represent (instead of 4 in UTF-8), and Perl extends it
1301 to a maximum of 7 bytes to encode pode points up to what can fit in a
1302 32-bit word (instead of 13 bytes and a 64-bit word in UTF-8).
1306 UTF-16, UTF-16BE, UTF-16LE, Surrogates, and C<BOM>'s (Byte Order Marks)
1308 The followings items are mostly for reference and general Unicode
1309 knowledge, Perl doesn't use these constructs internally.
1311 Like UTF-8, UTF-16 is a variable-width encoding, but where
1312 UTF-8 uses 8-bit code units, UTF-16 uses 16-bit code units.
1313 All code points occupy either 2 or 4 bytes in UTF-16: code points
1314 C<U+0000..U+FFFF> are stored in a single 16-bit unit, and code
1315 points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is
1316 using I<surrogates>, the first 16-bit unit being the I<high
1317 surrogate>, and the second being the I<low surrogate>.
1319 Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
1320 range of Unicode code points in pairs of 16-bit units. The I<high
1321 surrogates> are the range C<U+D800..U+DBFF> and the I<low surrogates>
1322 are the range C<U+DC00..U+DFFF>. The surrogate encoding is
1324 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1325 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
1329 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
1331 Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
1332 itself can be used for in-memory computations, but if storage or
1333 transfer is required either UTF-16BE (big-endian) or UTF-16LE
1334 (little-endian) encodings must be chosen.
1336 This introduces another problem: what if you just know that your data
1337 is UTF-16, but you don't know which endianness? Byte Order Marks, or
1338 C<BOM>'s, are a solution to this. A special character has been reserved
1339 in Unicode to function as a byte order marker: the character with the
1340 code point C<U+FEFF> is the C<BOM>.
1342 The trick is that if you read a C<BOM>, you will know the byte order,
1343 since if it was written on a big-endian platform, you will read the
1344 bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
1345 you will read the bytes C<0xFF 0xFE>. (And if the originating platform
1346 was writing in ASCII platform UTF-8, you will read the bytes
1349 The way this trick works is that the character with the code point
1350 C<U+FFFE> is not supposed to be in input streams, so the
1351 sequence of bytes C<0xFF 0xFE> is unambiguously "C<BOM>, represented in
1352 little-endian format" and cannot be C<U+FFFE>, represented in big-endian
1355 Surrogates have no meaning in Unicode outside their use in pairs to
1356 represent other code points. However, Perl allows them to be
1357 represented individually internally, for example by saying
1358 C<chr(0xD801)>, so that all code points, not just those valid for open
1360 representable. Unicode does define semantics for them, such as their
1361 C<L</General_Category>> is C<"Cs">. But because their use is somewhat dangerous,
1362 Perl will warn (using the warning category C<"surrogate">, which is a
1363 sub-category of C<"utf8">) if an attempt is made
1364 to do things like take the lower case of one, or match
1365 case-insensitively, or to output them. (But don't try this on Perls
1370 UTF-32, UTF-32BE, UTF-32LE
1372 The UTF-32 family is pretty much like the UTF-16 family, except that
1373 the units are 32-bit, and therefore the surrogate scheme is not
1374 needed. UTF-32 is a fixed-width encoding. The C<BOM> signatures are
1375 C<0x00 0x00 0xFE 0xFF> for BE and C<0xFF 0xFE 0x00 0x00> for LE.
1381 Legacy, fixed-width encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
1382 encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
1383 because it does not use surrogates. UCS-4 is a 32-bit encoding,
1384 functionally identical to UTF-32 (the difference being that
1385 UCS-4 forbids neither surrogates nor code points larger than C<0x10_FFFF>).
1391 A seven-bit safe (non-eight-bit) encoding, which is useful if the
1392 transport or storage is not eight-bit safe. Defined by RFC 2152.
1396 =head2 Noncharacter code points
1398 66 code points are set aside in Unicode as "noncharacter code points".
1399 These all have the C<Unassigned> (C<Cn>) C<L</General_Category>>, and
1400 no character will ever be assigned to any of them. They are the 32 code
1401 points between C<U+FDD0> and C<U+FDEF> inclusive, and the 34 code
1412 Until Unicode 7.0, the noncharacters were "B<forbidden> for use in open
1413 interchange of Unicode text data", so that code that processed those
1414 streams could use these code points as sentinels that could be mixed in
1415 with character data, and would always be distinguishable from that data.
1416 (Emphasis above and in the next paragraph are added in this document.)
1418 Unicode 7.0 changed the wording so that they are "B<not recommended> for
1419 use in open interchange of Unicode text data". The 7.0 Standard goes on
1424 "If a noncharacter is received in open interchange, an application is
1425 not required to interpret it in any way. It is good practice, however,
1426 to recognize it as a noncharacter and to take appropriate action, such
1427 as replacing it with C<U+FFFD> replacement character, to indicate the
1428 problem in the text. It is not recommended to simply delete
1429 noncharacter code points from such text, because of the potential
1430 security issues caused by deleting uninterpreted characters. (See
1431 conformance clause C7 in Section 3.2, Conformance Requirements, and
1432 L<Unicode Technical Report #36, "Unicode Security
1433 Considerations"|http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
1437 This change was made because it was found that various commercial tools
1438 like editors, or for things like source code control, had been written
1439 so that they would not handle program files that used these code points,
1440 effectively precluding their use almost entirely! And that was never
1441 the intent. They've always been meant to be usable within an
1442 application, or cooperating set of applications, at will.
1444 If you're writing code, such as an editor, that is supposed to be able
1445 to handle any Unicode text data, then you shouldn't be using these code
1446 points yourself, and instead allow them in the input. If you need
1447 sentinels, they should instead be something that isn't legal Unicode.
1448 For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
1449 they never appear in well-formed UTF-8. (There are equivalents for
1450 UTF-EBCDIC). You can also store your Unicode code points in integer
1451 variables and use negative values as sentinels.
1453 If you're not writing such a tool, then whether you accept noncharacters
1454 as input is up to you (though the Standard recommends that you not). If
1455 you do strict input stream checking with Perl, these code points
1456 continue to be forbidden. This is to maintain backward compatibility
1457 (otherwise potential security holes could open up, as an unsuspecting
1458 application that was written assuming the noncharacters would be
1459 filtered out before getting to it, could now, without warning, start
1460 getting them). To do strict checking, you can use the layer
1461 C<:encoding('UTF-8')>.
1463 Perl continues to warn (using the warning category C<"nonchar">, which
1464 is a sub-category of C<"utf8">) if an attempt is made to output
1467 =head2 Beyond Unicode code points
1469 The maximum Unicode code point is C<U+10FFFF>, and Unicode only defines
1470 operations on code points up through that. But Perl works on code
1471 points up to the maximum permissible unsigned number available on the
1472 platform. However, Perl will not accept these from input streams unless
1473 lax rules are being used, and will warn (using the warning category
1474 C<"non_unicode">, which is a sub-category of C<"utf8">) if any are output.
1476 Since Unicode rules are not defined on these code points, if a
1477 Unicode-defined operation is done on them, Perl uses what we believe are
1478 sensible rules, while generally warning, using the C<"non_unicode">
1479 category. For example, C<uc("\x{11_0000}")> will generate such a
1480 warning, returning the input parameter as its result, since Perl defines
1481 the uppercase of every non-Unicode code point to be the code point
1482 itself. (All the case changing operations, not just uppercasing, work
1485 The situation with matching Unicode properties in regular expressions,
1486 the C<\p{}> and C<\P{}> constructs, against these code points is not as
1487 clear cut, and how these are handled has changed as we've gained
1490 One possibility is to treat any match against these code points as
1491 undefined. But since Perl doesn't have the concept of a match being
1492 undefined, it converts this to failing or C<FALSE>. This is almost, but
1493 not quite, what Perl did from v5.14 (when use of these code points
1494 became generally reliable) through v5.18. The difference is that Perl
1495 treated all C<\p{}> matches as failing, but all C<\P{}> matches as
1498 One problem with this is that it leads to unexpected, and confusting
1499 results in some cases:
1501 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Failed on <= v5.18
1502 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Failed! on <= v5.18
1504 That is, it treated both matches as undefined, and converted that to
1505 false (raising a warning on each). The first case is the expected
1506 result, but the second is likely counterintuitive: "How could both be
1507 false when they are complements?" Another problem was that the
1508 implementation optimized many Unicode property matches down to already
1509 existing simpler, faster operations, which don't raise the warning. We
1510 chose to not forgo those optimizations, which help the vast majority of
1511 matches, just to generate a warning for the unlikely event that an
1512 above-Unicode code point is being matched against.
1514 As a result of these problems, starting in v5.20, what Perl does is
1515 to treat non-Unicode code points as just typical unassigned Unicode
1516 characters, and matches accordingly. (Note: Unicode has atypical
1517 unassigned code points. For example, it has noncharacter code points,
1518 and ones that, when they do get assigned, are destined to be written
1519 Right-to-left, as Arabic and Hebrew are. Perl assumes that no
1520 non-Unicode code point has any atypical properties.)
1522 Perl, in most cases, will raise a warning when matching an above-Unicode
1523 code point against a Unicode property when the result is C<TRUE> for
1524 C<\p{}>, and C<FALSE> for C<\P{}>. For example:
1526 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails, no warning
1527 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Succeeds, with warning
1529 In both these examples, the character being matched is non-Unicode, so
1530 Unicode doesn't define how it should match. It clearly isn't an ASCII
1531 hex digit, so the first example clearly should fail, and so it does,
1532 with no warning. But it is arguable that the second example should have
1533 an undefined, hence C<FALSE>, result. So a warning is raised for it.
1535 Thus the warning is raised for many fewer cases than in earlier Perls,
1536 and only when what the result is could be arguable. It turns out that
1537 none of the optimizations made by Perl (or are ever likely to be made)
1538 cause the warning to be skipped, so it solves both problems of Perl's
1539 earlier approach. The most commonly used property that is affected by
1540 this change is C<\p{Unassigned}> which is a short form for
1541 C<\p{General_Category=Unassigned}>. Starting in v5.20, all non-Unicode
1542 code points are considered C<Unassigned>. In earlier releases the
1543 matches failed because the result was considered undefined.
1545 The only place where the warning is not raised when it might ought to
1546 have been is if optimizations cause the whole pattern match to not even
1547 be attempted. For example, Perl may figure out that for a string to
1548 match a certain regular expression pattern, the string has to contain
1549 the substring C<"foobar">. Before attempting the match, Perl may look
1550 for that substring, and if not found, immediately fail the match without
1551 actually trying it; so no warning gets generated even if the string
1552 contains an above-Unicode code point.
1554 This behavior is more "Do what I mean" than in earlier Perls for most
1555 applications. But it catches fewer issues for code that needs to be
1556 strictly Unicode compliant. Therefore there is an additional mode of
1557 operation available to accommodate such code. This mode is enabled if a
1558 regular expression pattern is compiled within the lexical scope where
1559 the C<"non_unicode"> warning class has been made fatal, say by:
1561 use warnings FATAL => "non_unicode"
1563 (see L<warnings>). In this mode of operation, Perl will raise the
1564 warning for all matches against a non-Unicode code point (not just the
1565 arguable ones), and it skips the optimizations that might cause the
1566 warning to not be output. (It currently still won't warn if the match
1567 isn't even attempted, like in the C<"foobar"> example above.)
1569 In summary, Perl now normally treats non-Unicode code points as typical
1570 Unicode unassigned code points for regular expression matches, raising a
1571 warning only when it is arguable what the result should be. However, if
1572 this warning has been made fatal, it isn't skipped.
1574 There is one exception to all this. C<\p{All}> looks like a Unicode
1575 property, but it is a Perl extension that is defined to be true for all
1576 possible code points, Unicode or not, so no warning is ever generated
1577 when matching this against a non-Unicode code point. (Prior to v5.20,
1578 it was an exact synonym for C<\p{Any}>, matching code points C<0>
1579 through C<0x10FFFF>.)
1581 =head2 Security Implications of Unicode
1584 L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
1586 Also, note the following:
1594 Unfortunately, the original specification of UTF-8 leaves some room for
1595 interpretation of how many bytes of encoded output one should generate
1596 from one input Unicode character. Strictly speaking, the shortest
1597 possible sequence of UTF-8 bytes should be generated,
1598 because otherwise there is potential for an input buffer overflow at
1599 the receiving end of a UTF-8 connection. Perl always generates the
1600 shortest length UTF-8, and with warnings on, Perl will warn about
1601 non-shortest length UTF-8 along with other malformations, such as the
1602 surrogates, which are not Unicode code points valid for interchange.
1606 Regular expression pattern matching may surprise you if you're not
1607 accustomed to Unicode. Starting in Perl 5.14, several pattern
1608 modifiers are available to control this, called the character set
1609 modifiers. Details are given in L<perlre/Character set modifiers>.
1613 As discussed elsewhere, Perl has one foot (two hooves?) planted in
1614 each of two worlds: the old world of ASCII and single-byte locales, and
1615 the new world of Unicode, upgrading when necessary.
1616 If your legacy code does not explicitly use Unicode, no automatic
1617 switch-over to Unicode should happen.
1619 =head2 Unicode in Perl on EBCDIC
1621 Unicode is supported on EBCDIC platforms. See L<perlebcdic>.
1623 Unless ASCII vs. EBCDIC issues are specifically being discussed,
1624 references to UTF-8 encoding in this document and elsewhere should be
1625 read as meaning UTF-EBCDIC on EBCDIC platforms.
1626 See L<perlebcdic/Unicode and UTF>.
1628 Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
1629 hidden from you; S<C<use utf8>> (and NOT something like
1630 S<C<use utfebcdic>>) declares the the script is in the platform's
1631 "native" 8-bit encoding of Unicode. (Similarly for the C<":utf8">
1636 See L<perllocale/Unicode and UTF-8>
1638 =head2 When Unicode Does Not Happen
1640 There are still many places where Unicode (in some encoding or
1641 another) could be given as arguments or received as results, or both in
1642 Perl, but it is not, in spite of Perl having extensive ways to input and
1643 output in Unicode, and a few other "entry points" like the C<@ARGV>
1644 array (which can sometimes be interpreted as UTF-8).
1646 The following are such interfaces. Also, see L</The "Unicode Bug">.
1647 For all of these interfaces Perl
1648 currently (as of v5.16.0) simply assumes byte strings both as arguments
1649 and results, or UTF-8 strings if the (deprecated) C<encoding> pragma has been used.
1651 One reason that Perl does not attempt to resolve the role of Unicode in
1652 these situations is that the answers are highly dependent on the operating
1653 system and the file system(s). For example, whether filenames can be
1654 in Unicode and in exactly what kind of encoding, is not exactly a
1655 portable concept. Similarly for C<qx> and C<system>: how well will the
1656 "command-line interface" (and which of them?) handle Unicode?
1662 C<chdir>, C<chmod>, C<chown>, C<chroot>, C<exec>, C<link>, C<lstat>, C<mkdir>,
1663 C<rename>, C<rmdir>, C<stat>, C<symlink>, C<truncate>, C<unlink>, C<utime>, C<-X>
1671 C<glob> (aka the C<E<lt>*E<gt>>)
1675 C<open>, C<opendir>, C<sysopen>
1679 C<qx> (aka the backtick operator), C<system>
1683 C<readdir>, C<readlink>
1687 =head2 The "Unicode Bug"
1689 The term, "Unicode bug" has been applied to an inconsistency with the
1690 code points in the C<Latin-1 Supplement> block, that is, between
1691 128 and 255. Without a locale specified, unlike all other characters or
1692 code points, these characters can have very different semantics
1693 depending on the rules in effect. (Characters whose code points are
1694 above 255 force Unicode rules; whereas the rules for ASCII characters
1695 are the same under both ASCII and Unicode rules.)
1697 Under Unicode rules, these upper-Latin1 characters are interpreted as
1698 Unicode code points, which means they have the same semantics as Latin-1
1699 (ISO-8859-1) and C1 controls.
1701 As explained in L</ASCII Rules versus Unicode Rules>, under ASCII rules,
1702 they are considered to be unassigned characters.
1704 This can lead to unexpected results. For example, a string's
1705 semantics can suddenly change if a code point above 255 is appended to
1706 it, which changes the rules from ASCII to Unicode. As an
1707 example, consider the following program and its output:
1710 no feature 'unicode_strings';
1713 for ($s1, $s2, $s1.$s2) {
1721 If there's no C<\w> in C<s1> nor in C<s2>, why does their concatenation
1724 This anomaly stems from Perl's attempt to not disturb older programs that
1725 didn't use Unicode, along with Perl's desire to add Unicode support
1726 seamlessly. But the result turned out to not be seamless. (By the way,
1727 you can choose to be warned when things like this happen. See
1728 C<L<encoding::warnings>>.)
1730 L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
1731 was added, starting in Perl v5.12, to address this problem. It affects
1738 Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
1739 and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in double-quotish
1740 contexts, such as regular expression substitutions.
1742 Under C<unicode_strings> starting in Perl 5.12.0, Unicode rules are
1743 generally used. See L<perlfunc/lc> for details on how this works
1744 in combination with various other pragmas.
1748 Using caseless (C</i>) regular expression matching.
1750 Starting in Perl 5.14.0, regular expressions compiled within
1751 the scope of C<unicode_strings> use Unicode rules
1752 even when executed or compiled into larger
1753 regular expressions outside the scope.
1757 Matching any of several properties in regular expressions.
1759 These properties are C<\b> (without braces), C<\B> (without braces),
1760 C<\s>, C<\S>, C<\w>, C<\W>, and all the Posix character classes
1761 I<except> C<[[:ascii:]]>.
1763 Starting in Perl 5.14.0, regular expressions compiled within
1764 the scope of C<unicode_strings> use Unicode rules
1765 even when executed or compiled into larger
1766 regular expressions outside the scope.
1770 In C<quotemeta> or its inline equivalent C<\Q>.
1772 Starting in Perl 5.16.0, consistent quoting rules are used within the
1773 scope of C<unicode_strings>, as described in L<perlfunc/quotemeta>.
1774 Prior to that, or outside its scope, no code points above 127 are quoted
1775 in UTF-8 encoded strings, but in byte encoded strings, code points
1776 between 128-255 are always quoted.
1780 You can see from the above that the effect of C<unicode_strings>
1781 increased over several Perl releases. (And Perl's support for Unicode
1782 continues to improve; it's best to use the latest available release in
1783 order to get the most complete and accurate results possible.) Note that
1784 C<unicode_strings> is automatically chosen if you S<C<use 5.012>> or
1787 For Perls earlier than those described above, or when a string is passed
1788 to a function outside the scope of C<unicode_strings>, see the next section.
1790 =head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
1792 Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
1793 there are situations where you simply need to force a byte
1794 string into UTF-8, or vice versa. The standard module L<Encode> can be
1795 used for this, or the low-level calls
1796 L<C<utf8::upgrade($bytestring)>|utf8/Utility functions> and
1797 L<C<utf8::downgrade($utf8string[, FAIL_OK])>|utf8/Utility functions>.
1799 Note that C<utf8::downgrade()> can fail if the string contains characters
1800 that don't fit into a byte.
1802 Calling either function on a string that already is in the desired state is a
1805 L</ASCII Rules versus Unicode Rules> gives all the ways that a string is
1806 made to use Unicode rules.
1808 =head2 Using Unicode in XS
1810 See L<perlguts/"Unicode Support"> for an introduction to Unicode at
1811 the XS level, and L<perlapi/Unicode Support> for the API details.
1813 =head2 Hacking Perl to work on earlier Unicode versions (for very serious hackers only)
1815 Perl by default comes with the latest supported Unicode version built-in, but
1816 the goal is to allow you to change to use any earlier one. In Perls
1817 v5.20 and v5.22, however, the earliest usable version is Unicode 5.1.
1818 Perl v5.18 is able to handle all earlier versions.
1820 Download the files in the desired version of Unicode from the Unicode web
1821 site L<http://www.unicode.org>). These should replace the existing files in
1822 F<lib/unicore> in the Perl source tree. Follow the instructions in
1823 F<README.perl> in that directory to change some of their names, and then build
1824 perl (see L<INSTALL>).
1826 =head2 Porting code from perl-5.6.X
1828 Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6 the
1829 programmer was required to use the C<utf8> pragma to declare that a
1830 given scope expected to deal with Unicode data and had to make sure that
1831 only Unicode data were reaching that scope. If you have code that is
1832 working with 5.6, you will need some of the following adjustments to
1833 your code. The examples are written such that the code will continue to
1834 work under 5.6, so you should be safe to try them out.
1840 A filehandle that should read or write UTF-8
1843 binmode $fh, ":encoding(utf8)";
1848 A scalar that is going to be passed to some extension
1850 Be it C<Compress::Zlib>, C<Apache::Request> or any extension that has no
1851 mention of Unicode in the manpage, you need to make sure that the
1852 UTF8 flag is stripped off. Note that at the time of this writing
1853 (January 2012) the mentioned modules are not UTF-8-aware. Please
1854 check the documentation to verify if this is still true.
1858 $val = Encode::encode_utf8($val); # make octets
1863 A scalar we got back from an extension
1865 If you believe the scalar comes back as UTF-8, you will most likely
1866 want the UTF8 flag restored:
1870 $val = Encode::decode_utf8($val);
1875 Same thing, if you are really sure it is UTF-8
1879 Encode::_utf8_on($val);
1884 A wrapper for L<DBI> C<fetchrow_array> and C<fetchrow_hashref>
1886 When the database contains only UTF-8, a wrapper function or method is
1887 a convenient way to replace all your C<fetchrow_array> and
1888 C<fetchrow_hashref> calls. A wrapper function will also make it easier to
1889 adapt to future enhancements in your database driver. Note that at the
1890 time of this writing (January 2012), the DBI has no standardized way
1891 to deal with UTF-8 data. Please check the L<DBI documentation|DBI> to verify if
1895 # $what is one of fetchrow_{array,hashref}
1896 my($self, $sth, $what) = @_;
1902 my @arr = $sth->$what;
1904 defined && /[^\000-\177]/ && Encode::_utf8_on($_);
1908 my $ret = $sth->$what;
1910 for my $k (keys %$ret) {
1913 && Encode::_utf8_on($_) for $ret->{$k};
1917 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
1927 A large scalar that you know can only contain ASCII
1929 Scalars that contain only ASCII and are marked as UTF-8 are sometimes
1930 a drag to your program. If you recognize such a situation, just remove
1933 utf8::downgrade($val) if $] > 5.008;
1939 See also L</The "Unicode Bug"> above.
1941 =head2 Interaction with Extensions
1943 When Perl exchanges data with an extension, the extension should be
1944 able to understand the UTF8 flag and act accordingly. If the
1945 extension doesn't recognize that flag, it's likely that the extension
1946 will return incorrectly-flagged data.
1948 So if you're working with Unicode data, consult the documentation of
1949 every module you're using if there are any issues with Unicode data
1950 exchange. If the documentation does not talk about Unicode at all,
1951 suspect the worst and probably look at the source to learn how the
1952 module is implemented. Modules written completely in Perl shouldn't
1953 cause problems. Modules that directly or indirectly access code written
1954 in other programming languages are at risk.
1956 For affected functions, the simple strategy to avoid data corruption is
1957 to always make the encoding of the exchanged data explicit. Choose an
1958 encoding that you know the extension can handle. Convert arguments passed
1959 to the extensions to that encoding and convert results back from that
1960 encoding. Write wrapper functions that do the conversions for you, so
1961 you can later change the functions when the extension catches up.
1963 To provide an example, let's say the popular C<Foo::Bar::escape_html>
1964 function doesn't deal with Unicode data yet. The wrapper function
1965 would convert the argument to raw UTF-8 and convert the result back to
1966 Perl's internal representation like so:
1968 sub my_escape_html ($) {
1970 return unless defined $what;
1971 Encode::decode_utf8(Foo::Bar::escape_html(
1972 Encode::encode_utf8($what)));
1975 Sometimes, when the extension does not convert data but just stores
1976 and retrieves it, you will be able to use the otherwise
1977 dangerous L<C<Encode::_utf8_on()>|Encode/_utf8_on> function. Let's say
1978 the popular C<Foo::Bar> extension, written in C, provides a C<param>
1979 method that lets you store and retrieve data according to these prototypes:
1981 $self->param($name, $value); # set a scalar
1982 $value = $self->param($name); # retrieve a scalar
1984 If it does not yet provide support for any encoding, one could write a
1985 derived class with such a C<param> method:
1988 my($self,$name,$value) = @_;
1989 utf8::upgrade($name); # make sure it is UTF-8 encoded
1990 if (defined $value) {
1991 utf8::upgrade($value); # make sure it is UTF-8 encoded
1992 return $self->SUPER::param($name,$value);
1994 my $ret = $self->SUPER::param($name);
1995 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
2000 Some extensions provide filters on data entry/exit points, such as
2001 C<DB_File::filter_store_key> and family. Look out for such filters in
2002 the documentation of your extensions; they can make the transition to
2003 Unicode data much easier.
2007 Some functions are slower when working on UTF-8 encoded strings than
2008 on byte encoded strings. All functions that need to hop over
2009 characters such as C<length()>, C<substr()> or C<index()>, or matching
2010 regular expressions can work B<much> faster when the underlying data are
2013 In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
2014 a caching scheme was introduced which improved the situation. In general,
2015 operations with UTF-8 encoded strings are still slower. As an example,
2016 the Unicode properties (character classes) like C<\p{Nd}> are known to
2017 be quite a bit slower (5-20 times) than their simpler counterparts
2018 like C<[0-9]> (then again, there are hundreds of Unicode characters matching
2019 C<Nd> compared with the 10 ASCII characters matching C<[0-9]>).
2023 L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>,
2024 L<perlretut>, L<perlvar/"${^UNICODE}">,
2025 L<http://www.unicode.org/reports/tr44>).