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
40 obvious, see L</Security Implications of Unicode>.
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 same short form for another.
453 Thus, for the C<L</General_Category>> property, C<"L"> means
454 C<"Letter">, but for the L<C<Bidi_Class>|/Bidirectional Character Types>
455 property, C<"L"> means C<"Left">. A complete list of properties and
456 synonyms is in L<perluniprops>.
458 Upper/lower case differences in property names and values are irrelevant;
459 thus C<\p{Upper}> means the same thing as C<\p{upper}> or even C<\p{UpPeR}>.
460 Similarly, you can add or subtract underscores anywhere in the middle of a
461 word, so that these are also equivalent to C<\p{U_p_p_e_r}>. And white space
462 is irrelevant adjacent to non-word characters, such as the braces and the equals
463 or colon separators, so C<\p{ Upper }> and C<\p{ Upper_case : Y }> are
464 equivalent to these as well. In fact, white space and even
465 hyphens can usually be added or deleted anywhere. So even C<\p{ Up-per case = Yes}> is
466 equivalent. All this is called "loose-matching" by Unicode. The few places
467 where stricter matching is used is in the middle of numbers, and in the Perl
468 extension properties that begin or end with an underscore. Stricter matching
469 cares about white space (except adjacent to non-word characters),
470 hyphens, and non-interior underscores.
472 You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret
473 (C<^>) between the first brace and the property name: C<\p{^Tamil}> is
474 equal to C<\P{Tamil}>.
476 Almost all properties are immune to case-insensitive matching. That is,
477 adding a C</i> regular expression modifier does not change what they
478 match. There are two sets that are affected.
482 and C<Titlecase_Letter>,
483 all of which match C<Cased_Letter> under C</i> matching.
484 And the second set is
488 all of which match C<Cased> under C</i> matching.
489 This set also includes its subsets C<PosixUpper> and C<PosixLower> both
490 of which under C</i> match C<PosixAlpha>.
491 (The difference between these sets is that some things, such as Roman
492 numerals, come in both upper and lower case so they are C<Cased>, but
493 aren't considered letters, so they aren't C<Cased_Letter>'s.)
495 See L</Beyond Unicode code points> for special considerations when
496 matching Unicode properties against non-Unicode code points.
498 =head3 B<General_Category>
500 Every Unicode character is assigned a general category, which is the "most
501 usual categorization of a character" (from
502 L<http://www.unicode.org/reports/tr44>).
504 The compound way of writing these is like C<\p{General_Category=Number}>
505 (short: C<\p{gc:n}>). But Perl furnishes shortcuts in which everything up
506 through the equal or colon separator is omitted. So you can instead just write
509 Here are the short and long forms of the values the C<General Category> property
515 LC, L& Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
528 Nd Decimal_Number (also Digit)
532 P Punctuation (also Punct)
533 Pc Connector_Punctuation
537 Pi Initial_Punctuation
538 (may behave like Ps or Pe depending on usage)
540 (may behave like Ps or Pe depending on usage)
552 Zp Paragraph_Separator
555 Cc Control (also Cntrl)
561 Single-letter properties match all characters in any of the
562 two-letter sub-properties starting with the same letter.
563 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>.
565 =head3 B<Bidirectional Character Types>
567 Because scripts differ in their directionality (Hebrew and Arabic are
568 written right to left, for example) Unicode supplies a C<Bidi_Class> property.
569 Some of the values this property can have are:
574 LRE Left-to-Right Embedding
575 LRO Left-to-Right Override
578 RLE Right-to-Left Embedding
579 RLO Right-to-Left Override
580 PDF Pop Directional Format
582 ES European Separator
583 ET European Terminator
588 B Paragraph Separator
593 This property is always written in the compound form.
594 For example, C<\p{Bidi_Class:R}> matches characters that are normally
595 written right to left. Unlike the
596 C<L</General_Category>> property, this
597 property can have more values added in a future Unicode release. Those
598 listed above comprised the complete set for many Unicode releases, but
599 others were added in Unicode 6.3; you can always find what the
600 current ones are in L<perluniprops>. And
601 L<http://www.unicode.org/reports/tr9/> describes how to use them.
605 The world's languages are written in many different scripts. This sentence
606 (unless you're reading it in translation) is written in Latin, while Russian is
607 written in Cyrillic, and Greek is written in, well, Greek; Japanese mainly in
608 Hiragana or Katakana. There are many more.
610 The Unicode C<Script> and C<Script_Extensions> properties give what
611 script a given character is in. The C<Script_Extensions> property is an
612 improved version of C<Script>, as demonstrated below. Either property
613 can be specified with the compound form like
614 C<\p{Script=Hebrew}> (short: C<\p{sc=hebr}>), or
615 C<\p{Script_Extensions=Javanese}> (short: C<\p{scx=java}>).
616 In addition, Perl furnishes shortcuts for all
617 C<Script_Extensions> property names. You can omit everything up through
618 the equals (or colon), and simply write C<\p{Latin}> or C<\P{Cyrillic}>.
619 (This is not true for C<Script>, which is required to be
620 written in the compound form. Prior to Perl v5.26, the single form
621 returned the plain old C<Script> version, but was changed because
622 C<Script_Extensions> gives better results.)
624 The difference between these two properties involves characters that are
625 used in multiple scripts. For example the digits '0' through '9' are
626 used in many parts of the world. These are placed in a script named
627 C<Common>. Other characters are used in just a few scripts. For
628 example, the C<"KATAKANA-HIRAGANA DOUBLE HYPHEN"> is used in both Japanese
629 scripts, Katakana and Hiragana, but nowhere else. The C<Script>
630 property places all characters that are used in multiple scripts in the
631 C<Common> script, while the C<Script_Extensions> property places those
632 that are used in only a few scripts into each of those scripts; while
633 still using C<Common> for those used in many scripts. Thus both these
636 "0" =~ /\p{sc=Common}/ # Matches
637 "0" =~ /\p{scx=Common}/ # Matches
639 and only the first of these match:
641 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common} # Matches
642 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match
644 And only the last two of these match:
646 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana} # No match
647 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana} # No match
648 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
649 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches
651 C<Script_Extensions> is thus an improved C<Script>, in which there are
652 fewer characters in the C<Common> script, and correspondingly more in
653 other scripts. It is new in Unicode version 6.0, and its data are likely
654 to change significantly in later releases, as things get sorted out.
655 New code should probably be using C<Script_Extensions> and not plain
656 C<Script>. If you compile perl with a Unicode release that doesn't have
657 C<Script_Extensions>, the single form Perl extensions will instead refer
658 to the plain C<Script> property. If you compile with a version of
659 Unicode that doesn't have the C<Script> property, these extensions will
660 not be defined at all.
662 (Actually, besides C<Common>, the C<Inherited> script, contains
663 characters that are used in multiple scripts. These are modifier
664 characters which inherit the script value
665 of the controlling character. Some of these are used in many scripts,
666 and so go into C<Inherited> in both C<Script> and C<Script_Extensions>.
667 Others are used in just a few scripts, so are in C<Inherited> in
668 C<Script>, but not in C<Script_Extensions>.)
670 It is worth stressing that there are several different sets of digits in
671 Unicode that are equivalent to 0-9 and are matchable by C<\d> in a
672 regular expression. If they are used in a single language only, they
673 are in that language's C<Script> and C<Script_Extensions>. If they are
674 used in more than one script, they will be in C<sc=Common>, but only
675 if they are used in many scripts should they be in C<scx=Common>.
677 The explanation above has omitted some detail; refer to UAX#24 "Unicode
678 Script Property": L<http://www.unicode.org/reports/tr24>.
680 A complete list of scripts and their shortcuts is in L<perluniprops>.
682 =head3 B<Use of the C<"Is"> Prefix>
684 For backward compatibility (with ancient Perl 5.6), all properties writable
685 without using the compound form mentioned
686 so far may have C<Is> or C<Is_> prepended to their name, so C<\P{Is_Lu}>, for
687 example, is equal to C<\P{Lu}>, and C<\p{IsScript:Arabic}> is equal to
692 In addition to B<scripts>, Unicode also defines B<blocks> of
693 characters. The difference between scripts and blocks is that the
694 concept of scripts is closer to natural languages, while the concept
695 of blocks is more of an artificial grouping based on groups of Unicode
696 characters with consecutive ordinal values. For example, the C<"Basic Latin">
697 block is all the characters whose ordinals are between 0 and 127, inclusive; in
698 other words, the ASCII characters. The C<"Latin"> script contains some letters
699 from this as well as several other blocks, like C<"Latin-1 Supplement">,
700 C<"Latin Extended-A">, I<etc.>, but it does not contain all the characters from
701 those blocks. It does not, for example, contain the digits 0-9, because
702 those digits are shared across many scripts, and hence are in the
705 For more about scripts versus blocks, see UAX#24 "Unicode Script Property":
706 L<http://www.unicode.org/reports/tr24>
708 The C<Script_Extensions> or C<Script> properties are likely to be the
709 ones you want to use when processing
710 natural language; the C<Block> property may occasionally be useful in working
711 with the nuts and bolts of Unicode.
713 Block names are matched in the compound form, like C<\p{Block: Arrows}> or
714 C<\p{Blk=Hebrew}>. Unlike most other properties, only a few block names have a
715 Unicode-defined short name.
717 Perl also defines single form synonyms for the block property in cases
718 where these do not conflict with something else. But don't use any of
719 these, because they are unstable. Since these are Perl extensions, they
720 are subordinate to official Unicode property names; Unicode doesn't know
721 nor care about Perl's extensions. It may happen that a name that
722 currently means the Perl extension will later be changed without warning
723 to mean a different Unicode property in a future version of the perl
724 interpreter that uses a later Unicode release, and your code would no
725 longer work. The extensions are mentioned here for completeness: Take
726 the block name and prefix it with one of: C<In> (for example
727 C<\p{Blk=Arrows}> can currently be written as C<\p{In_Arrows}>); or
728 sometimes C<Is> (like C<\p{Is_Arrows}>); or sometimes no prefix at all
729 (C<\p{Arrows}>). As of this writing (Unicode 9.0) there are no
730 conflicts with using the C<In_> prefix, but there are plenty with the
731 other two forms. For example, C<\p{Is_Hebrew}> and C<\p{Hebrew}> mean
732 C<\p{Script_Extensions=Hebrew}> which is NOT the same thing as
733 C<\p{Blk=Hebrew}>. Our
734 advice used to be to use the C<In_> prefix as a single form way of
735 specifying a block. But Unicode 8.0 added properties whose names begin
736 with C<In>, and it's now clear that it's only luck that's so far
737 prevented a conflict. Using C<In> is only marginally less typing than
738 C<Blk:>, and the latter's meaning is clearer anyway, and guaranteed to
739 never conflict. So don't take chances. Use C<\p{Blk=foo}> for new
740 code. And be sure that block is what you really really want to do. In
741 most cases scripts are what you want instead.
743 A complete list of blocks is in L<perluniprops>.
745 =head3 B<Other Properties>
747 There are many more properties than the very basic ones described here.
748 A complete list is in L<perluniprops>.
750 Unicode defines all its properties in the compound form, so all single-form
751 properties are Perl extensions. Most of these are just synonyms for the
752 Unicode ones, but some are genuine extensions, including several that are in
753 the compound form. And quite a few of these are actually recommended by Unicode
754 (in L<http://www.unicode.org/reports/tr18>).
756 This section gives some details on all extensions that aren't just
757 synonyms for compound-form Unicode properties
758 (for those properties, you'll have to refer to the
759 L<Unicode Standard|http://www.unicode.org/reports/tr44>.
765 This matches every possible code point. It is equivalent to C<qr/./s>.
766 Unlike all the other non-user-defined C<\p{}> property matches, no
767 warning is ever generated if this is property is matched against a
768 non-Unicode code point (see L</Beyond Unicode code points> below).
770 =item B<C<\p{Alnum}>>
772 This matches any C<\p{Alphabetic}> or C<\p{Decimal_Number}> character.
776 This matches any of the 1_114_112 Unicode code points. It is a synonym
779 =item B<C<\p{ASCII}>>
781 This matches any of the 128 characters in the US-ASCII character set,
782 which is a subset of Unicode.
784 =item B<C<\p{Assigned}>>
786 This matches any assigned code point; that is, any code point whose L<general
787 category|/General_Category> is not C<Unassigned> (or equivalently, not C<Cn>).
789 =item B<C<\p{Blank}>>
791 This is the same as C<\h> and C<\p{HorizSpace}>: A character that changes the
792 spacing horizontally.
794 =item B<C<\p{Decomposition_Type: Non_Canonical}>> (Short: C<\p{Dt=NonCanon}>)
796 Matches a character that has a non-canonical decomposition.
798 The L</Extended Grapheme Clusters (Logical characters)> section above
799 talked about canonical decompositions. However, many more characters
800 have a different type of decomposition, a "compatible" or
801 "non-canonical" decomposition. The sequences that form these
802 decompositions are not considered canonically equivalent to the
803 pre-composed character. An example is the C<"SUPERSCRIPT ONE">. It is
804 somewhat like a regular digit 1, but not exactly; its decomposition into
805 the digit 1 is called a "compatible" decomposition, specifically a
806 "super" decomposition. There are several such compatibility
807 decompositions (see L<http://www.unicode.org/reports/tr44>), including
808 one called "compat", which means some miscellaneous type of
809 decomposition that doesn't fit into the other decomposition categories
810 that Unicode has chosen.
812 Note that most Unicode characters don't have a decomposition, so their
813 decomposition type is C<"None">.
815 For your convenience, Perl has added the C<Non_Canonical> decomposition
816 type to mean any of the several compatibility decompositions.
818 =item B<C<\p{Graph}>>
820 Matches any character that is graphic. Theoretically, this means a character
821 that on a printer would cause ink to be used.
823 =item B<C<\p{HorizSpace}>>
825 This is the same as C<\h> and C<\p{Blank}>: a character that changes the
826 spacing horizontally.
830 This is a synonym for C<\p{Present_In=*}>
832 =item B<C<\p{PerlSpace}>>
834 This is the same as C<\s>, restricted to ASCII, namely C<S<[ \f\n\r\t]>>
835 and starting in Perl v5.18, a vertical tab.
837 Mnemonic: Perl's (original) space
839 =item B<C<\p{PerlWord}>>
841 This is the same as C<\w>, restricted to ASCII, namely C<[A-Za-z0-9_]>
843 Mnemonic: Perl's (original) word.
845 =item B<C<\p{Posix...}>>
847 There are several of these, which are equivalents, using the C<\p{}>
848 notation, for Posix classes and are described in
849 L<perlrecharclass/POSIX Character Classes>.
851 =item B<C<\p{Present_In: *}>> (Short: C<\p{In=*}>)
853 This property is used when you need to know in what Unicode version(s) a
856 The "*" above stands for some two digit Unicode version number, such as
857 C<1.1> or C<4.0>; or the "*" can also be C<Unassigned>. This property will
858 match the code points whose final disposition has been settled as of the
859 Unicode release given by the version number; C<\p{Present_In: Unassigned}>
860 will match those code points whose meaning has yet to be assigned.
862 For example, C<U+0041> C<"LATIN CAPITAL LETTER A"> was present in the very first
863 Unicode release available, which is C<1.1>, so this property is true for all
864 valid "*" versions. On the other hand, C<U+1EFF> was not assigned until version
865 5.1 when it became C<"LATIN SMALL LETTER Y WITH LOOP">, so the only "*" that
866 would match it are 5.1, 5.2, and later.
868 Unicode furnishes the C<Age> property from which this is derived. The problem
869 with Age is that a strict interpretation of it (which Perl takes) has it
870 matching the precise release a code point's meaning is introduced in. Thus
871 C<U+0041> would match only 1.1; and C<U+1EFF> only 5.1. This is not usually what
874 Some non-Perl implementations of the Age property may change its meaning to be
875 the same as the Perl C<Present_In> property; just be aware of that.
877 Another confusion with both these properties is that the definition is not
878 that the code point has been I<assigned>, but that the meaning of the code point
879 has been I<determined>. This is because 66 code points will always be
880 unassigned, and so the C<Age> for them is the Unicode version in which the decision
881 to make them so was made. For example, C<U+FDD0> is to be permanently
882 unassigned to a character, and the decision to do that was made in version 3.1,
883 so C<\p{Age=3.1}> matches this character, as also does C<\p{Present_In: 3.1}> and up.
885 =item B<C<\p{Print}>>
887 This matches any character that is graphical or blank, except controls.
889 =item B<C<\p{SpacePerl}>>
891 This is the same as C<\s>, including beyond ASCII.
893 Mnemonic: Space, as modified by Perl. (It doesn't include the vertical tab
894 until v5.18, which both the Posix standard and Unicode consider white space.)
896 =item B<C<\p{Title}>> and B<C<\p{Titlecase}>>
898 Under case-sensitive matching, these both match the same code points as
899 C<\p{General Category=Titlecase_Letter}> (C<\p{gc=lt}>). The difference
900 is that under C</i> caseless matching, these match the same as
901 C<\p{Cased}>, whereas C<\p{gc=lt}> matches C<\p{Cased_Letter>).
903 =item B<C<\p{Unicode}>>
905 This matches any of the 1_114_112 Unicode code points.
908 =item B<C<\p{VertSpace}>>
910 This is the same as C<\v>: A character that changes the spacing vertically.
914 This is the same as C<\w>, including over 100_000 characters beyond ASCII.
916 =item B<C<\p{XPosix...}>>
918 There are several of these, which are the standard Posix classes
919 extended to the full Unicode range. They are described in
920 L<perlrecharclass/POSIX Character Classes>.
925 =head2 User-Defined Character Properties
927 You can define your own binary character properties by defining subroutines
928 whose names begin with C<"In"> or C<"Is">. (The experimental feature
929 L<perlre/(?[ ])> provides an alternative which allows more complex
930 definitions.) The subroutines can be defined in any
931 package. The user-defined properties can be used in the regular expression
932 C<\p{}> and C<\P{}> constructs; if you are using a user-defined property from a
933 package other than the one you are in, you must specify its package in the
934 C<\p{}> or C<\P{}> construct.
936 # assuming property Is_Foreign defined in Lang::
937 package main; # property package name required
938 if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
940 package Lang; # property package name not required
941 if ($txt =~ /\p{IsForeign}+/) { ... }
944 Note that the effect is compile-time and immutable once defined.
945 However, the subroutines are passed a single parameter, which is 0 if
946 case-sensitive matching is in effect and non-zero if caseless matching
947 is in effect. The subroutine may return different values depending on
948 the value of the flag, and one set of values will immutably be in effect
949 for all case-sensitive matches, and the other set for all case-insensitive
952 Note that if the regular expression is tainted, then Perl will die rather
953 than calling the subroutine when the name of the subroutine is
954 determined by the tainted data.
956 The subroutines must return a specially-formatted string, with one
957 or more newline-separated lines. Each line must be one of the following:
963 A single hexadecimal number denoting a code point to include.
967 Two hexadecimal numbers separated by horizontal whitespace (space or
968 tabular characters) denoting a range of code points to include.
972 Something to include, prefixed by C<"+">: a built-in character
973 property (prefixed by C<"utf8::">) or a fully qualified (including package
974 name) user-defined character property,
975 to represent all the characters in that property; two hexadecimal code
976 points for a range; or a single hexadecimal code point.
980 Something to exclude, prefixed by C<"-">: an existing character
981 property (prefixed by C<"utf8::">) or a fully qualified (including package
982 name) user-defined character property,
983 to represent all the characters in that property; two hexadecimal code
984 points for a range; or a single hexadecimal code point.
988 Something to negate, prefixed C<"!">: an existing character
989 property (prefixed by C<"utf8::">) or a fully qualified (including package
990 name) user-defined character property,
991 to represent all the characters in that property; two hexadecimal code
992 points for a range; or a single hexadecimal code point.
996 Something to intersect with, prefixed by C<"&">: an existing character
997 property (prefixed by C<"utf8::">) or a fully qualified (including package
998 name) user-defined character property,
999 for all the characters except the characters in the property; two
1000 hexadecimal code points for a range; or a single hexadecimal code point.
1004 For example, to define a property that covers both the Japanese
1005 syllabaries (hiragana and katakana), you can define
1014 Imagine that the here-doc end marker is at the beginning of the line.
1015 Now you can use C<\p{InKana}> and C<\P{InKana}>.
1017 You could also have used the existing block property names:
1026 Suppose you wanted to match only the allocated characters,
1027 not the raw block ranges: in other words, you want to remove
1028 the unassigned characters:
1038 The negation is useful for defining (surprise!) negated classes.
1048 This will match all non-Unicode code points, since every one of them is
1049 not in Kana. You can use intersection to exclude these, if desired, as
1050 this modified example shows:
1061 C<&utf8::Any> must be the last line in the definition.
1063 Intersection is used generally for getting the common characters matched
1064 by two (or more) classes. It's important to remember not to use C<"&"> for
1065 the first set; that would be intersecting with nothing, resulting in an
1066 empty set. (Similarly using C<"-"> for the first set does nothing).
1068 Unlike non-user-defined C<\p{}> property matches, no warning is ever
1069 generated if these properties are matched against a non-Unicode code
1070 point (see L</Beyond Unicode code points> below).
1072 =head2 User-Defined Case Mappings (for serious hackers only)
1074 B<This feature has been removed as of Perl 5.16.>
1075 The CPAN module C<L<Unicode::Casing>> provides better functionality without
1076 the drawbacks that this feature had. If you are using a Perl earlier
1077 than 5.16, this feature was most fully documented in the 5.14 version of
1079 L<http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>
1081 =head2 Character Encodings for Input and Output
1085 =head2 Unicode Regular Expression Support Level
1087 The following list of Unicode supported features for regular expressions describes
1088 all features currently directly supported by core Perl. The references
1089 to "Level I<N>" and the section numbers refer to
1090 L<UTS#18 "Unicode Regular Expressions"|http://www.unicode.org/reports/tr18>,
1091 version 13, November 2013.
1093 =head3 Level 1 - Basic Unicode Support
1095 RL1.1 Hex Notation - Done [1]
1096 RL1.2 Properties - Done [2]
1097 RL1.2a Compatibility Properties - Done [3]
1098 RL1.3 Subtraction and Intersection - Experimental [4]
1099 RL1.4 Simple Word Boundaries - Done [5]
1100 RL1.5 Simple Loose Matches - Done [6]
1101 RL1.6 Line Boundaries - Partial [7]
1102 RL1.7 Supplementary Code Points - Done [8]
1106 =item [1] C<\N{U+...}> and C<\x{...}>
1109 C<\p{...}> C<\P{...}>. This requirement is for a minimal list of
1110 properties. Perl supports these and all other Unicode character
1111 properties, as R2.7 asks (see L</"Unicode Character Properties"> above).
1114 Perl has C<\d> C<\D> C<\s> C<\S> C<\w> C<\W> C<\X> C<[:I<prop>:]>
1115 C<[:^I<prop>:]>, plus all the properties specified by
1116 L<http://www.unicode.org/reports/tr18/#Compatibility_Properties>. These
1117 are described above in L</Other Properties>
1121 The experimental feature C<"(?[...])"> starting in v5.18 accomplishes
1124 See L<perlre/(?[ ])>. If you don't want to use an experimental
1125 feature, you can use one of the following:
1130 Regular expression lookahead
1132 You can mimic class subtraction using lookahead.
1133 For example, what UTS#18 might write as
1135 [{Block=Greek}-[{UNASSIGNED}]]
1137 in Perl can be written as:
1139 (?!\p{Unassigned})\p{Block=Greek}
1140 (?=\p{Assigned})\p{Block=Greek}
1142 But in this particular example, you probably really want
1146 which will match assigned characters known to be part of the Greek script.
1150 CPAN module C<L<Unicode::Regex::Set>>
1152 It does implement the full UTS#18 grouping, intersection, union, and
1153 removal (subtraction) syntax.
1157 L</"User-Defined Character Properties">
1159 C<"+"> for union, C<"-"> for removal (set-difference), C<"&"> for intersection
1164 C<\b> C<\B> meet most, but not all, the details of this requirement, but
1165 C<\b{wb}> and C<\B{wb}> do, as well as the stricter R2.3.
1169 Note that Perl does Full case-folding in matching, not Simple:
1171 For example C<U+1F88> is equivalent to C<U+1F00 U+03B9>, instead of just
1172 C<U+1F80>. This difference matters mainly for certain Greek capital
1173 letters with certain modifiers: the Full case-folding decomposes the
1174 letter, while the Simple case-folding would map it to a single
1179 The reason this is considered to be only partially implemented is that
1180 Perl has L<C<qrE<sol>\b{lb}E<sol>>|perlrebackslash/\b{lb}> and
1181 C<L<Unicode::LineBreak>> that are conformant with
1182 L<UAX#14 "Unicode Line Breaking Algorithm"|http://www.unicode.org/reports/tr14>.
1183 The regular expression construct provides default behavior, while the
1184 heavier-weight module provides customizable line breaking.
1186 But Perl treats C<\n> as the start- and end-line
1187 delimiter, whereas Unicode specifies more characters that should be
1199 C<^> and C<$> in regular expression patterns are supposed to match all
1201 These characters also don't, but should, affect C<< <> >> C<$.>, and
1202 script line numbers.
1204 Also, lines should not be split within C<CRLF> (i.e. there is no
1205 empty line between C<\r> and C<\n>). For C<CRLF>, try the C<:crlf>
1206 layer (see L<PerlIO>).
1209 UTF-8/UTF-EBDDIC used in Perl allows not only C<U+10000> to
1210 C<U+10FFFF> but also beyond C<U+10FFFF>
1214 =head3 Level 2 - Extended Unicode Support
1216 RL2.1 Canonical Equivalents - Retracted [9]
1218 RL2.2 Extended Grapheme Clusters - Partial [10]
1219 RL2.3 Default Word Boundaries - Done [11]
1220 RL2.4 Default Case Conversion - Done
1221 RL2.5 Name Properties - Done
1222 RL2.6 Wildcard Properties - Missing
1223 RL2.7 Full Properties - Done
1228 Unicode has rewritten this portion of UTS#18 to say that getting
1229 canonical equivalence (see UAX#15
1230 L<"Unicode Normalization Forms"|http://www.unicode.org/reports/tr15>)
1231 is basically to be done at the programmer level. Use NFD to write
1232 both your regular expressions and text to match them against (you
1233 can use L<Unicode::Normalize>).
1236 Perl has C<\X> and C<\b{gcb}> but we don't have a "Grapheme Cluster Mode".
1239 L<UAX#29 "Unicode Text Segmentation"|http://www.unicode.org/reports/tr29>,
1243 =head3 Level 3 - Tailored Support
1245 RL3.1 Tailored Punctuation - Missing
1246 RL3.2 Tailored Grapheme Clusters - Missing [12]
1247 RL3.3 Tailored Word Boundaries - Missing
1248 RL3.4 Tailored Loose Matches - Retracted by Unicode
1249 RL3.5 Tailored Ranges - Retracted by Unicode
1250 RL3.6 Context Matching - Missing [13]
1251 RL3.7 Incremental Matches - Missing
1252 RL3.8 Unicode Set Sharing - Unicode is proposing
1254 RL3.9 Possible Match Sets - Missing
1255 RL3.10 Folded Matching - Retracted by Unicode
1256 RL3.11 Submatchers - Missing
1261 Perl has L<Unicode::Collate>, but it isn't integrated with regular
1263 L<UTS#10 "Unicode Collation Algorithms"|http://www.unicode.org/reports/tr10>.
1266 Perl has C<(?<=x)> and C<(?=x)>, but lookaheads or lookbehinds should
1267 see outside of the target substring
1271 =head2 Unicode Encodings
1273 Unicode characters are assigned to I<code points>, which are abstract
1274 numbers. To use these numbers, various encodings are needed.
1282 UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
1283 encoding. In most of Perl's documentation, including elsewhere in this
1284 document, the term "UTF-8" means also "UTF-EBCDIC". But in this section,
1285 "UTF-8" refers only to the encoding used on ASCII platforms. It is a
1286 superset of 7-bit US-ASCII, so anything encoded in ASCII has the
1287 identical representation when encoded in UTF-8.
1289 The following table is from Unicode 3.2.
1291 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
1293 U+0000..U+007F 00..7F
1294 U+0080..U+07FF * C2..DF 80..BF
1295 U+0800..U+0FFF E0 * A0..BF 80..BF
1296 U+1000..U+CFFF E1..EC 80..BF 80..BF
1297 U+D000..U+D7FF ED 80..9F 80..BF
1298 U+D800..U+DFFF +++++ utf16 surrogates, not legal utf8 +++++
1299 U+E000..U+FFFF EE..EF 80..BF 80..BF
1300 U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF
1301 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
1302 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
1304 Note the gaps marked by "*" before several of the byte entries above. These are
1305 caused by legal UTF-8 avoiding non-shortest encodings: it is technically
1306 possible to UTF-8-encode a single code point in different ways, but that is
1307 explicitly forbidden, and the shortest possible encoding should always be used
1308 (and that is what Perl does).
1310 Another way to look at it is via bits:
1312 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
1315 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
1316 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
1317 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
1319 As you can see, the continuation bytes all begin with C<"10">, and the
1320 leading bits of the start byte tell how many bytes there are in the
1323 The original UTF-8 specification allowed up to 6 bytes, to allow
1324 encoding of numbers up to C<0x7FFF_FFFF>. Perl continues to allow those,
1325 and has extended that up to 13 bytes to encode code points up to what
1326 can fit in a 64-bit word. However, Perl will warn if you output any of
1327 these as being non-portable; and under strict UTF-8 input protocols,
1328 they are forbidden. In addition, it is deprecated to use a code point
1329 larger than what a signed integer variable on your system can hold. On
1330 32-bit ASCII systems, this means C<0x7FFF_FFFF> is the legal maximum
1331 going forward (much higher on 64-bit systems).
1337 Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
1338 This means that all the basic characters (which includes all
1339 those that have ASCII equivalents (like C<"A">, C<"0">, C<"%">, I<etc.>)
1340 are the same in both EBCDIC and UTF-EBCDIC.)
1342 UTF-EBCDIC is used on EBCDIC platforms. It generally requires more
1343 bytes to represent a given code point than UTF-8 does; the largest
1344 Unicode code points take 5 bytes to represent (instead of 4 in UTF-8),
1345 and, extended for 64-bit words, it uses 14 bytes instead of 13 bytes in
1350 UTF-16, UTF-16BE, UTF-16LE, Surrogates, and C<BOM>'s (Byte Order Marks)
1352 The followings items are mostly for reference and general Unicode
1353 knowledge, Perl doesn't use these constructs internally.
1355 Like UTF-8, UTF-16 is a variable-width encoding, but where
1356 UTF-8 uses 8-bit code units, UTF-16 uses 16-bit code units.
1357 All code points occupy either 2 or 4 bytes in UTF-16: code points
1358 C<U+0000..U+FFFF> are stored in a single 16-bit unit, and code
1359 points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is
1360 using I<surrogates>, the first 16-bit unit being the I<high
1361 surrogate>, and the second being the I<low surrogate>.
1363 Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
1364 range of Unicode code points in pairs of 16-bit units. The I<high
1365 surrogates> are the range C<U+D800..U+DBFF> and the I<low surrogates>
1366 are the range C<U+DC00..U+DFFF>. The surrogate encoding is
1368 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1369 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
1373 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
1375 Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
1376 itself can be used for in-memory computations, but if storage or
1377 transfer is required either UTF-16BE (big-endian) or UTF-16LE
1378 (little-endian) encodings must be chosen.
1380 This introduces another problem: what if you just know that your data
1381 is UTF-16, but you don't know which endianness? Byte Order Marks, or
1382 C<BOM>'s, are a solution to this. A special character has been reserved
1383 in Unicode to function as a byte order marker: the character with the
1384 code point C<U+FEFF> is the C<BOM>.
1386 The trick is that if you read a C<BOM>, you will know the byte order,
1387 since if it was written on a big-endian platform, you will read the
1388 bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
1389 you will read the bytes C<0xFF 0xFE>. (And if the originating platform
1390 was writing in ASCII platform UTF-8, you will read the bytes
1393 The way this trick works is that the character with the code point
1394 C<U+FFFE> is not supposed to be in input streams, so the
1395 sequence of bytes C<0xFF 0xFE> is unambiguously "C<BOM>, represented in
1396 little-endian format" and cannot be C<U+FFFE>, represented in big-endian
1399 Surrogates have no meaning in Unicode outside their use in pairs to
1400 represent other code points. However, Perl allows them to be
1401 represented individually internally, for example by saying
1402 C<chr(0xD801)>, so that all code points, not just those valid for open
1404 representable. Unicode does define semantics for them, such as their
1405 C<L</General_Category>> is C<"Cs">. But because their use is somewhat dangerous,
1406 Perl will warn (using the warning category C<"surrogate">, which is a
1407 sub-category of C<"utf8">) if an attempt is made
1408 to do things like take the lower case of one, or match
1409 case-insensitively, or to output them. (But don't try this on Perls
1414 UTF-32, UTF-32BE, UTF-32LE
1416 The UTF-32 family is pretty much like the UTF-16 family, except that
1417 the units are 32-bit, and therefore the surrogate scheme is not
1418 needed. UTF-32 is a fixed-width encoding. The C<BOM> signatures are
1419 C<0x00 0x00 0xFE 0xFF> for BE and C<0xFF 0xFE 0x00 0x00> for LE.
1425 Legacy, fixed-width encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
1426 encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
1427 because it does not use surrogates. UCS-4 is a 32-bit encoding,
1428 functionally identical to UTF-32 (the difference being that
1429 UCS-4 forbids neither surrogates nor code points larger than C<0x10_FFFF>).
1435 A seven-bit safe (non-eight-bit) encoding, which is useful if the
1436 transport or storage is not eight-bit safe. Defined by RFC 2152.
1440 =head2 Noncharacter code points
1442 66 code points are set aside in Unicode as "noncharacter code points".
1443 These all have the C<Unassigned> (C<Cn>) C<L</General_Category>>, and
1444 no character will ever be assigned to any of them. They are the 32 code
1445 points between C<U+FDD0> and C<U+FDEF> inclusive, and the 34 code
1456 Until Unicode 7.0, the noncharacters were "B<forbidden> for use in open
1457 interchange of Unicode text data", so that code that processed those
1458 streams could use these code points as sentinels that could be mixed in
1459 with character data, and would always be distinguishable from that data.
1460 (Emphasis above and in the next paragraph are added in this document.)
1462 Unicode 7.0 changed the wording so that they are "B<not recommended> for
1463 use in open interchange of Unicode text data". The 7.0 Standard goes on
1468 "If a noncharacter is received in open interchange, an application is
1469 not required to interpret it in any way. It is good practice, however,
1470 to recognize it as a noncharacter and to take appropriate action, such
1471 as replacing it with C<U+FFFD> replacement character, to indicate the
1472 problem in the text. It is not recommended to simply delete
1473 noncharacter code points from such text, because of the potential
1474 security issues caused by deleting uninterpreted characters. (See
1475 conformance clause C7 in Section 3.2, Conformance Requirements, and
1476 L<Unicode Technical Report #36, "Unicode Security
1477 Considerations"|http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
1481 This change was made because it was found that various commercial tools
1482 like editors, or for things like source code control, had been written
1483 so that they would not handle program files that used these code points,
1484 effectively precluding their use almost entirely! And that was never
1485 the intent. They've always been meant to be usable within an
1486 application, or cooperating set of applications, at will.
1488 If you're writing code, such as an editor, that is supposed to be able
1489 to handle any Unicode text data, then you shouldn't be using these code
1490 points yourself, and instead allow them in the input. If you need
1491 sentinels, they should instead be something that isn't legal Unicode.
1492 For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
1493 they never appear in well-formed UTF-8. (There are equivalents for
1494 UTF-EBCDIC). You can also store your Unicode code points in integer
1495 variables and use negative values as sentinels.
1497 If you're not writing such a tool, then whether you accept noncharacters
1498 as input is up to you (though the Standard recommends that you not). If
1499 you do strict input stream checking with Perl, these code points
1500 continue to be forbidden. This is to maintain backward compatibility
1501 (otherwise potential security holes could open up, as an unsuspecting
1502 application that was written assuming the noncharacters would be
1503 filtered out before getting to it, could now, without warning, start
1504 getting them). To do strict checking, you can use the layer
1505 C<:encoding('UTF-8')>.
1507 Perl continues to warn (using the warning category C<"nonchar">, which
1508 is a sub-category of C<"utf8">) if an attempt is made to output
1511 =head2 Beyond Unicode code points
1513 The maximum Unicode code point is C<U+10FFFF>, and Unicode only defines
1514 operations on code points up through that. But Perl works on code
1515 points up to the maximum permissible unsigned number available on the
1516 platform. However, Perl will not accept these from input streams unless
1517 lax rules are being used, and will warn (using the warning category
1518 C<"non_unicode">, which is a sub-category of C<"utf8">) if any are output.
1520 Since Unicode rules are not defined on these code points, if a
1521 Unicode-defined operation is done on them, Perl uses what we believe are
1522 sensible rules, while generally warning, using the C<"non_unicode">
1523 category. For example, C<uc("\x{11_0000}")> will generate such a
1524 warning, returning the input parameter as its result, since Perl defines
1525 the uppercase of every non-Unicode code point to be the code point
1526 itself. (All the case changing operations, not just uppercasing, work
1529 The situation with matching Unicode properties in regular expressions,
1530 the C<\p{}> and C<\P{}> constructs, against these code points is not as
1531 clear cut, and how these are handled has changed as we've gained
1534 One possibility is to treat any match against these code points as
1535 undefined. But since Perl doesn't have the concept of a match being
1536 undefined, it converts this to failing or C<FALSE>. This is almost, but
1537 not quite, what Perl did from v5.14 (when use of these code points
1538 became generally reliable) through v5.18. The difference is that Perl
1539 treated all C<\p{}> matches as failing, but all C<\P{}> matches as
1542 One problem with this is that it leads to unexpected, and confusing
1543 results in some cases:
1545 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Failed on <= v5.18
1546 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Failed! on <= v5.18
1548 That is, it treated both matches as undefined, and converted that to
1549 false (raising a warning on each). The first case is the expected
1550 result, but the second is likely counterintuitive: "How could both be
1551 false when they are complements?" Another problem was that the
1552 implementation optimized many Unicode property matches down to already
1553 existing simpler, faster operations, which don't raise the warning. We
1554 chose to not forgo those optimizations, which help the vast majority of
1555 matches, just to generate a warning for the unlikely event that an
1556 above-Unicode code point is being matched against.
1558 As a result of these problems, starting in v5.20, what Perl does is
1559 to treat non-Unicode code points as just typical unassigned Unicode
1560 characters, and matches accordingly. (Note: Unicode has atypical
1561 unassigned code points. For example, it has noncharacter code points,
1562 and ones that, when they do get assigned, are destined to be written
1563 Right-to-left, as Arabic and Hebrew are. Perl assumes that no
1564 non-Unicode code point has any atypical properties.)
1566 Perl, in most cases, will raise a warning when matching an above-Unicode
1567 code point against a Unicode property when the result is C<TRUE> for
1568 C<\p{}>, and C<FALSE> for C<\P{}>. For example:
1570 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails, no warning
1571 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Succeeds, with warning
1573 In both these examples, the character being matched is non-Unicode, so
1574 Unicode doesn't define how it should match. It clearly isn't an ASCII
1575 hex digit, so the first example clearly should fail, and so it does,
1576 with no warning. But it is arguable that the second example should have
1577 an undefined, hence C<FALSE>, result. So a warning is raised for it.
1579 Thus the warning is raised for many fewer cases than in earlier Perls,
1580 and only when what the result is could be arguable. It turns out that
1581 none of the optimizations made by Perl (or are ever likely to be made)
1582 cause the warning to be skipped, so it solves both problems of Perl's
1583 earlier approach. The most commonly used property that is affected by
1584 this change is C<\p{Unassigned}> which is a short form for
1585 C<\p{General_Category=Unassigned}>. Starting in v5.20, all non-Unicode
1586 code points are considered C<Unassigned>. In earlier releases the
1587 matches failed because the result was considered undefined.
1589 The only place where the warning is not raised when it might ought to
1590 have been is if optimizations cause the whole pattern match to not even
1591 be attempted. For example, Perl may figure out that for a string to
1592 match a certain regular expression pattern, the string has to contain
1593 the substring C<"foobar">. Before attempting the match, Perl may look
1594 for that substring, and if not found, immediately fail the match without
1595 actually trying it; so no warning gets generated even if the string
1596 contains an above-Unicode code point.
1598 This behavior is more "Do what I mean" than in earlier Perls for most
1599 applications. But it catches fewer issues for code that needs to be
1600 strictly Unicode compliant. Therefore there is an additional mode of
1601 operation available to accommodate such code. This mode is enabled if a
1602 regular expression pattern is compiled within the lexical scope where
1603 the C<"non_unicode"> warning class has been made fatal, say by:
1605 use warnings FATAL => "non_unicode"
1607 (see L<warnings>). In this mode of operation, Perl will raise the
1608 warning for all matches against a non-Unicode code point (not just the
1609 arguable ones), and it skips the optimizations that might cause the
1610 warning to not be output. (It currently still won't warn if the match
1611 isn't even attempted, like in the C<"foobar"> example above.)
1613 In summary, Perl now normally treats non-Unicode code points as typical
1614 Unicode unassigned code points for regular expression matches, raising a
1615 warning only when it is arguable what the result should be. However, if
1616 this warning has been made fatal, it isn't skipped.
1618 There is one exception to all this. C<\p{All}> looks like a Unicode
1619 property, but it is a Perl extension that is defined to be true for all
1620 possible code points, Unicode or not, so no warning is ever generated
1621 when matching this against a non-Unicode code point. (Prior to v5.20,
1622 it was an exact synonym for C<\p{Any}>, matching code points C<0>
1623 through C<0x10FFFF>.)
1625 =head2 Security Implications of Unicode
1628 L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
1630 Also, note the following:
1638 UTF-8 is very structured, so many combinations of bytes are invalid. In
1639 the past, Perl tried to soldier on and make some sense of invalid
1640 combinations, but this can lead to security holes, so now, if the Perl
1641 core needs to process an invalid combination, it will either raise a
1642 fatal error, or will replace those bytes by the sequence that forms the
1643 Unicode REPLACEMENT CHARACTER, for which purpose Unicode created it.
1645 Every code point can be represented by more than one possible
1646 syntactically valid UTF-8 sequence. Early on, both Unicode and Perl
1647 considered any of these to be valid, but now, all sequences longer
1648 than the shortest possible one are considered to be malformed.
1650 Unicode considers many code points to be illegal, or to be avoided.
1651 Perl generally accepts them, once they have passed through any input
1652 filters that may try to exclude them. These have been discussed above
1653 (see "Surrogates" under UTF-16 in L</Unicode Encodings>,
1654 L</Noncharacter code points>, and L</Beyond Unicode code points>).
1658 Regular expression pattern matching may surprise you if you're not
1659 accustomed to Unicode. Starting in Perl 5.14, several pattern
1660 modifiers are available to control this, called the character set
1661 modifiers. Details are given in L<perlre/Character set modifiers>.
1665 As discussed elsewhere, Perl has one foot (two hooves?) planted in
1666 each of two worlds: the old world of ASCII and single-byte locales, and
1667 the new world of Unicode, upgrading when necessary.
1668 If your legacy code does not explicitly use Unicode, no automatic
1669 switch-over to Unicode should happen.
1671 =head2 Unicode in Perl on EBCDIC
1673 Unicode is supported on EBCDIC platforms. See L<perlebcdic>.
1675 Unless ASCII vs. EBCDIC issues are specifically being discussed,
1676 references to UTF-8 encoding in this document and elsewhere should be
1677 read as meaning UTF-EBCDIC on EBCDIC platforms.
1678 See L<perlebcdic/Unicode and UTF>.
1680 Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
1681 hidden from you; S<C<use utf8>> (and NOT something like
1682 S<C<use utfebcdic>>) declares the script is in the platform's
1683 "native" 8-bit encoding of Unicode. (Similarly for the C<":utf8">
1688 See L<perllocale/Unicode and UTF-8>
1690 =head2 When Unicode Does Not Happen
1692 There are still many places where Unicode (in some encoding or
1693 another) could be given as arguments or received as results, or both in
1694 Perl, but it is not, in spite of Perl having extensive ways to input and
1695 output in Unicode, and a few other "entry points" like the C<@ARGV>
1696 array (which can sometimes be interpreted as UTF-8).
1698 The following are such interfaces. Also, see L</The "Unicode Bug">.
1699 For all of these interfaces Perl
1700 currently (as of v5.16.0) simply assumes byte strings both as arguments
1701 and results, or UTF-8 strings if the (deprecated) C<encoding> pragma has been used.
1703 One reason that Perl does not attempt to resolve the role of Unicode in
1704 these situations is that the answers are highly dependent on the operating
1705 system and the file system(s). For example, whether filenames can be
1706 in Unicode and in exactly what kind of encoding, is not exactly a
1707 portable concept. Similarly for C<qx> and C<system>: how well will the
1708 "command-line interface" (and which of them?) handle Unicode?
1714 C<chdir>, C<chmod>, C<chown>, C<chroot>, C<exec>, C<link>, C<lstat>, C<mkdir>,
1715 C<rename>, C<rmdir>, C<stat>, C<symlink>, C<truncate>, C<unlink>, C<utime>, C<-X>
1723 C<glob> (aka the C<E<lt>*E<gt>>)
1727 C<open>, C<opendir>, C<sysopen>
1731 C<qx> (aka the backtick operator), C<system>
1735 C<readdir>, C<readlink>
1739 =head2 The "Unicode Bug"
1741 The term, "Unicode bug" has been applied to an inconsistency with the
1742 code points in the C<Latin-1 Supplement> block, that is, between
1743 128 and 255. Without a locale specified, unlike all other characters or
1744 code points, these characters can have very different semantics
1745 depending on the rules in effect. (Characters whose code points are
1746 above 255 force Unicode rules; whereas the rules for ASCII characters
1747 are the same under both ASCII and Unicode rules.)
1749 Under Unicode rules, these upper-Latin1 characters are interpreted as
1750 Unicode code points, which means they have the same semantics as Latin-1
1751 (ISO-8859-1) and C1 controls.
1753 As explained in L</ASCII Rules versus Unicode Rules>, under ASCII rules,
1754 they are considered to be unassigned characters.
1756 This can lead to unexpected results. For example, a string's
1757 semantics can suddenly change if a code point above 255 is appended to
1758 it, which changes the rules from ASCII to Unicode. As an
1759 example, consider the following program and its output:
1762 no feature "unicode_strings";
1765 for ($s1, $s2, $s1.$s2) {
1773 If there's no C<\w> in C<s1> nor in C<s2>, why does their concatenation
1776 This anomaly stems from Perl's attempt to not disturb older programs that
1777 didn't use Unicode, along with Perl's desire to add Unicode support
1778 seamlessly. But the result turned out to not be seamless. (By the way,
1779 you can choose to be warned when things like this happen. See
1780 C<L<encoding::warnings>>.)
1782 L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
1783 was added, starting in Perl v5.12, to address this problem. It affects
1790 Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
1791 and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in double-quotish
1792 contexts, such as regular expression substitutions.
1794 Under C<unicode_strings> starting in Perl 5.12.0, Unicode rules are
1795 generally used. See L<perlfunc/lc> for details on how this works
1796 in combination with various other pragmas.
1800 Using caseless (C</i>) regular expression matching.
1802 Starting in Perl 5.14.0, regular expressions compiled within
1803 the scope of C<unicode_strings> use Unicode rules
1804 even when executed or compiled into larger
1805 regular expressions outside the scope.
1809 Matching any of several properties in regular expressions.
1811 These properties are C<\b> (without braces), C<\B> (without braces),
1812 C<\s>, C<\S>, C<\w>, C<\W>, and all the Posix character classes
1813 I<except> C<[[:ascii:]]>.
1815 Starting in Perl 5.14.0, regular expressions compiled within
1816 the scope of C<unicode_strings> use Unicode rules
1817 even when executed or compiled into larger
1818 regular expressions outside the scope.
1822 In C<quotemeta> or its inline equivalent C<\Q>.
1824 Starting in Perl 5.16.0, consistent quoting rules are used within the
1825 scope of C<unicode_strings>, as described in L<perlfunc/quotemeta>.
1826 Prior to that, or outside its scope, no code points above 127 are quoted
1827 in UTF-8 encoded strings, but in byte encoded strings, code points
1828 between 128-255 are always quoted.
1832 In the C<..> or L<range|perlop/Range Operators> operator.
1834 Starting in Perl 5.26.0, the range operator on strings treats their lengths
1835 consistently within the scope of C<unicode_strings>. Prior to that, or
1836 outside its scope, it could produce strings whose length in characters
1837 exceeded that of the right-hand side, where the right-hand side took up more
1838 bytes than the correct range endpoint.
1842 In L<< C<split>'s special-case whitespace splitting|perlfunc/split >>.
1844 Starting in Perl 5.28.0, the C<split> function with a pattern specified as
1845 a string containing a single space handles whitespace characters consistently
1846 within the scope of of C<unicode_strings>. Prior to that, or outside its scope,
1847 characters that are whitespace according to Unicode rules but not according to
1848 ASCII rules were treated as field contents rather than field separators when
1849 they appear in byte-encoded strings.
1853 You can see from the above that the effect of C<unicode_strings>
1854 increased over several Perl releases. (And Perl's support for Unicode
1855 continues to improve; it's best to use the latest available release in
1856 order to get the most complete and accurate results possible.) Note that
1857 C<unicode_strings> is automatically chosen if you S<C<use 5.012>> or
1860 For Perls earlier than those described above, or when a string is passed
1861 to a function outside the scope of C<unicode_strings>, see the next section.
1863 =head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
1865 Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
1866 there are situations where you simply need to force a byte
1867 string into UTF-8, or vice versa. The standard module L<Encode> can be
1868 used for this, or the low-level calls
1869 L<C<utf8::upgrade($bytestring)>|utf8/Utility functions> and
1870 L<C<utf8::downgrade($utf8string[, FAIL_OK])>|utf8/Utility functions>.
1872 Note that C<utf8::downgrade()> can fail if the string contains characters
1873 that don't fit into a byte.
1875 Calling either function on a string that already is in the desired state is a
1878 L</ASCII Rules versus Unicode Rules> gives all the ways that a string is
1879 made to use Unicode rules.
1881 =head2 Using Unicode in XS
1883 See L<perlguts/"Unicode Support"> for an introduction to Unicode at
1884 the XS level, and L<perlapi/Unicode Support> for the API details.
1886 =head2 Hacking Perl to work on earlier Unicode versions (for very serious hackers only)
1888 Perl by default comes with the latest supported Unicode version built-in, but
1889 the goal is to allow you to change to use any earlier one. In Perls
1890 v5.20 and v5.22, however, the earliest usable version is Unicode 5.1.
1891 Perl v5.18 and v5.24 are able to handle all earlier versions.
1893 Download the files in the desired version of Unicode from the Unicode web
1894 site L<http://www.unicode.org>). These should replace the existing files in
1895 F<lib/unicore> in the Perl source tree. Follow the instructions in
1896 F<README.perl> in that directory to change some of their names, and then build
1897 perl (see L<INSTALL>).
1899 =head2 Porting code from perl-5.6.X
1901 Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6 the
1902 programmer was required to use the C<utf8> pragma to declare that a
1903 given scope expected to deal with Unicode data and had to make sure that
1904 only Unicode data were reaching that scope. If you have code that is
1905 working with 5.6, you will need some of the following adjustments to
1906 your code. The examples are written such that the code will continue to
1907 work under 5.6, so you should be safe to try them out.
1913 A filehandle that should read or write UTF-8
1916 binmode $fh, ":encoding(UTF-8)";
1921 A scalar that is going to be passed to some extension
1923 Be it C<Compress::Zlib>, C<Apache::Request> or any extension that has no
1924 mention of Unicode in the manpage, you need to make sure that the
1925 UTF8 flag is stripped off. Note that at the time of this writing
1926 (January 2012) the mentioned modules are not UTF-8-aware. Please
1927 check the documentation to verify if this is still true.
1931 $val = Encode::encode("UTF-8", $val); # make octets
1936 A scalar we got back from an extension
1938 If you believe the scalar comes back as UTF-8, you will most likely
1939 want the UTF8 flag restored:
1943 $val = Encode::decode("UTF-8", $val);
1948 Same thing, if you are really sure it is UTF-8
1952 Encode::_utf8_on($val);
1957 A wrapper for L<DBI> C<fetchrow_array> and C<fetchrow_hashref>
1959 When the database contains only UTF-8, a wrapper function or method is
1960 a convenient way to replace all your C<fetchrow_array> and
1961 C<fetchrow_hashref> calls. A wrapper function will also make it easier to
1962 adapt to future enhancements in your database driver. Note that at the
1963 time of this writing (January 2012), the DBI has no standardized way
1964 to deal with UTF-8 data. Please check the L<DBI documentation|DBI> to verify if
1968 # $what is one of fetchrow_{array,hashref}
1969 my($self, $sth, $what) = @_;
1975 my @arr = $sth->$what;
1977 defined && /[^\000-\177]/ && Encode::_utf8_on($_);
1981 my $ret = $sth->$what;
1983 for my $k (keys %$ret) {
1986 && Encode::_utf8_on($_) for $ret->{$k};
1990 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
2000 A large scalar that you know can only contain ASCII
2002 Scalars that contain only ASCII and are marked as UTF-8 are sometimes
2003 a drag to your program. If you recognize such a situation, just remove
2006 utf8::downgrade($val) if $] > 5.008;
2012 See also L</The "Unicode Bug"> above.
2014 =head2 Interaction with Extensions
2016 When Perl exchanges data with an extension, the extension should be
2017 able to understand the UTF8 flag and act accordingly. If the
2018 extension doesn't recognize that flag, it's likely that the extension
2019 will return incorrectly-flagged data.
2021 So if you're working with Unicode data, consult the documentation of
2022 every module you're using if there are any issues with Unicode data
2023 exchange. If the documentation does not talk about Unicode at all,
2024 suspect the worst and probably look at the source to learn how the
2025 module is implemented. Modules written completely in Perl shouldn't
2026 cause problems. Modules that directly or indirectly access code written
2027 in other programming languages are at risk.
2029 For affected functions, the simple strategy to avoid data corruption is
2030 to always make the encoding of the exchanged data explicit. Choose an
2031 encoding that you know the extension can handle. Convert arguments passed
2032 to the extensions to that encoding and convert results back from that
2033 encoding. Write wrapper functions that do the conversions for you, so
2034 you can later change the functions when the extension catches up.
2036 To provide an example, let's say the popular C<Foo::Bar::escape_html>
2037 function doesn't deal with Unicode data yet. The wrapper function
2038 would convert the argument to raw UTF-8 and convert the result back to
2039 Perl's internal representation like so:
2041 sub my_escape_html ($) {
2043 return unless defined $what;
2044 Encode::decode("UTF-8", Foo::Bar::escape_html(
2045 Encode::encode("UTF-8", $what)));
2048 Sometimes, when the extension does not convert data but just stores
2049 and retrieves it, you will be able to use the otherwise
2050 dangerous L<C<Encode::_utf8_on()>|Encode/_utf8_on> function. Let's say
2051 the popular C<Foo::Bar> extension, written in C, provides a C<param>
2052 method that lets you store and retrieve data according to these prototypes:
2054 $self->param($name, $value); # set a scalar
2055 $value = $self->param($name); # retrieve a scalar
2057 If it does not yet provide support for any encoding, one could write a
2058 derived class with such a C<param> method:
2061 my($self,$name,$value) = @_;
2062 utf8::upgrade($name); # make sure it is UTF-8 encoded
2063 if (defined $value) {
2064 utf8::upgrade($value); # make sure it is UTF-8 encoded
2065 return $self->SUPER::param($name,$value);
2067 my $ret = $self->SUPER::param($name);
2068 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
2073 Some extensions provide filters on data entry/exit points, such as
2074 C<DB_File::filter_store_key> and family. Look out for such filters in
2075 the documentation of your extensions; they can make the transition to
2076 Unicode data much easier.
2080 Some functions are slower when working on UTF-8 encoded strings than
2081 on byte encoded strings. All functions that need to hop over
2082 characters such as C<length()>, C<substr()> or C<index()>, or matching
2083 regular expressions can work B<much> faster when the underlying data are
2086 In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
2087 a caching scheme was introduced which improved the situation. In general,
2088 operations with UTF-8 encoded strings are still slower. As an example,
2089 the Unicode properties (character classes) like C<\p{Nd}> are known to
2090 be quite a bit slower (5-20 times) than their simpler counterparts
2091 like C<[0-9]> (then again, there are hundreds of Unicode characters matching
2092 C<Nd> compared with the 10 ASCII characters matching C<[0-9]>).
2096 L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>,
2097 L<perlretut>, L<perlvar/"${^UNICODE}">,
2098 L<http://www.unicode.org/reports/tr44>).