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<https://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
40 obvious, see L</Security Implications of Unicode> below.
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 must convert your non-ASCII, non-UTF-8 Perl scripts to be
66 The L<encoding> module has been deprecated since perl 5.18 and the
67 perl internals it requires have been removed with perl 5.26.
69 =item C<use utf8> still needed to enable L<UTF-8|/Unicode Encodings> in scripts
71 If your Perl script is itself encoded in L<UTF-8|/Unicode Encodings>,
72 the S<C<use utf8>> pragma must be explicitly included to enable
73 recognition of that (in string or regular expression literals, or in
74 identifier names). B<This is the only time when an explicit S<C<use
75 utf8>> is needed.> (See L<utf8>).
77 If a Perl script begins with the bytes that form the UTF-8 encoding of
78 the Unicode BYTE ORDER MARK (C<BOM>, see L</Unicode Encodings>), those
79 bytes are completely ignored.
81 =item L<UTF-16|/Unicode Encodings> scripts autodetected
83 If a Perl script begins with the Unicode C<BOM> (UTF-16LE,
84 UTF16-BE), or if the script looks like non-C<BOM>-marked
85 UTF-16 of either endianness, Perl will correctly read in the script as
86 the appropriate Unicode encoding.
90 =head2 Byte and Character Semantics
92 Before Unicode, most encodings used 8 bits (a single byte) to encode
93 each character. Thus a character was a byte, and a byte was a
94 character, and there could be only 256 or fewer possible characters.
95 "Byte Semantics" in the title of this section refers to
96 this behavior. There was no need to distinguish between "Byte" and
99 Then along comes Unicode which has room for over a million characters
100 (and Perl allows for even more). This means that a character may
101 require more than a single byte to represent it, and so the two terms
102 are no longer equivalent. What matter are the characters as whole
103 entities, and not usually the bytes that comprise them. That's what the
104 term "Character Semantics" in the title of this section refers to.
106 Perl had to change internally to decouple "bytes" from "characters".
107 It is important that you too change your ideas, if you haven't already,
108 so that "byte" and "character" no longer mean the same thing in your
111 The basic building block of Perl strings has always been a "character".
112 The changes basically come down to that the implementation no longer
113 thinks that a character is always just a single byte.
115 There are various things to note:
121 String handling functions, for the most part, continue to operate in
122 terms of characters. C<length()>, for example, returns the number of
123 characters in a string, just as before. But that number no longer is
124 necessarily the same as the number of bytes in the string (there may be
125 more bytes than characters). The other such functions include
126 C<chop()>, C<chomp()>, C<substr()>, C<pos()>, C<index()>, C<rindex()>,
127 C<sort()>, C<sprintf()>, and C<write()>.
135 the bit-oriented C<vec>
141 the byte-oriented C<pack>/C<unpack> C<"C"> format
143 However, the C<W> specifier does operate on whole characters, as does the
148 some operators that interact with the platform's operating system
150 Operators dealing with filenames are examples.
154 when the functions are called from within the scope of the
155 S<C<L<use bytes|bytes>>> pragma
157 Likely, you should use this only for debugging anyway.
163 Strings--including hash keys--and regular expression patterns may
164 contain characters that have ordinal values larger than 255.
166 If you use a Unicode editor to edit your program, Unicode characters may
167 occur directly within the literal strings in UTF-8 encoding, or UTF-16.
168 (The former requires a C<use utf8>, the latter may require a C<BOM>.)
170 L<perluniintro/Creating Unicode> gives other ways to place non-ASCII
171 characters in your strings.
175 The C<chr()> and C<ord()> functions work on whole characters.
179 Regular expressions match whole characters. For example, C<"."> matches
180 a whole character instead of only a single byte.
184 The C<tr///> operator translates whole characters. (Note that the
185 C<tr///CU> functionality has been removed. For similar functionality to
186 that, see C<pack('U0', ...)> and C<pack('C0', ...)>).
190 C<scalar reverse()> reverses by character rather than by byte.
194 The bit string operators, C<& | ^ ~> and (starting in v5.22)
195 C<&. |. ^. ~.> can operate on bit strings encoded in UTF-8, but this
196 can give unexpected results if any of the strings contain code points
197 above 0xFF. Starting in v5.28, it is a fatal error to have such an
198 operand. Otherwise, the operation is performed on a non-UTF-8 copy of
199 the operand. If you're not sure about the encoding of a string,
200 downgrade it before using any of these operators; you can use
201 L<C<utf8::utf8_downgrade()>|utf8/Utility functions>.
205 The bottom line is that Perl has always practiced "Character Semantics",
206 but with the advent of Unicode, that is now different than "Byte
209 =head2 ASCII Rules versus Unicode Rules
211 Before Unicode, when a character was a byte was a character,
212 Perl knew only about the 128 characters defined by ASCII, code points 0
213 through 127 (except for under L<S<C<use locale>>|perllocale>). That
215 points 128 to 255 as unassigned, and available for whatever use a
216 program might want. The only semantics they have is their ordinal
217 numbers, and that they are members of none of the non-negative character
218 classes. None are considered to match C<\w> for example, but all match
221 Unicode, of course, assigns each of those code points a particular
222 meaning (along with ones above 255). To preserve backward
223 compatibility, Perl only uses the Unicode meanings when there is some
224 indication that Unicode is what is intended; otherwise the non-ASCII
225 code points remain treated as if they are unassigned.
227 Here are the ways that Perl knows that a string should be treated as
234 Within the scope of S<C<use utf8>>
236 If the whole program is Unicode (signified by using 8-bit B<U>nicode
237 B<T>ransformation B<F>ormat), then all literal strings within it must be
243 L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
245 This pragma was created so you can explicitly tell Perl that operations
246 executed within its scope are to use Unicode rules. More operations are
247 affected with newer perls. See L</The "Unicode Bug">.
251 Within the scope of S<C<use 5.012>> or higher
253 This implicitly turns on S<C<use feature 'unicode_strings'>>.
258 L<S<C<use locale 'not_characters'>>|perllocale/Unicode and UTF-8>,
259 or L<S<C<use locale>>|perllocale> and the current
260 locale is a UTF-8 locale.
262 The former is defined to imply Unicode handling; and the latter
263 indicates a Unicode locale, hence a Unicode interpretation of all
268 When the string contains a Unicode-only code point
270 Perl has never accepted code points above 255 without them being
271 Unicode, so their use implies Unicode for the whole string.
275 When the string contains a Unicode named code point C<\N{...}>
277 The C<\N{...}> construct explicitly refers to a Unicode code point,
278 even if it is one that is also in ASCII. Therefore the string
279 containing it must be Unicode.
283 When the string has come from an external source marked as
286 The L<C<-C>|perlrun/-C [numberE<sol>list]> command line option can
287 specify that certain inputs to the program are Unicode, and the values
288 of this can be read by your Perl code, see L<perlvar/"${^UNICODE}">.
290 =item * When the string has been upgraded to UTF-8
292 The function L<C<utf8::utf8_upgrade()>|utf8/Utility functions>
293 can be explicitly used to permanently (unless a subsequent
294 C<utf8::utf8_downgrade()> is called) cause a string to be treated as
297 =item * There are additional methods for regular expression patterns
299 A pattern that is compiled with the C<< /u >> or C<< /a >> modifiers is
300 treated as Unicode (though there are some restrictions with C<< /a >>).
301 Under the C<< /d >> and C<< /l >> modifiers, there are several other
302 indications for Unicode; see L<perlre/Character set modifiers>.
306 Note that all of the above are overridden within the scope of
307 C<L<use bytes|bytes>>; but you should be using this pragma only for
310 Note also that some interactions with the platform's operating system
311 never use Unicode rules.
313 When Unicode rules are in effect:
319 Case translation operators use the Unicode case translation tables.
321 Note that C<uc()>, or C<\U> in interpolated strings, translates to
322 uppercase, while C<ucfirst>, or C<\u> in interpolated strings,
323 translates to titlecase in languages that make the distinction (which is
324 equivalent to uppercase in languages without the distinction).
326 There is a CPAN module, C<L<Unicode::Casing>>, which allows you to
327 define your own mappings to be used in C<lc()>, C<lcfirst()>, C<uc()>,
328 C<ucfirst()>, and C<fc> (or their double-quoted string inlined versions
329 such as C<\U>). (Prior to Perl 5.16, this functionality was partially
330 provided in the Perl core, but suffered from a number of insurmountable
331 drawbacks, so the CPAN module was written instead.)
335 Character classes in regular expressions match based on the character
336 properties specified in the Unicode properties database.
338 C<\w> can be used to match a Japanese ideograph, for instance; and
339 C<[[:digit:]]> a Bengali number.
343 Named Unicode properties, scripts, and block ranges may be used (like
344 bracketed character classes) by using the C<\p{}> "matches property"
345 construct and the C<\P{}> negation, "doesn't match property".
347 See L</"Unicode Character Properties"> for more details.
349 You can define your own character properties and use them
350 in the regular expression with the C<\p{}> or C<\P{}> construct.
351 See L</"User-Defined Character Properties"> for more details.
355 =head2 Extended Grapheme Clusters (Logical characters)
357 Consider a character, say C<H>. It could appear with various marks around it,
358 such as an acute accent, or a circumflex, or various hooks, circles, arrows,
359 I<etc.>, above, below, to one side or the other, I<etc>. There are many
360 possibilities among the world's languages. The number of combinations is
361 astronomical, and if there were a character for each combination, it would
362 soon exhaust Unicode's more than a million possible characters. So Unicode
363 took a different approach: there is a character for the base C<H>, and a
364 character for each of the possible marks, and these can be variously combined
365 to get a final logical character. So a logical character--what appears to be a
366 single character--can be a sequence of more than one individual characters.
367 The Unicode standard calls these "extended grapheme clusters" (which
368 is an improved version of the no-longer much used "grapheme cluster");
369 Perl furnishes the C<\X> regular expression construct to match such
370 sequences in their entirety.
372 But Unicode's intent is to unify the existing character set standards and
373 practices, and several pre-existing standards have single characters that
374 mean the same thing as some of these combinations, like ISO-8859-1,
375 which has quite a few of them. For example, C<"LATIN CAPITAL LETTER E
376 WITH ACUTE"> was already in this standard when Unicode came along.
377 Unicode therefore added it to its repertoire as that single character.
378 But this character is considered by Unicode to be equivalent to the
379 sequence consisting of the character C<"LATIN CAPITAL LETTER E">
380 followed by the character C<"COMBINING ACUTE ACCENT">.
382 C<"LATIN CAPITAL LETTER E WITH ACUTE"> is called a "pre-composed"
383 character, and its equivalence with the "E" and the "COMBINING ACCENT"
384 sequence is called canonical equivalence. All pre-composed characters
385 are said to have a decomposition (into the equivalent sequence), and the
386 decomposition type is also called canonical. A string may be comprised
387 as much as possible of precomposed characters, or it may be comprised of
388 entirely decomposed characters. Unicode calls these respectively,
389 "Normalization Form Composed" (NFC) and "Normalization Form Decomposed".
390 The C<L<Unicode::Normalize>> module contains functions that convert
391 between the two. A string may also have both composed characters and
392 decomposed characters; this module can be used to make it all one or the
395 You may be presented with strings in any of these equivalent forms.
396 There is currently nothing in Perl 5 that ignores the differences. So
397 you'll have to specially handle it. The usual advice is to convert your
398 inputs to C<NFD> before processing further.
400 For more detailed information, see L<http://unicode.org/reports/tr15/>.
402 =head2 Unicode Character Properties
404 (The only time that Perl considers a sequence of individual code
405 points as a single logical character is in the C<\X> construct, already
406 mentioned above. Therefore "character" in this discussion means a single
409 Very nearly all Unicode character properties are accessible through
410 regular expressions by using the C<\p{}> "matches property" construct
411 and the C<\P{}> "doesn't match property" for its negation.
413 For instance, C<\p{Uppercase}> matches any single character with the Unicode
414 C<"Uppercase"> property, while C<\p{L}> matches any character with a
415 C<General_Category> of C<"L"> (letter) property (see
416 L</General_Category> below). Brackets are not
417 required for single letter property names, so C<\p{L}> is equivalent to C<\pL>.
419 More formally, C<\p{Uppercase}> matches any single character whose Unicode
420 C<Uppercase> property value is C<True>, and C<\P{Uppercase}> matches any character
421 whose C<Uppercase> property value is C<False>, and they could have been written as
422 C<\p{Uppercase=True}> and C<\p{Uppercase=False}>, respectively.
424 This formality is needed when properties are not binary; that is, if they can
425 take on more values than just C<True> and C<False>. For example, the
426 C<Bidi_Class> property (see L</"Bidirectional Character Types"> below),
427 can take on several different
428 values, such as C<Left>, C<Right>, C<Whitespace>, and others. To match these, one needs
429 to specify both the property name (C<Bidi_Class>), AND the value being
431 (C<Left>, C<Right>, I<etc.>). This is done, as in the examples above, by having the
432 two components separated by an equal sign (or interchangeably, a colon), like
433 C<\p{Bidi_Class: Left}>.
435 All Unicode-defined character properties may be written in these compound forms
436 of C<\p{I<property>=I<value>}> or C<\p{I<property>:I<value>}>, but Perl provides some
437 additional properties that are written only in the single form, as well as
438 single-form short-cuts for all binary properties and certain others described
439 below, in which you may omit the property name and the equals or colon
442 Most Unicode character properties have at least two synonyms (or aliases if you
443 prefer): a short one that is easier to type and a longer one that is more
444 descriptive and hence easier to understand. Thus the C<"L"> and
445 C<"Letter"> properties above are equivalent and can be used
446 interchangeably. Likewise, C<"Upper"> is a synonym for C<"Uppercase">,
447 and we could have written C<\p{Uppercase}> equivalently as C<\p{Upper}>.
448 Also, there are typically various synonyms for the values the property
449 can be. For binary properties, C<"True"> has 3 synonyms: C<"T">,
450 C<"Yes">, and C<"Y">; and C<"False"> has correspondingly C<"F">,
451 C<"No">, and C<"N">. But be careful. A short form of a value for one
452 property may not mean the same thing as the short form spelled the same
454 Thus, for the C<L</General_Category>> property, C<"L"> means
455 C<"Letter">, but for the L<C<Bidi_Class>|/Bidirectional Character Types>
456 property, C<"L"> means C<"Left">. A complete list of properties and
457 synonyms is in L<perluniprops>.
459 Upper/lower case differences in property names and values are irrelevant;
460 thus C<\p{Upper}> means the same thing as C<\p{upper}> or even C<\p{UpPeR}>.
461 Similarly, you can add or subtract underscores anywhere in the middle of a
462 word, so that these are also equivalent to C<\p{U_p_p_e_r}>. And white space
463 is generally irrelevant adjacent to non-word characters, such as the
464 braces and the equals or colon separators, so C<\p{ Upper }> and
465 C<\p{ Upper_case : Y }> are equivalent to these as well. In fact, white
466 space and even hyphens can usually be added or deleted anywhere. So
467 even C<\p{ Up-per case = Yes}> is equivalent. All this is called
468 "loose-matching" by Unicode. The "name" property has some restrictions
469 on this due to a few outlier names. Full details are given in
470 L<https://www.unicode.org/reports/tr44/tr44-24.html#UAX44-LM2>.
472 The few places where stricter matching is
473 used is in the middle of numbers, the "name" property, and in the Perl
474 extension properties that begin or end with an underscore. Stricter
475 matching cares about white space (except adjacent to non-word
476 characters), hyphens, and non-interior underscores.
478 You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret
479 (C<^>) between the first brace and the property name: C<\p{^Tamil}> is
480 equal to C<\P{Tamil}>.
482 Almost all properties are immune to case-insensitive matching. That is,
483 adding a C</i> regular expression modifier does not change what they
484 match. There are two sets that are affected.
488 and C<Titlecase_Letter>,
489 all of which match C<Cased_Letter> under C</i> matching.
490 And the second set is
494 all of which match C<Cased> under C</i> matching.
495 This set also includes its subsets C<PosixUpper> and C<PosixLower> both
496 of which under C</i> match C<PosixAlpha>.
497 (The difference between these sets is that some things, such as Roman
498 numerals, come in both upper and lower case so they are C<Cased>, but
499 aren't considered letters, so they aren't C<Cased_Letter>'s.)
501 See L</Beyond Unicode code points> for special considerations when
502 matching Unicode properties against non-Unicode code points.
504 =head3 B<General_Category>
506 Every Unicode character is assigned a general category, which is the "most
507 usual categorization of a character" (from
508 L<https://www.unicode.org/reports/tr44>).
510 The compound way of writing these is like C<\p{General_Category=Number}>
511 (short: C<\p{gc:n}>). But Perl furnishes shortcuts in which everything up
512 through the equal or colon separator is omitted. So you can instead just write
515 Here are the short and long forms of the values the C<General Category> property
521 LC, L& Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
534 Nd Decimal_Number (also Digit)
538 P Punctuation (also Punct)
539 Pc Connector_Punctuation
543 Pi Initial_Punctuation
544 (may behave like Ps or Pe depending on usage)
546 (may behave like Ps or Pe depending on usage)
558 Zp Paragraph_Separator
561 Cc Control (also Cntrl)
567 Single-letter properties match all characters in any of the
568 two-letter sub-properties starting with the same letter.
569 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>.
571 =head3 B<Bidirectional Character Types>
573 Because scripts differ in their directionality (Hebrew and Arabic are
574 written right to left, for example) Unicode supplies a C<Bidi_Class> property.
575 Some of the values this property can have are:
580 LRE Left-to-Right Embedding
581 LRO Left-to-Right Override
584 RLE Right-to-Left Embedding
585 RLO Right-to-Left Override
586 PDF Pop Directional Format
588 ES European Separator
589 ET European Terminator
594 B Paragraph Separator
599 This property is always written in the compound form.
600 For example, C<\p{Bidi_Class:R}> matches characters that are normally
601 written right to left. Unlike the
602 C<L</General_Category>> property, this
603 property can have more values added in a future Unicode release. Those
604 listed above comprised the complete set for many Unicode releases, but
605 others were added in Unicode 6.3; you can always find what the
606 current ones are in L<perluniprops>. And
607 L<https://www.unicode.org/reports/tr9/> describes how to use them.
611 The world's languages are written in many different scripts. This sentence
612 (unless you're reading it in translation) is written in Latin, while Russian is
613 written in Cyrillic, and Greek is written in, well, Greek; Japanese mainly in
614 Hiragana or Katakana. There are many more.
616 The Unicode C<Script> and C<Script_Extensions> properties give what
617 script a given character is in. The C<Script_Extensions> property is an
618 improved version of C<Script>, as demonstrated below. Either property
619 can be specified with the compound form like
620 C<\p{Script=Hebrew}> (short: C<\p{sc=hebr}>), or
621 C<\p{Script_Extensions=Javanese}> (short: C<\p{scx=java}>).
622 In addition, Perl furnishes shortcuts for all
623 C<Script_Extensions> property names. You can omit everything up through
624 the equals (or colon), and simply write C<\p{Latin}> or C<\P{Cyrillic}>.
625 (This is not true for C<Script>, which is required to be
626 written in the compound form. Prior to Perl v5.26, the single form
627 returned the plain old C<Script> version, but was changed because
628 C<Script_Extensions> gives better results.)
630 The difference between these two properties involves characters that are
631 used in multiple scripts. For example the digits '0' through '9' are
632 used in many parts of the world. These are placed in a script named
633 C<Common>. Other characters are used in just a few scripts. For
634 example, the C<"KATAKANA-HIRAGANA DOUBLE HYPHEN"> is used in both Japanese
635 scripts, Katakana and Hiragana, but nowhere else. The C<Script>
636 property places all characters that are used in multiple scripts in the
637 C<Common> script, while the C<Script_Extensions> property places those
638 that are used in only a few scripts into each of those scripts; while
639 still using C<Common> for those used in many scripts. Thus both these
642 "0" =~ /\p{sc=Common}/ # Matches
643 "0" =~ /\p{scx=Common}/ # Matches
645 and only the first of these match:
647 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common} # Matches
648 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match
650 And only the last two of these match:
652 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana} # No match
653 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana} # No match
654 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
655 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches
657 C<Script_Extensions> is thus an improved C<Script>, in which there are
658 fewer characters in the C<Common> script, and correspondingly more in
659 other scripts. It is new in Unicode version 6.0, and its data are likely
660 to change significantly in later releases, as things get sorted out.
661 New code should probably be using C<Script_Extensions> and not plain
662 C<Script>. If you compile perl with a Unicode release that doesn't have
663 C<Script_Extensions>, the single form Perl extensions will instead refer
664 to the plain C<Script> property. If you compile with a version of
665 Unicode that doesn't have the C<Script> property, these extensions will
666 not be defined at all.
668 (Actually, besides C<Common>, the C<Inherited> script, contains
669 characters that are used in multiple scripts. These are modifier
670 characters which inherit the script value
671 of the controlling character. Some of these are used in many scripts,
672 and so go into C<Inherited> in both C<Script> and C<Script_Extensions>.
673 Others are used in just a few scripts, so are in C<Inherited> in
674 C<Script>, but not in C<Script_Extensions>.)
676 It is worth stressing that there are several different sets of digits in
677 Unicode that are equivalent to 0-9 and are matchable by C<\d> in a
678 regular expression. If they are used in a single language only, they
679 are in that language's C<Script> and C<Script_Extensions>. If they are
680 used in more than one script, they will be in C<sc=Common>, but only
681 if they are used in many scripts should they be in C<scx=Common>.
683 The explanation above has omitted some detail; refer to UAX#24 "Unicode
684 Script Property": L<https://www.unicode.org/reports/tr24>.
686 A complete list of scripts and their shortcuts is in L<perluniprops>.
688 =head3 B<Use of the C<"Is"> Prefix>
690 For backward compatibility (with ancient Perl 5.6), all properties writable
691 without using the compound form mentioned
692 so far may have C<Is> or C<Is_> prepended to their name, so C<\P{Is_Lu}>, for
693 example, is equal to C<\P{Lu}>, and C<\p{IsScript:Arabic}> is equal to
698 In addition to B<scripts>, Unicode also defines B<blocks> of
699 characters. The difference between scripts and blocks is that the
700 concept of scripts is closer to natural languages, while the concept
701 of blocks is more of an artificial grouping based on groups of Unicode
702 characters with consecutive ordinal values. For example, the C<"Basic Latin">
703 block is all the characters whose ordinals are between 0 and 127, inclusive; in
704 other words, the ASCII characters. The C<"Latin"> script contains some letters
705 from this as well as several other blocks, like C<"Latin-1 Supplement">,
706 C<"Latin Extended-A">, I<etc.>, but it does not contain all the characters from
707 those blocks. It does not, for example, contain the digits 0-9, because
708 those digits are shared across many scripts, and hence are in the
711 For more about scripts versus blocks, see UAX#24 "Unicode Script Property":
712 L<https://www.unicode.org/reports/tr24>
714 The C<Script_Extensions> or C<Script> properties are likely to be the
715 ones you want to use when processing
716 natural language; the C<Block> property may occasionally be useful in working
717 with the nuts and bolts of Unicode.
719 Block names are matched in the compound form, like C<\p{Block: Arrows}> or
720 C<\p{Blk=Hebrew}>. Unlike most other properties, only a few block names have a
721 Unicode-defined short name.
723 Perl also defines single form synonyms for the block property in cases
724 where these do not conflict with something else. But don't use any of
725 these, because they are unstable. Since these are Perl extensions, they
726 are subordinate to official Unicode property names; Unicode doesn't know
727 nor care about Perl's extensions. It may happen that a name that
728 currently means the Perl extension will later be changed without warning
729 to mean a different Unicode property in a future version of the perl
730 interpreter that uses a later Unicode release, and your code would no
731 longer work. The extensions are mentioned here for completeness: Take
732 the block name and prefix it with one of: C<In> (for example
733 C<\p{Blk=Arrows}> can currently be written as C<\p{In_Arrows}>); or
734 sometimes C<Is> (like C<\p{Is_Arrows}>); or sometimes no prefix at all
735 (C<\p{Arrows}>). As of this writing (Unicode 9.0) there are no
736 conflicts with using the C<In_> prefix, but there are plenty with the
737 other two forms. For example, C<\p{Is_Hebrew}> and C<\p{Hebrew}> mean
738 C<\p{Script_Extensions=Hebrew}> which is NOT the same thing as
739 C<\p{Blk=Hebrew}>. Our
740 advice used to be to use the C<In_> prefix as a single form way of
741 specifying a block. But Unicode 8.0 added properties whose names begin
742 with C<In>, and it's now clear that it's only luck that's so far
743 prevented a conflict. Using C<In> is only marginally less typing than
744 C<Blk:>, and the latter's meaning is clearer anyway, and guaranteed to
745 never conflict. So don't take chances. Use C<\p{Blk=foo}> for new
746 code. And be sure that block is what you really really want to do. In
747 most cases scripts are what you want instead.
749 A complete list of blocks is in L<perluniprops>.
751 =head3 B<Other Properties>
753 There are many more properties than the very basic ones described here.
754 A complete list is in L<perluniprops>.
756 Unicode defines all its properties in the compound form, so all single-form
757 properties are Perl extensions. Most of these are just synonyms for the
758 Unicode ones, but some are genuine extensions, including several that are in
759 the compound form. And quite a few of these are actually recommended by Unicode
760 (in L<https://www.unicode.org/reports/tr18>).
762 This section gives some details on all extensions that aren't just
763 synonyms for compound-form Unicode properties
764 (for those properties, you'll have to refer to the
765 L<Unicode Standard|https://www.unicode.org/reports/tr44>.
771 This matches every possible code point. It is equivalent to C<qr/./s>.
772 Unlike all the other non-user-defined C<\p{}> property matches, no
773 warning is ever generated if this is property is matched against a
774 non-Unicode code point (see L</Beyond Unicode code points> below).
776 =item B<C<\p{Alnum}>>
778 This matches any C<\p{Alphabetic}> or C<\p{Decimal_Number}> character.
782 This matches any of the 1_114_112 Unicode code points. It is a synonym
785 =item B<C<\p{ASCII}>>
787 This matches any of the 128 characters in the US-ASCII character set,
788 which is a subset of Unicode.
790 =item B<C<\p{Assigned}>>
792 This matches any assigned code point; that is, any code point whose L<general
793 category|/General_Category> is not C<Unassigned> (or equivalently, not C<Cn>).
795 =item B<C<\p{Blank}>>
797 This is the same as C<\h> and C<\p{HorizSpace}>: A character that changes the
798 spacing horizontally.
800 =item B<C<\p{Decomposition_Type: Non_Canonical}>> (Short: C<\p{Dt=NonCanon}>)
802 Matches a character that has a non-canonical decomposition.
804 The L</Extended Grapheme Clusters (Logical characters)> section above
805 talked about canonical decompositions. However, many more characters
806 have a different type of decomposition, a "compatible" or
807 "non-canonical" decomposition. The sequences that form these
808 decompositions are not considered canonically equivalent to the
809 pre-composed character. An example is the C<"SUPERSCRIPT ONE">. It is
810 somewhat like a regular digit 1, but not exactly; its decomposition into
811 the digit 1 is called a "compatible" decomposition, specifically a
812 "super" decomposition. There are several such compatibility
813 decompositions (see L<https://www.unicode.org/reports/tr44>), including
814 one called "compat", which means some miscellaneous type of
815 decomposition that doesn't fit into the other decomposition categories
816 that Unicode has chosen.
818 Note that most Unicode characters don't have a decomposition, so their
819 decomposition type is C<"None">.
821 For your convenience, Perl has added the C<Non_Canonical> decomposition
822 type to mean any of the several compatibility decompositions.
824 =item B<C<\p{Graph}>>
826 Matches any character that is graphic. Theoretically, this means a character
827 that on a printer would cause ink to be used.
829 =item B<C<\p{HorizSpace}>>
831 This is the same as C<\h> and C<\p{Blank}>: a character that changes the
832 spacing horizontally.
836 This is a synonym for C<\p{Present_In=*}>
838 =item B<C<\p{PerlSpace}>>
840 This is the same as C<\s>, restricted to ASCII, namely C<S<[ \f\n\r\t]>>
841 and starting in Perl v5.18, a vertical tab.
843 Mnemonic: Perl's (original) space
845 =item B<C<\p{PerlWord}>>
847 This is the same as C<\w>, restricted to ASCII, namely C<[A-Za-z0-9_]>
849 Mnemonic: Perl's (original) word.
851 =item B<C<\p{Posix...}>>
853 There are several of these, which are equivalents, using the C<\p{}>
854 notation, for Posix classes and are described in
855 L<perlrecharclass/POSIX Character Classes>.
857 =item B<C<\p{Present_In: *}>> (Short: C<\p{In=*}>)
859 This property is used when you need to know in what Unicode version(s) a
862 The "*" above stands for some Unicode version number, such as
863 C<1.1> or C<12.0>; or the "*" can also be C<Unassigned>. This property will
864 match the code points whose final disposition has been settled as of the
865 Unicode release given by the version number; C<\p{Present_In: Unassigned}>
866 will match those code points whose meaning has yet to be assigned.
868 For example, C<U+0041> C<"LATIN CAPITAL LETTER A"> was present in the very first
869 Unicode release available, which is C<1.1>, so this property is true for all
870 valid "*" versions. On the other hand, C<U+1EFF> was not assigned until version
871 5.1 when it became C<"LATIN SMALL LETTER Y WITH LOOP">, so the only "*" that
872 would match it are 5.1, 5.2, and later.
874 Unicode furnishes the C<Age> property from which this is derived. The problem
875 with Age is that a strict interpretation of it (which Perl takes) has it
876 matching the precise release a code point's meaning is introduced in. Thus
877 C<U+0041> would match only 1.1; and C<U+1EFF> only 5.1. This is not usually what
880 Some non-Perl implementations of the Age property may change its meaning to be
881 the same as the Perl C<Present_In> property; just be aware of that.
883 Another confusion with both these properties is that the definition is not
884 that the code point has been I<assigned>, but that the meaning of the code point
885 has been I<determined>. This is because 66 code points will always be
886 unassigned, and so the C<Age> for them is the Unicode version in which the decision
887 to make them so was made. For example, C<U+FDD0> is to be permanently
888 unassigned to a character, and the decision to do that was made in version 3.1,
889 so C<\p{Age=3.1}> matches this character, as also does C<\p{Present_In: 3.1}> and up.
891 =item B<C<\p{Print}>>
893 This matches any character that is graphical or blank, except controls.
895 =item B<C<\p{SpacePerl}>>
897 This is the same as C<\s>, including beyond ASCII.
899 Mnemonic: Space, as modified by Perl. (It doesn't include the vertical tab
900 until v5.18, which both the Posix standard and Unicode consider white space.)
902 =item B<C<\p{Title}>> and B<C<\p{Titlecase}>>
904 Under case-sensitive matching, these both match the same code points as
905 C<\p{General Category=Titlecase_Letter}> (C<\p{gc=lt}>). The difference
906 is that under C</i> caseless matching, these match the same as
907 C<\p{Cased}>, whereas C<\p{gc=lt}> matches C<\p{Cased_Letter>).
909 =item B<C<\p{Unicode}>>
911 This matches any of the 1_114_112 Unicode code points.
914 =item B<C<\p{VertSpace}>>
916 This is the same as C<\v>: A character that changes the spacing vertically.
920 This is the same as C<\w>, including over 100_000 characters beyond ASCII.
922 =item B<C<\p{XPosix...}>>
924 There are several of these, which are the standard Posix classes
925 extended to the full Unicode range. They are described in
926 L<perlrecharclass/POSIX Character Classes>.
930 =head2 Comparison of C<\N{...}> and C<\p{name=...}>
932 Starting in Perl 5.32, you can specify a character by its name in
933 regular expression patterns using C<\p{name=...}>. This is in addition
934 to the longstanding method of using C<\N{...}>. The following
935 summarizes the differences between these two:
938 can interpolate only with eval yes [1]
939 custom names yes no [2]
940 name aliases yes yes [3]
941 named sequences yes yes [4]
942 name value parsing exact Unicode loose [5]
948 The ability to interpolate means you can do something like
950 qr/\p{na=latin capital letter $which}/
952 and specify C<$which> elsewhere.
956 You can create your own names for characters, and override official
957 ones when using C<\N{...}>. See L<charnames/CUSTOM ALIASES>.
961 Some characters have multiple names (synonyms).
965 Some particular sequences of characters are given a single name, in
966 addition to their individual ones.
970 Exact name value matching means you have to specify case, hyphens,
971 underscores, and spaces precisely in the name you want. Loose matching
972 follows the Unicode rules
973 L<https://www.unicode.org/reports/tr44/tr44-24.html#UAX44-LM2>,
974 where these are mostly irrelevant. Except for a few outlier character
975 names, these are the same rules as are already used for any other
980 =head2 Wildcards in Property Values
982 Starting in Perl 5.30, it is possible to do something like this:
984 qr!\p{numeric_value=/\A[0-5]\z/}!
986 or, by abbreviating and adding C</x>,
988 qr! \p{nv= /(?x) \A [0-5] \z / }!
990 This matches all code points whose numeric value is one of 0, 1, 2, 3,
991 4, or 5. This particular example could instead have been written as
993 qr! \A [ \p{nv=0}\p{nv=1}\p{nv=2}\p{nv=3}\p{nv=4}\p{nv=5} ] \z !xx
995 in earlier perls, so in this case this feature just makes things easier
996 and shorter to write. If we hadn't included the C<\A> and C<\z>, these
997 would have matched things like C<1E<sol>2> because that contains a 1 (as
998 well as a 2). As written, it matches things like subscripts that have
999 these numeric values. If we only wanted the decimal digits with those
1000 numeric values, we could say,
1002 qr! (?[ \d & \p{nv=/[0-5]/ ]) }!x
1004 The C<\d> gets rid of needing to anchor the pattern, since it forces the
1005 result to only match C<[0-9]>, and the C<[0-5]> further restricts it.
1007 The text in the above examples enclosed between the C<"E<sol>">
1008 characters can be just about any regular expression. It is independent
1009 of the main pattern, so doesn't share any capturing groups, I<etc>. The
1010 delimiters for it must be ASCII punctuation, but it may NOT be
1011 delimited by C<"{">, nor C<"}"> nor contain a literal C<"}">, as that
1012 delimits the end of the enclosing C<\p{}>. Like any pattern, certain
1013 other delimiters are terminated by their mirror images. These are
1014 C<"(">, C<"[>", and C<"E<lt>">. If the delimiter is any of C<"-">,
1015 C<"_">, C<"+">, or C<"\">, or is the same delimiter as is used for the
1016 enclosing pattern, it must be preceded by a backslash escape, both
1019 Beware of using C<"$"> to indicate to match the end of the string. It
1020 can too easily be interpreted as being a punctuation variable, like
1023 No modifiers may follow the final delimiter. Instead, use
1024 L<perlre/(?adlupimnsx-imnsx)> and/or
1025 L<perlre/(?adluimnsx-imnsx:pattern)> to specify modifiers.
1026 However, certain modifiers are illegal in your wildcard subpattern.
1027 The only character set modifier specifiable is C</aa>;
1028 any other character set, and C<-m>, and C<p>, and C<s> are all illegal.
1029 Specifying modifiers like C<qr/.../gc> that aren't legal in the
1030 C<(?...)> notation normally raise a warning, but with wildcard
1031 subpatterns, their use is an error. The C<m> modifier is ineffective;
1032 everything that matches will be a single line.
1034 By default, your pattern is matched case-insensitively, as if C</i> had
1035 been specified. You can change this by saying C<(?-i)> in your pattern.
1037 There are also certain operations that are illegal. You can't nest
1038 C<\p{...}> and C<\P{...}> calls within a wildcard subpattern, and C<\G>
1039 doesn't make sense, so is also prohibited.
1041 And the C<*> quantifier (or its equivalent C<(0,}>) is illegal.
1043 This feature is not available when the left-hand side is prefixed by
1044 C<Is_>, nor for any form that is marked as "Discouraged" in
1045 L<perluniprops/Discouraged>.
1047 This experimental feature has been added to begin to implement
1048 L<https://www.unicode.org/reports/tr18/#Wildcard_Properties>. Using it
1049 will raise a (default-on) warning in the
1050 C<experimental::uniprop_wildcards> category. We reserve the right to
1051 change its operation as we gain experience.
1053 Your subpattern can be just about anything, but for it to have some
1054 utility, it should match when called with either or both of
1055 a) the full name of the property value with underscores (and/or spaces
1056 in the Block property) and some things uppercase; or b) the property
1057 value in all lowercase with spaces and underscores squeezed out. For
1060 qr!\p{Blk=/Old I.*/}!
1061 qr!\p{Blk=/oldi.*/}!
1063 would match the same things.
1065 Another example that shows that within C<\p{...}>, C</x> isn't needed to
1068 qr!\p{scx= /Hebrew|Greek/ }!
1070 To be safe, we should have anchored the above example, to prevent
1071 matches for something like C<Hebrew_Braille>, but there aren't
1072 any script names like that, so far.
1073 A warning is issued if none of the legal values for a property are
1074 matched by your pattern. It's likely that a future release will raise a
1075 warning if your pattern ends up causing every possible code point to
1078 Starting in 5.32, the Name, Name Aliases, and Named Sequences properties
1079 are allowed to be matched. They are considered to be a single
1080 combination property, just as has long been the case for C<\N{}>. Loose
1081 matching doesn't work in exactly the same way for these as it does for
1082 the values of other properties. The rules are given in
1083 L<https://www.unicode.org/reports/tr44/tr44-24.html#UAX44-LM2>. As a
1084 result, Perl doesn't try loose matching for you, like it does in other
1085 properties. All letters in names are uppercase, but you can add C<(?i)>
1086 to your subpattern to ignore case. If you're uncertain where a blank
1087 is, you can use C< ?> in your subpattern. No character name contains an
1088 underscore, so don't bother trying to match one. The use of hyphens is
1089 particularly problematic; refer to the above link. But note that, as of
1090 Unicode 13.0, the only script in modern usage which has weirdnesses with
1091 these is Tibetan; also the two Korean characters U+116C HANGUL JUNGSEONG
1092 OE and U+1180 HANGUL JUNGSEONG O-E. Unicode makes no promises to not
1093 add hyphen-problematic names in the future.
1095 Using wildcards on these is resource intensive, given the hundreds of
1096 thousands of legal names that must be checked against.
1098 An example of using Name property wildcards is
1100 qr!\p{name=/(SMILING|GRINNING) FACE/}!
1104 qr/(?[ \p{name=\/CJK\/} - \p{ideographic} ])/
1106 which is the 200-ish (as of Unicode 13.0) CJK characters that aren't
1109 There are certain properties that wildcard subpatterns don't currently
1110 work with. These are:
1112 Bidi Mirroring Glyph
1115 Decomposition Mapping
1116 Equivalent Unified Ideograph
1122 Nor is the C<@I<unicode_property>@> form implemented.
1124 Here's a complete example of matching IPV4 internet protocol addresses
1125 in any (single) script
1127 no warnings 'experimental::regex_sets';
1128 no warnings 'experimental::uniprop_wildcards';
1130 # Can match a substring, so this intermediate regex needs to have
1131 # context or anchoring in its final use. Using nt=de yields decimal
1132 # digits. When specifying a subset of these, we must include \d to
1133 # prevent things like U+00B2 SUPERSCRIPT TWO from matching
1134 my $zero_through_255 =
1135 qr/ \b (*sr: # All from same sript
1136 (?[ \p{nv=0} & \d ])* # Optional leading zeros
1139 | (?[ \p{nv=1} & \d ]) \d{2} # 100 - 199
1140 | (?[ \p{nv=2} & \d ])
1141 ( (?[ \p{nv=:[0-4]:} & \d ]) \d # 200 - 249
1142 | (?[ \p{nv=5} & \d ])
1143 (?[ \p{nv=:[0-5]:} & \d ]) # 250 - 255
1150 my $ipv4 = qr/ \A (*sr: $zero_through_255
1151 (?: [.] $zero_through_255 ) {3}
1156 =head2 User-Defined Character Properties
1158 You can define your own binary character properties by defining subroutines
1159 whose names begin with C<"In"> or C<"Is">. (The experimental feature
1160 L<perlre/(?[ ])> provides an alternative which allows more complex
1161 definitions.) The subroutines can be defined in any
1162 package. They override any Unicode properties expressed as the same
1163 names. The user-defined properties can be used in the regular
1165 C<\p{}> and C<\P{}> constructs; if you are using a user-defined property from a
1166 package other than the one you are in, you must specify its package in the
1167 C<\p{}> or C<\P{}> construct.
1169 # assuming property Is_Foreign defined in Lang::
1170 package main; # property package name required
1171 if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
1173 package Lang; # property package name not required
1174 if ($txt =~ /\p{IsForeign}+/) { ... }
1177 Note that the effect is compile-time and immutable once defined.
1178 However, the subroutines are passed a single parameter, which is 0 if
1179 case-sensitive matching is in effect and non-zero if caseless matching
1180 is in effect. The subroutine may return different values depending on
1181 the value of the flag, and one set of values will immutably be in effect
1182 for all case-sensitive matches, and the other set for all case-insensitive
1185 Note that if the regular expression is tainted, then Perl will die rather
1186 than calling the subroutine when the name of the subroutine is
1187 determined by the tainted data.
1189 The subroutines must return a specially-formatted string, with one
1190 or more newline-separated lines. Each line must be one of the following:
1196 A single hexadecimal number denoting a code point to include.
1200 Two hexadecimal numbers separated by horizontal whitespace (space or
1201 tabular characters) denoting a range of code points to include. The
1202 second number must not be smaller than the first.
1206 Something to include, prefixed by C<"+">: a built-in character
1207 property (prefixed by C<"utf8::">) or a fully qualified (including package
1208 name) user-defined character property,
1209 to represent all the characters in that property; two hexadecimal code
1210 points for a range; or a single hexadecimal code point.
1214 Something to exclude, prefixed by C<"-">: an existing character
1215 property (prefixed by C<"utf8::">) or a fully qualified (including package
1216 name) user-defined character property,
1217 to represent all the characters in that property; two hexadecimal code
1218 points for a range; or a single hexadecimal code point.
1222 Something to negate, prefixed C<"!">: an existing character
1223 property (prefixed by C<"utf8::">) or a fully qualified (including package
1224 name) user-defined character property,
1225 to represent all the characters in that property; two hexadecimal code
1226 points for a range; or a single hexadecimal code point.
1230 Something to intersect with, prefixed by C<"&">: an existing character
1231 property (prefixed by C<"utf8::">) or a fully qualified (including package
1232 name) user-defined character property,
1233 for all the characters except the characters in the property; two
1234 hexadecimal code points for a range; or a single hexadecimal code point.
1238 For example, to define a property that covers both the Japanese
1239 syllabaries (hiragana and katakana), you can define
1248 Imagine that the here-doc end marker is at the beginning of the line.
1249 Now you can use C<\p{InKana}> and C<\P{InKana}>.
1251 You could also have used the existing block property names:
1260 Suppose you wanted to match only the allocated characters,
1261 not the raw block ranges: in other words, you want to remove
1262 the unassigned characters:
1272 The negation is useful for defining (surprise!) negated classes.
1282 This will match all non-Unicode code points, since every one of them is
1283 not in Kana. You can use intersection to exclude these, if desired, as
1284 this modified example shows:
1295 C<&utf8::Any> must be the last line in the definition.
1297 Intersection is used generally for getting the common characters matched
1298 by two (or more) classes. It's important to remember not to use C<"&"> for
1299 the first set; that would be intersecting with nothing, resulting in an
1300 empty set. (Similarly using C<"-"> for the first set does nothing).
1302 Unlike non-user-defined C<\p{}> property matches, no warning is ever
1303 generated if these properties are matched against a non-Unicode code
1304 point (see L</Beyond Unicode code points> below).
1306 =head2 User-Defined Case Mappings (for serious hackers only)
1308 B<This feature has been removed as of Perl 5.16.>
1309 The CPAN module C<L<Unicode::Casing>> provides better functionality without
1310 the drawbacks that this feature had. If you are using a Perl earlier
1311 than 5.16, this feature was most fully documented in the 5.14 version of
1313 L<http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>
1315 =head2 Character Encodings for Input and Output
1319 =head2 Unicode Regular Expression Support Level
1321 The following list of Unicode supported features for regular expressions describes
1322 all features currently directly supported by core Perl. The references
1323 to "Level I<N>" and the section numbers refer to
1324 L<UTS#18 "Unicode Regular Expressions"|https://www.unicode.org/reports/tr18>,
1325 version 18, October 2016.
1327 =head3 Level 1 - Basic Unicode Support
1329 RL1.1 Hex Notation - Done [1]
1330 RL1.2 Properties - Done [2]
1331 RL1.2a Compatibility Properties - Done [3]
1332 RL1.3 Subtraction and Intersection - Experimental [4]
1333 RL1.4 Simple Word Boundaries - Done [5]
1334 RL1.5 Simple Loose Matches - Done [6]
1335 RL1.6 Line Boundaries - Partial [7]
1336 RL1.7 Supplementary Code Points - Done [8]
1340 =item [1] C<\N{U+...}> and C<\x{...}>
1343 C<\p{...}> C<\P{...}>. This requirement is for a minimal list of
1344 properties. Perl supports these. See R2.7 for other properties.
1347 Perl has C<\d> C<\D> C<\s> C<\S> C<\w> C<\W> C<\X> C<[:I<prop>:]>
1348 C<[:^I<prop>:]>, plus all the properties specified by
1349 L<https://www.unicode.org/reports/tr18/#Compatibility_Properties>. These
1350 are described above in L</Other Properties>
1354 The experimental feature C<"(?[...])"> starting in v5.18 accomplishes
1357 See L<perlre/(?[ ])>. If you don't want to use an experimental
1358 feature, you can use one of the following:
1363 Regular expression lookahead
1365 You can mimic class subtraction using lookahead.
1366 For example, what UTS#18 might write as
1368 [{Block=Greek}-[{UNASSIGNED}]]
1370 in Perl can be written as:
1372 (?!\p{Unassigned})\p{Block=Greek}
1373 (?=\p{Assigned})\p{Block=Greek}
1375 But in this particular example, you probably really want
1379 which will match assigned characters known to be part of the Greek script.
1383 CPAN module C<L<Unicode::Regex::Set>>
1385 It does implement the full UTS#18 grouping, intersection, union, and
1386 removal (subtraction) syntax.
1390 L</"User-Defined Character Properties">
1392 C<"+"> for union, C<"-"> for removal (set-difference), C<"&"> for intersection
1397 C<\b> C<\B> meet most, but not all, the details of this requirement, but
1398 C<\b{wb}> and C<\B{wb}> do, as well as the stricter R2.3.
1402 Note that Perl does Full case-folding in matching, not Simple:
1404 For example C<U+1F88> is equivalent to C<U+1F00 U+03B9>, instead of just
1405 C<U+1F80>. This difference matters mainly for certain Greek capital
1406 letters with certain modifiers: the Full case-folding decomposes the
1407 letter, while the Simple case-folding would map it to a single
1412 The reason this is considered to be only partially implemented is that
1413 Perl has L<C<qrE<sol>\b{lb}E<sol>>|perlrebackslash/\b{lb}> and
1414 C<L<Unicode::LineBreak>> that are conformant with
1415 L<UAX#14 "Unicode Line Breaking Algorithm"|https://www.unicode.org/reports/tr14>.
1416 The regular expression construct provides default behavior, while the
1417 heavier-weight module provides customizable line breaking.
1419 But Perl treats C<\n> as the start- and end-line
1420 delimiter, whereas Unicode specifies more characters that should be
1432 C<^> and C<$> in regular expression patterns are supposed to match all
1434 These characters also don't, but should, affect C<< <> >> C<$.>, and
1435 script line numbers.
1437 Also, lines should not be split within C<CRLF> (i.e. there is no
1438 empty line between C<\r> and C<\n>). For C<CRLF>, try the C<:crlf>
1439 layer (see L<PerlIO>).
1442 UTF-8/UTF-EBDDIC used in Perl allows not only C<U+10000> to
1443 C<U+10FFFF> but also beyond C<U+10FFFF>
1447 =head3 Level 2 - Extended Unicode Support
1449 RL2.1 Canonical Equivalents - Retracted [9]
1451 RL2.2 Extended Grapheme Clusters and - Partial [10]
1452 Character Classes with Strings
1453 RL2.3 Default Word Boundaries - Done [11]
1454 RL2.4 Default Case Conversion - Done
1455 RL2.5 Name Properties - Done
1456 RL2.6 Wildcards in Property Values - Partial [12]
1457 RL2.7 Full Properties - Partial [13]
1458 RL2.8 Optional Properties - Partial [14]
1463 Unicode has rewritten this portion of UTS#18 to say that getting
1464 canonical equivalence (see UAX#15
1465 L<"Unicode Normalization Forms"|https://www.unicode.org/reports/tr15>)
1466 is basically to be done at the programmer level. Use NFD to write
1467 both your regular expressions and text to match them against (you
1468 can use L<Unicode::Normalize>).
1471 Perl has C<\X> and C<\b{gcb}>. Unicode has retracted their "Grapheme
1472 Cluster Mode", and recently added string properties, which Perl does not
1476 L<UAX#29 "Unicode Text Segmentation"|https://www.unicode.org/reports/tr29>,
1479 L</Wildcards in Property Values> above.
1482 Perl supports all the properties in the Unicode Character Database
1483 (UCD). It does not yet support the listed properties that come from
1484 other Unicode sources.
1487 The only optional property that Perl supports is Named Sequence. None
1488 of these properties are in the UCD.
1492 =head3 Level 3 - Tailored Support
1494 This has been retracted by Unicode.
1496 =head2 Unicode Encodings
1498 Unicode characters are assigned to I<code points>, which are abstract
1499 numbers. To use these numbers, various encodings are needed.
1507 UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
1508 encoding. In most of Perl's documentation, including elsewhere in this
1509 document, the term "UTF-8" means also "UTF-EBCDIC". But in this section,
1510 "UTF-8" refers only to the encoding used on ASCII platforms. It is a
1511 superset of 7-bit US-ASCII, so anything encoded in ASCII has the
1512 identical representation when encoded in UTF-8.
1514 The following table is from Unicode 3.2.
1516 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
1518 U+0000..U+007F 00..7F
1519 U+0080..U+07FF * C2..DF 80..BF
1520 U+0800..U+0FFF E0 * A0..BF 80..BF
1521 U+1000..U+CFFF E1..EC 80..BF 80..BF
1522 U+D000..U+D7FF ED 80..9F 80..BF
1523 U+D800..U+DFFF +++++ utf16 surrogates, not legal utf8 +++++
1524 U+E000..U+FFFF EE..EF 80..BF 80..BF
1525 U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF
1526 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
1527 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
1529 Note the gaps marked by "*" before several of the byte entries above. These are
1530 caused by legal UTF-8 avoiding non-shortest encodings: it is technically
1531 possible to UTF-8-encode a single code point in different ways, but that is
1532 explicitly forbidden, and the shortest possible encoding should always be used
1533 (and that is what Perl does).
1535 Another way to look at it is via bits:
1537 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
1540 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
1541 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
1542 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
1544 As you can see, the continuation bytes all begin with C<"10">, and the
1545 leading bits of the start byte tell how many bytes there are in the
1548 The original UTF-8 specification allowed up to 6 bytes, to allow
1549 encoding of numbers up to C<0x7FFF_FFFF>. Perl continues to allow those,
1550 and has extended that up to 13 bytes to encode code points up to what
1551 can fit in a 64-bit word. However, Perl will warn if you output any of
1552 these as being non-portable; and under strict UTF-8 input protocols,
1553 they are forbidden. In addition, it is now illegal to use a code point
1554 larger than what a signed integer variable on your system can hold. On
1555 32-bit ASCII systems, this means C<0x7FFF_FFFF> is the legal maximum
1556 (much higher on 64-bit systems).
1562 Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
1563 This means that all the basic characters (which includes all
1564 those that have ASCII equivalents (like C<"A">, C<"0">, C<"%">, I<etc.>)
1565 are the same in both EBCDIC and UTF-EBCDIC.)
1567 UTF-EBCDIC is used on EBCDIC platforms. It generally requires more
1568 bytes to represent a given code point than UTF-8 does; the largest
1569 Unicode code points take 5 bytes to represent (instead of 4 in UTF-8),
1570 and, extended for 64-bit words, it uses 14 bytes instead of 13 bytes in
1575 UTF-16, UTF-16BE, UTF-16LE, Surrogates, and C<BOM>'s (Byte Order Marks)
1577 The followings items are mostly for reference and general Unicode
1578 knowledge, Perl doesn't use these constructs internally.
1580 Like UTF-8, UTF-16 is a variable-width encoding, but where
1581 UTF-8 uses 8-bit code units, UTF-16 uses 16-bit code units.
1582 All code points occupy either 2 or 4 bytes in UTF-16: code points
1583 C<U+0000..U+FFFF> are stored in a single 16-bit unit, and code
1584 points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is
1585 using I<surrogates>, the first 16-bit unit being the I<high
1586 surrogate>, and the second being the I<low surrogate>.
1588 Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
1589 range of Unicode code points in pairs of 16-bit units. The I<high
1590 surrogates> are the range C<U+D800..U+DBFF> and the I<low surrogates>
1591 are the range C<U+DC00..U+DFFF>. The surrogate encoding is
1593 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1594 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
1598 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
1600 Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
1601 itself can be used for in-memory computations, but if storage or
1602 transfer is required either UTF-16BE (big-endian) or UTF-16LE
1603 (little-endian) encodings must be chosen.
1605 This introduces another problem: what if you just know that your data
1606 is UTF-16, but you don't know which endianness? Byte Order Marks, or
1607 C<BOM>'s, are a solution to this. A special character has been reserved
1608 in Unicode to function as a byte order marker: the character with the
1609 code point C<U+FEFF> is the C<BOM>.
1611 The trick is that if you read a C<BOM>, you will know the byte order,
1612 since if it was written on a big-endian platform, you will read the
1613 bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
1614 you will read the bytes C<0xFF 0xFE>. (And if the originating platform
1615 was writing in ASCII platform UTF-8, you will read the bytes
1618 The way this trick works is that the character with the code point
1619 C<U+FFFE> is not supposed to be in input streams, so the
1620 sequence of bytes C<0xFF 0xFE> is unambiguously "C<BOM>, represented in
1621 little-endian format" and cannot be C<U+FFFE>, represented in big-endian
1624 Surrogates have no meaning in Unicode outside their use in pairs to
1625 represent other code points. However, Perl allows them to be
1626 represented individually internally, for example by saying
1627 C<chr(0xD801)>, so that all code points, not just those valid for open
1629 representable. Unicode does define semantics for them, such as their
1630 C<L</General_Category>> is C<"Cs">. But because their use is somewhat dangerous,
1631 Perl will warn (using the warning category C<"surrogate">, which is a
1632 sub-category of C<"utf8">) if an attempt is made
1633 to do things like take the lower case of one, or match
1634 case-insensitively, or to output them. (But don't try this on Perls
1639 UTF-32, UTF-32BE, UTF-32LE
1641 The UTF-32 family is pretty much like the UTF-16 family, except that
1642 the units are 32-bit, and therefore the surrogate scheme is not
1643 needed. UTF-32 is a fixed-width encoding. The C<BOM> signatures are
1644 C<0x00 0x00 0xFE 0xFF> for BE and C<0xFF 0xFE 0x00 0x00> for LE.
1650 Legacy, fixed-width encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
1651 encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
1652 because it does not use surrogates. UCS-4 is a 32-bit encoding,
1653 functionally identical to UTF-32 (the difference being that
1654 UCS-4 forbids neither surrogates nor code points larger than C<0x10_FFFF>).
1660 A seven-bit safe (non-eight-bit) encoding, which is useful if the
1661 transport or storage is not eight-bit safe. Defined by RFC 2152.
1665 =head2 Noncharacter code points
1667 66 code points are set aside in Unicode as "noncharacter code points".
1668 These all have the C<Unassigned> (C<Cn>) C<L</General_Category>>, and
1669 no character will ever be assigned to any of them. They are the 32 code
1670 points between C<U+FDD0> and C<U+FDEF> inclusive, and the 34 code
1681 Until Unicode 7.0, the noncharacters were "B<forbidden> for use in open
1682 interchange of Unicode text data", so that code that processed those
1683 streams could use these code points as sentinels that could be mixed in
1684 with character data, and would always be distinguishable from that data.
1685 (Emphasis above and in the next paragraph are added in this document.)
1687 Unicode 7.0 changed the wording so that they are "B<not recommended> for
1688 use in open interchange of Unicode text data". The 7.0 Standard goes on
1693 "If a noncharacter is received in open interchange, an application is
1694 not required to interpret it in any way. It is good practice, however,
1695 to recognize it as a noncharacter and to take appropriate action, such
1696 as replacing it with C<U+FFFD> replacement character, to indicate the
1697 problem in the text. It is not recommended to simply delete
1698 noncharacter code points from such text, because of the potential
1699 security issues caused by deleting uninterpreted characters. (See
1700 conformance clause C7 in Section 3.2, Conformance Requirements, and
1701 L<Unicode Technical Report #36, "Unicode Security
1702 Considerations"|https://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
1706 This change was made because it was found that various commercial tools
1707 like editors, or for things like source code control, had been written
1708 so that they would not handle program files that used these code points,
1709 effectively precluding their use almost entirely! And that was never
1710 the intent. They've always been meant to be usable within an
1711 application, or cooperating set of applications, at will.
1713 If you're writing code, such as an editor, that is supposed to be able
1714 to handle any Unicode text data, then you shouldn't be using these code
1715 points yourself, and instead allow them in the input. If you need
1716 sentinels, they should instead be something that isn't legal Unicode.
1717 For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
1718 they never appear in well-formed UTF-8. (There are equivalents for
1719 UTF-EBCDIC). You can also store your Unicode code points in integer
1720 variables and use negative values as sentinels.
1722 If you're not writing such a tool, then whether you accept noncharacters
1723 as input is up to you (though the Standard recommends that you not). If
1724 you do strict input stream checking with Perl, these code points
1725 continue to be forbidden. This is to maintain backward compatibility
1726 (otherwise potential security holes could open up, as an unsuspecting
1727 application that was written assuming the noncharacters would be
1728 filtered out before getting to it, could now, without warning, start
1729 getting them). To do strict checking, you can use the layer
1730 C<:encoding('UTF-8')>.
1732 Perl continues to warn (using the warning category C<"nonchar">, which
1733 is a sub-category of C<"utf8">) if an attempt is made to output
1736 =head2 Beyond Unicode code points
1738 The maximum Unicode code point is C<U+10FFFF>, and Unicode only defines
1739 operations on code points up through that. But Perl works on code
1740 points up to the maximum permissible signed number available on the
1741 platform. However, Perl will not accept these from input streams unless
1742 lax rules are being used, and will warn (using the warning category
1743 C<"non_unicode">, which is a sub-category of C<"utf8">) if any are output.
1745 Since Unicode rules are not defined on these code points, if a
1746 Unicode-defined operation is done on them, Perl uses what we believe are
1747 sensible rules, while generally warning, using the C<"non_unicode">
1748 category. For example, C<uc("\x{11_0000}")> will generate such a
1749 warning, returning the input parameter as its result, since Perl defines
1750 the uppercase of every non-Unicode code point to be the code point
1751 itself. (All the case changing operations, not just uppercasing, work
1754 The situation with matching Unicode properties in regular expressions,
1755 the C<\p{}> and C<\P{}> constructs, against these code points is not as
1756 clear cut, and how these are handled has changed as we've gained
1759 One possibility is to treat any match against these code points as
1760 undefined. But since Perl doesn't have the concept of a match being
1761 undefined, it converts this to failing or C<FALSE>. This is almost, but
1762 not quite, what Perl did from v5.14 (when use of these code points
1763 became generally reliable) through v5.18. The difference is that Perl
1764 treated all C<\p{}> matches as failing, but all C<\P{}> matches as
1767 One problem with this is that it leads to unexpected, and confusing
1768 results in some cases:
1770 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Failed on <= v5.18
1771 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Failed! on <= v5.18
1773 That is, it treated both matches as undefined, and converted that to
1774 false (raising a warning on each). The first case is the expected
1775 result, but the second is likely counterintuitive: "How could both be
1776 false when they are complements?" Another problem was that the
1777 implementation optimized many Unicode property matches down to already
1778 existing simpler, faster operations, which don't raise the warning. We
1779 chose to not forgo those optimizations, which help the vast majority of
1780 matches, just to generate a warning for the unlikely event that an
1781 above-Unicode code point is being matched against.
1783 As a result of these problems, starting in v5.20, what Perl does is
1784 to treat non-Unicode code points as just typical unassigned Unicode
1785 characters, and matches accordingly. (Note: Unicode has atypical
1786 unassigned code points. For example, it has noncharacter code points,
1787 and ones that, when they do get assigned, are destined to be written
1788 Right-to-left, as Arabic and Hebrew are. Perl assumes that no
1789 non-Unicode code point has any atypical properties.)
1791 Perl, in most cases, will raise a warning when matching an above-Unicode
1792 code point against a Unicode property when the result is C<TRUE> for
1793 C<\p{}>, and C<FALSE> for C<\P{}>. For example:
1795 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails, no warning
1796 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Succeeds, with warning
1798 In both these examples, the character being matched is non-Unicode, so
1799 Unicode doesn't define how it should match. It clearly isn't an ASCII
1800 hex digit, so the first example clearly should fail, and so it does,
1801 with no warning. But it is arguable that the second example should have
1802 an undefined, hence C<FALSE>, result. So a warning is raised for it.
1804 Thus the warning is raised for many fewer cases than in earlier Perls,
1805 and only when what the result is could be arguable. It turns out that
1806 none of the optimizations made by Perl (or are ever likely to be made)
1807 cause the warning to be skipped, so it solves both problems of Perl's
1808 earlier approach. The most commonly used property that is affected by
1809 this change is C<\p{Unassigned}> which is a short form for
1810 C<\p{General_Category=Unassigned}>. Starting in v5.20, all non-Unicode
1811 code points are considered C<Unassigned>. In earlier releases the
1812 matches failed because the result was considered undefined.
1814 The only place where the warning is not raised when it might ought to
1815 have been is if optimizations cause the whole pattern match to not even
1816 be attempted. For example, Perl may figure out that for a string to
1817 match a certain regular expression pattern, the string has to contain
1818 the substring C<"foobar">. Before attempting the match, Perl may look
1819 for that substring, and if not found, immediately fail the match without
1820 actually trying it; so no warning gets generated even if the string
1821 contains an above-Unicode code point.
1823 This behavior is more "Do what I mean" than in earlier Perls for most
1824 applications. But it catches fewer issues for code that needs to be
1825 strictly Unicode compliant. Therefore there is an additional mode of
1826 operation available to accommodate such code. This mode is enabled if a
1827 regular expression pattern is compiled within the lexical scope where
1828 the C<"non_unicode"> warning class has been made fatal, say by:
1830 use warnings FATAL => "non_unicode"
1832 (see L<warnings>). In this mode of operation, Perl will raise the
1833 warning for all matches against a non-Unicode code point (not just the
1834 arguable ones), and it skips the optimizations that might cause the
1835 warning to not be output. (It currently still won't warn if the match
1836 isn't even attempted, like in the C<"foobar"> example above.)
1838 In summary, Perl now normally treats non-Unicode code points as typical
1839 Unicode unassigned code points for regular expression matches, raising a
1840 warning only when it is arguable what the result should be. However, if
1841 this warning has been made fatal, it isn't skipped.
1843 There is one exception to all this. C<\p{All}> looks like a Unicode
1844 property, but it is a Perl extension that is defined to be true for all
1845 possible code points, Unicode or not, so no warning is ever generated
1846 when matching this against a non-Unicode code point. (Prior to v5.20,
1847 it was an exact synonym for C<\p{Any}>, matching code points C<0>
1848 through C<0x10FFFF>.)
1850 =head2 Security Implications of Unicode
1853 L<Unicode Security Considerations|https://www.unicode.org/reports/tr36>.
1855 Also, note the following:
1863 UTF-8 is very structured, so many combinations of bytes are invalid. In
1864 the past, Perl tried to soldier on and make some sense of invalid
1865 combinations, but this can lead to security holes, so now, if the Perl
1866 core needs to process an invalid combination, it will either raise a
1867 fatal error, or will replace those bytes by the sequence that forms the
1868 Unicode REPLACEMENT CHARACTER, for which purpose Unicode created it.
1870 Every code point can be represented by more than one possible
1871 syntactically valid UTF-8 sequence. Early on, both Unicode and Perl
1872 considered any of these to be valid, but now, all sequences longer
1873 than the shortest possible one are considered to be malformed.
1875 Unicode considers many code points to be illegal, or to be avoided.
1876 Perl generally accepts them, once they have passed through any input
1877 filters that may try to exclude them. These have been discussed above
1878 (see "Surrogates" under UTF-16 in L</Unicode Encodings>,
1879 L</Noncharacter code points>, and L</Beyond Unicode code points>).
1883 Regular expression pattern matching may surprise you if you're not
1884 accustomed to Unicode. Starting in Perl 5.14, several pattern
1885 modifiers are available to control this, called the character set
1886 modifiers. Details are given in L<perlre/Character set modifiers>.
1890 As discussed elsewhere, Perl has one foot (two hooves?) planted in
1891 each of two worlds: the old world of ASCII and single-byte locales, and
1892 the new world of Unicode, upgrading when necessary.
1893 If your legacy code does not explicitly use Unicode, no automatic
1894 switch-over to Unicode should happen.
1896 =head2 Unicode in Perl on EBCDIC
1898 Unicode is supported on EBCDIC platforms. See L<perlebcdic>.
1900 Unless ASCII vs. EBCDIC issues are specifically being discussed,
1901 references to UTF-8 encoding in this document and elsewhere should be
1902 read as meaning UTF-EBCDIC on EBCDIC platforms.
1903 See L<perlebcdic/Unicode and UTF>.
1905 Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
1906 hidden from you; S<C<use utf8>> (and NOT something like
1907 S<C<use utfebcdic>>) declares the script is in the platform's
1908 "native" 8-bit encoding of Unicode. (Similarly for the C<":utf8">
1913 See L<perllocale/Unicode and UTF-8>
1915 =head2 When Unicode Does Not Happen
1917 There are still many places where Unicode (in some encoding or
1918 another) could be given as arguments or received as results, or both in
1919 Perl, but it is not, in spite of Perl having extensive ways to input and
1920 output in Unicode, and a few other "entry points" like the C<@ARGV>
1921 array (which can sometimes be interpreted as UTF-8).
1923 The following are such interfaces. Also, see L</The "Unicode Bug">.
1924 For all of these interfaces Perl
1925 currently (as of v5.16.0) simply assumes byte strings both as arguments
1926 and results, or UTF-8 strings if the (deprecated) C<encoding> pragma has been used.
1928 One reason that Perl does not attempt to resolve the role of Unicode in
1929 these situations is that the answers are highly dependent on the operating
1930 system and the file system(s). For example, whether filenames can be
1931 in Unicode and in exactly what kind of encoding, is not exactly a
1932 portable concept. Similarly for C<qx> and C<system>: how well will the
1933 "command-line interface" (and which of them?) handle Unicode?
1939 C<chdir>, C<chmod>, C<chown>, C<chroot>, C<exec>, C<link>, C<lstat>, C<mkdir>,
1940 C<rename>, C<rmdir>, C<stat>, C<symlink>, C<truncate>, C<unlink>, C<utime>, C<-X>
1948 C<glob> (aka the C<E<lt>*E<gt>>)
1952 C<open>, C<opendir>, C<sysopen>
1956 C<qx> (aka the backtick operator), C<system>
1960 C<readdir>, C<readlink>
1964 =head2 The "Unicode Bug"
1966 The term, "Unicode bug" has been applied to an inconsistency with the
1967 code points in the C<Latin-1 Supplement> block, that is, between
1968 128 and 255. Without a locale specified, unlike all other characters or
1969 code points, these characters can have very different semantics
1970 depending on the rules in effect. (Characters whose code points are
1971 above 255 force Unicode rules; whereas the rules for ASCII characters
1972 are the same under both ASCII and Unicode rules.)
1974 Under Unicode rules, these upper-Latin1 characters are interpreted as
1975 Unicode code points, which means they have the same semantics as Latin-1
1976 (ISO-8859-1) and C1 controls.
1978 As explained in L</ASCII Rules versus Unicode Rules>, under ASCII rules,
1979 they are considered to be unassigned characters.
1981 This can lead to unexpected results. For example, a string's
1982 semantics can suddenly change if a code point above 255 is appended to
1983 it, which changes the rules from ASCII to Unicode. As an
1984 example, consider the following program and its output:
1987 no feature "unicode_strings";
1990 for ($s1, $s2, $s1.$s2) {
1998 If there's no C<\w> in C<s1> nor in C<s2>, why does their concatenation
2001 This anomaly stems from Perl's attempt to not disturb older programs that
2002 didn't use Unicode, along with Perl's desire to add Unicode support
2003 seamlessly. But the result turned out to not be seamless. (By the way,
2004 you can choose to be warned when things like this happen. See
2005 C<L<encoding::warnings>>.)
2007 L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
2008 was added, starting in Perl v5.12, to address this problem. It affects
2015 Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
2016 and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in double-quotish
2017 contexts, such as regular expression substitutions.
2019 Under C<unicode_strings> starting in Perl 5.12.0, Unicode rules are
2020 generally used. See L<perlfunc/lc> for details on how this works
2021 in combination with various other pragmas.
2025 Using caseless (C</i>) regular expression matching.
2027 Starting in Perl 5.14.0, regular expressions compiled within
2028 the scope of C<unicode_strings> use Unicode rules
2029 even when executed or compiled into larger
2030 regular expressions outside the scope.
2034 Matching any of several properties in regular expressions.
2036 These properties are C<\b> (without braces), C<\B> (without braces),
2037 C<\s>, C<\S>, C<\w>, C<\W>, and all the Posix character classes
2038 I<except> C<[[:ascii:]]>.
2040 Starting in Perl 5.14.0, regular expressions compiled within
2041 the scope of C<unicode_strings> use Unicode rules
2042 even when executed or compiled into larger
2043 regular expressions outside the scope.
2047 In C<quotemeta> or its inline equivalent C<\Q>.
2049 Starting in Perl 5.16.0, consistent quoting rules are used within the
2050 scope of C<unicode_strings>, as described in L<perlfunc/quotemeta>.
2051 Prior to that, or outside its scope, no code points above 127 are quoted
2052 in UTF-8 encoded strings, but in byte encoded strings, code points
2053 between 128-255 are always quoted.
2057 In the C<..> or L<range|perlop/Range Operators> operator.
2059 Starting in Perl 5.26.0, the range operator on strings treats their lengths
2060 consistently within the scope of C<unicode_strings>. Prior to that, or
2061 outside its scope, it could produce strings whose length in characters
2062 exceeded that of the right-hand side, where the right-hand side took up more
2063 bytes than the correct range endpoint.
2067 In L<< C<split>'s special-case whitespace splitting|perlfunc/split >>.
2069 Starting in Perl 5.28.0, the C<split> function with a pattern specified as
2070 a string containing a single space handles whitespace characters consistently
2071 within the scope of C<unicode_strings>. Prior to that, or outside its scope,
2072 characters that are whitespace according to Unicode rules but not according to
2073 ASCII rules were treated as field contents rather than field separators when
2074 they appear in byte-encoded strings.
2078 You can see from the above that the effect of C<unicode_strings>
2079 increased over several Perl releases. (And Perl's support for Unicode
2080 continues to improve; it's best to use the latest available release in
2081 order to get the most complete and accurate results possible.) Note that
2082 C<unicode_strings> is automatically chosen if you S<C<use 5.012>> or
2085 For Perls earlier than those described above, or when a string is passed
2086 to a function outside the scope of C<unicode_strings>, see the next section.
2088 =head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
2090 Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
2091 there are situations where you simply need to force a byte
2092 string into UTF-8, or vice versa. The standard module L<Encode> can be
2093 used for this, or the low-level calls
2094 L<C<utf8::upgrade($bytestring)>|utf8/Utility functions> and
2095 L<C<utf8::downgrade($utf8string[, FAIL_OK])>|utf8/Utility functions>.
2097 Note that C<utf8::downgrade()> can fail if the string contains characters
2098 that don't fit into a byte.
2100 Calling either function on a string that already is in the desired state is a
2103 L</ASCII Rules versus Unicode Rules> gives all the ways that a string is
2104 made to use Unicode rules.
2106 =head2 Using Unicode in XS
2108 See L<perlguts/"Unicode Support"> for an introduction to Unicode at
2109 the XS level, and L<perlapi/Unicode Support> for the API details.
2111 =head2 Hacking Perl to work on earlier Unicode versions (for very serious hackers only)
2113 Perl by default comes with the latest supported Unicode version built-in, but
2114 the goal is to allow you to change to use any earlier one. In Perls
2115 v5.20 and v5.22, however, the earliest usable version is Unicode 5.1.
2116 Perl v5.18 and v5.24 are able to handle all earlier versions.
2118 Download the files in the desired version of Unicode from the Unicode web
2119 site L<https://www.unicode.org>). These should replace the existing files in
2120 F<lib/unicore> in the Perl source tree. Follow the instructions in
2121 F<README.perl> in that directory to change some of their names, and then build
2122 perl (see L<INSTALL>).
2124 =head2 Porting code from perl-5.6.X
2126 Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6 the
2127 programmer was required to use the C<utf8> pragma to declare that a
2128 given scope expected to deal with Unicode data and had to make sure that
2129 only Unicode data were reaching that scope. If you have code that is
2130 working with 5.6, you will need some of the following adjustments to
2131 your code. The examples are written such that the code will continue to
2132 work under 5.6, so you should be safe to try them out.
2138 A filehandle that should read or write UTF-8
2141 binmode $fh, ":encoding(UTF-8)";
2146 A scalar that is going to be passed to some extension
2148 Be it C<Compress::Zlib>, C<Apache::Request> or any extension that has no
2149 mention of Unicode in the manpage, you need to make sure that the
2150 UTF8 flag is stripped off. Note that at the time of this writing
2151 (January 2012) the mentioned modules are not UTF-8-aware. Please
2152 check the documentation to verify if this is still true.
2156 $val = Encode::encode("UTF-8", $val); # make octets
2161 A scalar we got back from an extension
2163 If you believe the scalar comes back as UTF-8, you will most likely
2164 want the UTF8 flag restored:
2168 $val = Encode::decode("UTF-8", $val);
2173 Same thing, if you are really sure it is UTF-8
2177 Encode::_utf8_on($val);
2182 A wrapper for L<DBI> C<fetchrow_array> and C<fetchrow_hashref>
2184 When the database contains only UTF-8, a wrapper function or method is
2185 a convenient way to replace all your C<fetchrow_array> and
2186 C<fetchrow_hashref> calls. A wrapper function will also make it easier to
2187 adapt to future enhancements in your database driver. Note that at the
2188 time of this writing (January 2012), the DBI has no standardized way
2189 to deal with UTF-8 data. Please check the L<DBI documentation|DBI> to verify if
2193 # $what is one of fetchrow_{array,hashref}
2194 my($self, $sth, $what) = @_;
2200 my @arr = $sth->$what;
2202 defined && /[^\000-\177]/ && Encode::_utf8_on($_);
2206 my $ret = $sth->$what;
2208 for my $k (keys %$ret) {
2211 && Encode::_utf8_on($_) for $ret->{$k};
2215 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
2225 A large scalar that you know can only contain ASCII
2227 Scalars that contain only ASCII and are marked as UTF-8 are sometimes
2228 a drag to your program. If you recognize such a situation, just remove
2231 utf8::downgrade($val) if $] > 5.008;
2237 See also L</The "Unicode Bug"> above.
2239 =head2 Interaction with Extensions
2241 When Perl exchanges data with an extension, the extension should be
2242 able to understand the UTF8 flag and act accordingly. If the
2243 extension doesn't recognize that flag, it's likely that the extension
2244 will return incorrectly-flagged data.
2246 So if you're working with Unicode data, consult the documentation of
2247 every module you're using if there are any issues with Unicode data
2248 exchange. If the documentation does not talk about Unicode at all,
2249 suspect the worst and probably look at the source to learn how the
2250 module is implemented. Modules written completely in Perl shouldn't
2251 cause problems. Modules that directly or indirectly access code written
2252 in other programming languages are at risk.
2254 For affected functions, the simple strategy to avoid data corruption is
2255 to always make the encoding of the exchanged data explicit. Choose an
2256 encoding that you know the extension can handle. Convert arguments passed
2257 to the extensions to that encoding and convert results back from that
2258 encoding. Write wrapper functions that do the conversions for you, so
2259 you can later change the functions when the extension catches up.
2261 To provide an example, let's say the popular C<Foo::Bar::escape_html>
2262 function doesn't deal with Unicode data yet. The wrapper function
2263 would convert the argument to raw UTF-8 and convert the result back to
2264 Perl's internal representation like so:
2266 sub my_escape_html ($) {
2268 return unless defined $what;
2269 Encode::decode("UTF-8", Foo::Bar::escape_html(
2270 Encode::encode("UTF-8", $what)));
2273 Sometimes, when the extension does not convert data but just stores
2274 and retrieves it, you will be able to use the otherwise
2275 dangerous L<C<Encode::_utf8_on()>|Encode/_utf8_on> function. Let's say
2276 the popular C<Foo::Bar> extension, written in C, provides a C<param>
2277 method that lets you store and retrieve data according to these prototypes:
2279 $self->param($name, $value); # set a scalar
2280 $value = $self->param($name); # retrieve a scalar
2282 If it does not yet provide support for any encoding, one could write a
2283 derived class with such a C<param> method:
2286 my($self,$name,$value) = @_;
2287 utf8::upgrade($name); # make sure it is UTF-8 encoded
2288 if (defined $value) {
2289 utf8::upgrade($value); # make sure it is UTF-8 encoded
2290 return $self->SUPER::param($name,$value);
2292 my $ret = $self->SUPER::param($name);
2293 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
2298 Some extensions provide filters on data entry/exit points, such as
2299 C<DB_File::filter_store_key> and family. Look out for such filters in
2300 the documentation of your extensions; they can make the transition to
2301 Unicode data much easier.
2305 Some functions are slower when working on UTF-8 encoded strings than
2306 on byte encoded strings. All functions that need to hop over
2307 characters such as C<length()>, C<substr()> or C<index()>, or matching
2308 regular expressions can work B<much> faster when the underlying data are
2311 In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
2312 a caching scheme was introduced which improved the situation. In general,
2313 operations with UTF-8 encoded strings are still slower. As an example,
2314 the Unicode properties (character classes) like C<\p{Nd}> are known to
2315 be quite a bit slower (5-20 times) than their simpler counterparts
2316 like C<[0-9]> (then again, there are hundreds of Unicode characters matching
2317 C<Nd> compared with the 10 ASCII characters matching C<[0-9]>).
2321 L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>,
2322 L<perlretut>, L<perlvar/"${^UNICODE}">,
2323 L<https://www.unicode.org/reports/tr44>).