1PERLUNICODE(1) Perl Programmers Reference Guide PERLUNICODE(1)
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6 perlunicode - Unicode support in Perl
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9 If you haven't already, before reading this document, you should become
10 familiar with both perlunitut and perluniintro.
11 Unicode aims to UNI-fy the en-CODE-ings of all the world's character
13 sets into a single Standard. For quite a few of the various coding
14 standards that existed when Unicode was first created, converting from
15 each to Unicode essentially meant adding a constant to each code point
16 in the original standard, and converting back meant just subtracting
17 that same constant. For ASCII and ISO-8859-1, the constant is 0. For
18 ISO-8859-5, (Cyrillic) the constant is 864; for Hebrew (ISO-8859-8),
19 it's 1488; Thai (ISO-8859-11), 3424; and so forth. This made it easy
20 to do the conversions, and facilitated the adoption of Unicode.
21 And it worked; nowadays, those legacy standards are rarely used. Most
23 everyone uses Unicode.
24 Unicode is a comprehensive standard. It specifies many things outside
26 the scope of Perl, such as how to display sequences of characters. For
27 a full discussion of all aspects of Unicode, see
28 <http://www.unicode.org>.
29 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:
34 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.
38 Also, the use of Unicode may present security issues that aren't
40 obvious, see "Security Implications of Unicode".
41 Safest if you "use feature 'unicode_strings'"
43 In order to preserve backward compatibility, Perl does not turn on
44 full internal Unicode support unless the pragma
45 "use feature 'unicode_strings'" is specified. (This is
46 automatically selected if you "use 5.012" or higher.) Failure to
47 do this can trigger unexpected surprises. See "The "Unicode Bug""
48 below.
49 This pragma doesn't affect I/O. Nor does it change the internal
51 representation of strings, only their interpretation. There are
52 still several places where Unicode isn't fully supported, such as
53 in filenames.
54 Input and Output Layers
56 Use the ":encoding(...)" layer to read from and write to
57 filehandles using the specified encoding. (See open.)
58 You must convert your non-ASCII, non-UTF-8 Perl scripts to be UTF-8.
60 The encoding module has been deprecated since perl 5.18 and the
61 perl internals it requires have been removed with perl 5.26.
62 "use utf8" still needed to enable UTF-8 in scripts
64 If your Perl script is itself encoded in UTF-8, the "use utf8"
65 pragma must be explicitly included to enable recognition of that
66 (in string or regular expression literals, or in identifier names).
67 This is the only time when an explicit "use utf8" is needed. (See
68 utf8).
69 If a Perl script begins with the bytes that form the UTF-8 encoding
71 of the Unicode BYTE ORDER MARK ("BOM", see "Unicode Encodings"),
72 those bytes are completely ignored.
73 UTF-16 scripts autodetected
75 If a Perl script begins with the Unicode "BOM" (UTF-16LE,
76 UTF16-BE), or if the script looks like non-"BOM"-marked UTF-16 of
77 either endianness, Perl will correctly read in the script as the
78 appropriate Unicode encoding.
79 Byte and Character Semantics
81 Before Unicode, most encodings used 8 bits (a single byte) to encode
82 each character. Thus a character was a byte, and a byte was a
83 character, and there could be only 256 or fewer possible characters.
84 "Byte Semantics" in the title of this section refers to this behavior.
85 There was no need to distinguish between "Byte" and "Character".
86 Then along comes Unicode which has room for over a million characters
88 (and Perl allows for even more). This means that a character may
89 require more than a single byte to represent it, and so the two terms
90 are no longer equivalent. What matter are the characters as whole
91 entities, and not usually the bytes that comprise them. That's what
92 the term "Character Semantics" in the title of this section refers to.
93 Perl had to change internally to decouple "bytes" from "characters".
95 It is important that you too change your ideas, if you haven't already,
96 so that "byte" and "character" no longer mean the same thing in your
97 mind.
98 The basic building block of Perl strings has always been a "character".
100 The changes basically come down to that the implementation no longer
101 thinks that a character is always just a single byte.
102 There are various things to note:
104 · String handling functions, for the most part, continue to operate
106 in terms of characters. "length()", for example, returns the
107 number of characters in a string, just as before. But that number
108 no longer is necessarily the same as the number of bytes in the
109 string (there may be more bytes than characters). The other such
110 functions include "chop()", "chomp()", "substr()", "pos()",
111 "index()", "rindex()", "sort()", "sprintf()", and "write()".
112 The exceptions are:
114 · the bit-oriented "vec"
116 · the byte-oriented "pack"/"unpack" "C" format
120 However, the "W" specifier does operate on whole characters, as
122 does the "U" specifier.
123 · some operators that interact with the platform's operating
125 system
126 Operators dealing with filenames are examples.
128 · when the functions are called from within the scope of the
130 "use bytes" pragma
131 Likely, you should use this only for debugging anyway.
133 · Strings--including hash keys--and regular expression patterns may
135 contain characters that have ordinal values larger than 255.
136 If you use a Unicode editor to edit your program, Unicode
138 characters may occur directly within the literal strings in UTF-8
139 encoding, or UTF-16. (The former requires a "use utf8", the latter
140 may require a "BOM".)
141 "Creating Unicode" in perluniintro gives other ways to place non-
143 ASCII characters in your strings.
144 · The "chr()" and "ord()" functions work on whole characters.
146 · Regular expressions match whole characters. For example, "."
148 matches a whole character instead of only a single byte.
149 · The "tr///" operator translates whole characters. (Note that the
151 "tr///CU" functionality has been removed. For similar
152 functionality to that, see "pack('U0', ...)" and "pack('C0',
153 ...)").
154 · "scalar reverse()" reverses by character rather than by byte.
156 · The bit string operators, "& | ^ ~" and (starting in v5.22) "&. |.
158 ^. ~." can operate on bit strings encoded in UTF-8, but this can
159 give unexpected results if any of the strings contain code points
160 above 0xFF. Starting in v5.28, it is a fatal error to have such an
161 operand. Otherwise, the operation is performed on a non-UTF-8 copy
162 of the operand. If you're not sure about the encoding of a string,
163 downgrade it before using any of these operators; you can use
164 "utf8::utf8_downgrade()".
165 The bottom line is that Perl has always practiced "Character
167 Semantics", but with the advent of Unicode, that is now different than
168 "Byte Semantics".
169 ASCII Rules versus Unicode Rules
171 Before Unicode, when a character was a byte was a character, Perl knew
172 only about the 128 characters defined by ASCII, code points 0 through
173 127 (except for under "use locale"). That left the code points 128 to
174 255 as unassigned, and available for whatever use a program might want.
175 The only semantics they have is their ordinal numbers, and that they
176 are members of none of the non-negative character classes. None are
177 considered to match "\w" for example, but all match "\W".
178 Unicode, of course, assigns each of those code points a particular
180 meaning (along with ones above 255). To preserve backward
181 compatibility, Perl only uses the Unicode meanings when there is some
182 indication that Unicode is what is intended; otherwise the non-ASCII
183 code points remain treated as if they are unassigned.
184 Here are the ways that Perl knows that a string should be treated as
186 Unicode:
187 · Within the scope of "use utf8"
189 If the whole program is Unicode (signified by using 8-bit Unicode
191 Transformation Format), then all literal strings within it must be
192 Unicode.
193 · Within the scope of "use feature 'unicode_strings'"
195 This pragma was created so you can explicitly tell Perl that
197 operations executed within its scope are to use Unicode rules.
198 More operations are affected with newer perls. See "The "Unicode
199 Bug"".
200 · Within the scope of "use 5.012" or higher
202 This implicitly turns on "use feature 'unicode_strings'".
204 · Within the scope of "use locale 'not_characters'", or "use locale"
206 and the current locale is a UTF-8 locale.
207 The former is defined to imply Unicode handling; and the latter
209 indicates a Unicode locale, hence a Unicode interpretation of all
210 strings within it.
211 · When the string contains a Unicode-only code point
213 Perl has never accepted code points above 255 without them being
215 Unicode, so their use implies Unicode for the whole string.
216 · When the string contains a Unicode named code point "\N{...}"
218 The "\N{...}" construct explicitly refers to a Unicode code point,
220 even if it is one that is also in ASCII. Therefore the string
221 containing it must be Unicode.
222 · When the string has come from an external source marked as Unicode
224 The "-C" command line option can specify that certain inputs to the
226 program are Unicode, and the values of this can be read by your
227 Perl code, see "${^UNICODE}" in perlvar.
228 · When the string has been upgraded to UTF-8
230 The function "utf8::utf8_upgrade()" can be explicitly used to
232 permanently (unless a subsequent "utf8::utf8_downgrade()" is
233 called) cause a string to be treated as Unicode.
234 · There are additional methods for regular expression patterns
236 A pattern that is compiled with the "/u" or "/a" modifiers is
238 treated as Unicode (though there are some restrictions with "/a").
239 Under the "/d" and "/l" modifiers, there are several other
240 indications for Unicode; see "Character set modifiers" in perlre.
241 Note that all of the above are overridden within the scope of "use
243 bytes"; but you should be using this pragma only for debugging.
244 Note also that some interactions with the platform's operating system
246 never use Unicode rules.
247 When Unicode rules are in effect:
249 · Case translation operators use the Unicode case translation tables.
251 Note that "uc()", or "\U" in interpolated strings, translates to
253 uppercase, while "ucfirst", or "\u" in interpolated strings,
254 translates to titlecase in languages that make the distinction
255 (which is equivalent to uppercase in languages without the
256 distinction).
257 There is a CPAN module, "Unicode::Casing", which allows you to
259 define your own mappings to be used in "lc()", "lcfirst()", "uc()",
260 "ucfirst()", and "fc" (or their double-quoted string inlined
261 versions such as "\U"). (Prior to Perl 5.16, this functionality
262 was partially provided in the Perl core, but suffered from a number
263 of insurmountable drawbacks, so the CPAN module was written
264 instead.)
265 · Character classes in regular expressions match based on the
267 character properties specified in the Unicode properties database.
268 "\w" can be used to match a Japanese ideograph, for instance; and
270 "[[:digit:]]" a Bengali number.
271 · Named Unicode properties, scripts, and block ranges may be used
273 (like bracketed character classes) by using the "\p{}" "matches
274 property" construct and the "\P{}" negation, "doesn't match
275 property".
276 See "Unicode Character Properties" for more details.
278 You can define your own character properties and use them in the
280 regular expression with the "\p{}" or "\P{}" construct. See "User-
281 Defined Character Properties" for more details.
282 Extended Grapheme Clusters (Logical characters)
284 Consider a character, say "H". It could appear with various marks
285 around it, such as an acute accent, or a circumflex, or various hooks,
286 circles, arrows, etc., above, below, to one side or the other, etc.
287 There are many possibilities among the world's languages. The number
288 of combinations is astronomical, and if there were a character for each
289 combination, it would soon exhaust Unicode's more than a million
290 possible characters. So Unicode took a different approach: there is a
291 character for the base "H", and a character for each of the possible
292 marks, and these can be variously combined to get a final logical
293 character. So a logical character--what appears to be a single
294 character--can be a sequence of more than one individual characters.
295 The Unicode standard calls these "extended grapheme clusters" (which is
296 an improved version of the no-longer much used "grapheme cluster");
297 Perl furnishes the "\X" regular expression construct to match such
298 sequences in their entirety.
299 But Unicode's intent is to unify the existing character set standards
301 and practices, and several pre-existing standards have single
302 characters that mean the same thing as some of these combinations, like
303 ISO-8859-1, which has quite a few of them. For example, "LATIN CAPITAL
304 LETTER E WITH ACUTE" was already in this standard when Unicode came
305 along. Unicode therefore added it to its repertoire as that single
306 character. But this character is considered by Unicode to be
307 equivalent to the sequence consisting of the character "LATIN CAPITAL
308 LETTER E" followed by the character "COMBINING ACUTE ACCENT".
309 "LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed"
311 character, and its equivalence with the "E" and the "COMBINING ACCENT"
312 sequence is called canonical equivalence. All pre-composed characters
313 are said to have a decomposition (into the equivalent sequence), and
314 the decomposition type is also called canonical. A string may be
315 comprised as much as possible of precomposed characters, or it may be
316 comprised of entirely decomposed characters. Unicode calls these
317 respectively, "Normalization Form Composed" (NFC) and "Normalization
318 Form Decomposed". The "Unicode::Normalize" module contains functions
319 that convert between the two. A string may also have both composed
320 characters and decomposed characters; this module can be used to make
321 it all one or the other.
322 You may be presented with strings in any of these equivalent forms.
324 There is currently nothing in Perl 5 that ignores the differences. So
325 you'll have to specially handle it. The usual advice is to convert
326 your inputs to "NFD" before processing further.
327 For more detailed information, see <http://unicode.org/reports/tr15/>.
329 Unicode Character Properties
331 (The only time that Perl considers a sequence of individual code points
332 as a single logical character is in the "\X" construct, already
333 mentioned above. Therefore "character" in this discussion means a
334 single Unicode code point.)
335 Very nearly all Unicode character properties are accessible through
337 regular expressions by using the "\p{}" "matches property" construct
338 and the "\P{}" "doesn't match property" for its negation.
339 For instance, "\p{Uppercase}" matches any single character with the
341 Unicode "Uppercase" property, while "\p{L}" matches any character with
342 a "General_Category" of "L" (letter) property (see "General_Category"
343 below). Brackets are not required for single letter property names, so
344 "\p{L}" is equivalent to "\pL".
345 More formally, "\p{Uppercase}" matches any single character whose
347 Unicode "Uppercase" property value is "True", and "\P{Uppercase}"
348 matches any character whose "Uppercase" property value is "False", and
349 they could have been written as "\p{Uppercase=True}" and
350 "\p{Uppercase=False}", respectively.
351 This formality is needed when properties are not binary; that is, if
353 they can take on more values than just "True" and "False". For
354 example, the "Bidi_Class" property (see "Bidirectional Character Types"
355 below), can take on several different values, such as "Left", "Right",
356 "Whitespace", and others. To match these, one needs to specify both
357 the property name ("Bidi_Class"), AND the value being matched against
358 ("Left", "Right", etc.). This is done, as in the examples above, by
359 having the two components separated by an equal sign (or
360 interchangeably, a colon), like "\p{Bidi_Class: Left}".
361 All Unicode-defined character properties may be written in these
363 compound forms of "\p{property=value}" or "\p{property:value}", but
364 Perl provides some additional properties that are written only in the
365 single form, as well as single-form short-cuts for all binary
366 properties and certain others described below, in which you may omit
367 the property name and the equals or colon separator.
368 Most Unicode character properties have at least two synonyms (or
370 aliases if you prefer): a short one that is easier to type and a longer
371 one that is more descriptive and hence easier to understand. Thus the
372 "L" and "Letter" properties above are equivalent and can be used
373 interchangeably. Likewise, "Upper" is a synonym for "Uppercase", and
374 we could have written "\p{Uppercase}" equivalently as "\p{Upper}".
375 Also, there are typically various synonyms for the values the property
376 can be. For binary properties, "True" has 3 synonyms: "T", "Yes", and
377 "Y"; and "False" has correspondingly "F", "No", and "N". But be
378 careful. A short form of a value for one property may not mean the
379 same thing as the same short form for another. Thus, for the
380 "General_Category" property, "L" means "Letter", but for the
381 "Bidi_Class" property, "L" means "Left". A complete list of properties
382 and synonyms is in perluniprops.
383 Upper/lower case differences in property names and values are
385 irrelevant; thus "\p{Upper}" means the same thing as "\p{upper}" or
386 even "\p{UpPeR}". Similarly, you can add or subtract underscores
387 anywhere in the middle of a word, so that these are also equivalent to
388 "\p{U_p_p_e_r}". And white space is irrelevant adjacent to non-word
389 characters, such as the braces and the equals or colon separators, so
390 "\p{ Upper }" and "\p{ Upper_case : Y }" are equivalent to these as
391 well. In fact, white space and even hyphens can usually be added or
392 deleted anywhere. So even "\p{ Up-per case = Yes}" is equivalent. All
393 this is called "loose-matching" by Unicode. The few places where
394 stricter matching is used is in the middle of numbers, and in the Perl
395 extension properties that begin or end with an underscore. Stricter
396 matching cares about white space (except adjacent to non-word
397 characters), hyphens, and non-interior underscores.
398 You can also use negation in both "\p{}" and "\P{}" by introducing a
400 caret ("^") between the first brace and the property name: "\p{^Tamil}"
401 is equal to "\P{Tamil}".
402 Almost all properties are immune to case-insensitive matching. That
404 is, adding a "/i" regular expression modifier does not change what they
405 match. There are two sets that are affected. The first set is
406 "Uppercase_Letter", "Lowercase_Letter", and "Titlecase_Letter", all of
407 which match "Cased_Letter" under "/i" matching. And the second set is
408 "Uppercase", "Lowercase", and "Titlecase", all of which match "Cased"
409 under "/i" matching. This set also includes its subsets "PosixUpper"
410 and "PosixLower" both of which under "/i" match "PosixAlpha". (The
411 difference between these sets is that some things, such as Roman
412 numerals, come in both upper and lower case so they are "Cased", but
413 aren't considered letters, so they aren't "Cased_Letter"'s.)
414 See "Beyond Unicode code points" for special considerations when
416 matching Unicode properties against non-Unicode code points.
417 General_Category
419 Every Unicode character is assigned a general category, which is the
421 "most usual categorization of a character" (from
422 <http://www.unicode.org/reports/tr44>).
423 The compound way of writing these is like "\p{General_Category=Number}"
425 (short: "\p{gc:n}"). But Perl furnishes shortcuts in which everything
426 up through the equal or colon separator is omitted. So you can instead
427 just write "\pN".
428 Here are the short and long forms of the values the "General Category"
430 property can have:
431 Short Long
433 L Letter
435 LC, L& Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
436 Lu Uppercase_Letter
437 Ll Lowercase_Letter
438 Lt Titlecase_Letter
439 Lm Modifier_Letter
440 Lo Other_Letter
441 M Mark
443 Mn Nonspacing_Mark
444 Mc Spacing_Mark
445 Me Enclosing_Mark
446 N Number
448 Nd Decimal_Number (also Digit)
449 Nl Letter_Number
450 No Other_Number
451 P Punctuation (also Punct)
453 Pc Connector_Punctuation
454 Pd Dash_Punctuation
455 Ps Open_Punctuation
456 Pe Close_Punctuation
457 Pi Initial_Punctuation
458 (may behave like Ps or Pe depending on usage)
459 Pf Final_Punctuation
460 (may behave like Ps or Pe depending on usage)
461 Po Other_Punctuation
462 S Symbol
464 Sm Math_Symbol
465 Sc Currency_Symbol
466 Sk Modifier_Symbol
467 So Other_Symbol
468 Z Separator
470 Zs Space_Separator
471 Zl Line_Separator
472 Zp Paragraph_Separator
473 C Other
475 Cc Control (also Cntrl)
476 Cf Format
477 Cs Surrogate
478 Co Private_Use
479 Cn Unassigned
480 Single-letter properties match all characters in any of the two-letter
482 sub-properties starting with the same letter. "LC" and "L&" are
483 special: both are aliases for the set consisting of everything matched
484 by "Ll", "Lu", and "Lt".
485 Bidirectional Character Types
487 Because scripts differ in their directionality (Hebrew and Arabic are
489 written right to left, for example) Unicode supplies a "Bidi_Class"
490 property. Some of the values this property can have are:
491 Value Meaning
493 L Left-to-Right
495 LRE Left-to-Right Embedding
496 LRO Left-to-Right Override
497 R Right-to-Left
498 AL Arabic Letter
499 RLE Right-to-Left Embedding
500 RLO Right-to-Left Override
501 PDF Pop Directional Format
502 EN European Number
503 ES European Separator
504 ET European Terminator
505 AN Arabic Number
506 CS Common Separator
507 NSM Non-Spacing Mark
508 BN Boundary Neutral
509 B Paragraph Separator
510 S Segment Separator
511 WS Whitespace
512 ON Other Neutrals
513 This property is always written in the compound form. For example,
515 "\p{Bidi_Class:R}" matches characters that are normally written right
516 to left. Unlike the "General_Category" property, this property can
517 have more values added in a future Unicode release. Those listed above
518 comprised the complete set for many Unicode releases, but others were
519 added in Unicode 6.3; you can always find what the current ones are in
520 perluniprops. And <http://www.unicode.org/reports/tr9/> describes how
521 to use them.
522 Scripts
524 The world's languages are written in many different scripts. This
526 sentence (unless you're reading it in translation) is written in Latin,
527 while Russian is written in Cyrillic, and Greek is written in, well,
528 Greek; Japanese mainly in Hiragana or Katakana. There are many more.
529 The Unicode "Script" and "Script_Extensions" properties give what
531 script a given character is in. The "Script_Extensions" property is an
532 improved version of "Script", as demonstrated below. Either property
533 can be specified with the compound form like "\p{Script=Hebrew}"
534 (short: "\p{sc=hebr}"), or "\p{Script_Extensions=Javanese}" (short:
535 "\p{scx=java}"). In addition, Perl furnishes shortcuts for all
536 "Script_Extensions" property names. You can omit everything up through
537 the equals (or colon), and simply write "\p{Latin}" or "\P{Cyrillic}".
538 (This is not true for "Script", which is required to be written in the
539 compound form. Prior to Perl v5.26, the single form returned the plain
540 old "Script" version, but was changed because "Script_Extensions" gives
541 better results.)
542 The difference between these two properties involves characters that
544 are used in multiple scripts. For example the digits '0' through '9'
545 are used in many parts of the world. These are placed in a script
546 named "Common". Other characters are used in just a few scripts. For
547 example, the "KATAKANA-HIRAGANA DOUBLE HYPHEN" is used in both Japanese
548 scripts, Katakana and Hiragana, but nowhere else. The "Script"
549 property places all characters that are used in multiple scripts in the
550 "Common" script, while the "Script_Extensions" property places those
551 that are used in only a few scripts into each of those scripts; while
552 still using "Common" for those used in many scripts. Thus both these
553 match:
554 "0" =~ /\p{sc=Common}/ # Matches
556 "0" =~ /\p{scx=Common}/ # Matches
557 and only the first of these match:
559 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common} # Matches
561 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match
562 And only the last two of these match:
564 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana} # No match
566 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana} # No match
567 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
568 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches
569 "Script_Extensions" is thus an improved "Script", in which there are
571 fewer characters in the "Common" script, and correspondingly more in
572 other scripts. It is new in Unicode version 6.0, and its data are
573 likely to change significantly in later releases, as things get sorted
574 out. New code should probably be using "Script_Extensions" and not
575 plain "Script". If you compile perl with a Unicode release that
576 doesn't have "Script_Extensions", the single form Perl extensions will
577 instead refer to the plain "Script" property. If you compile with a
578 version of Unicode that doesn't have the "Script" property, these
579 extensions will not be defined at all.
580 (Actually, besides "Common", the "Inherited" script, contains
582 characters that are used in multiple scripts. These are modifier
583 characters which inherit the script value of the controlling character.
584 Some of these are used in many scripts, and so go into "Inherited" in
585 both "Script" and "Script_Extensions". Others are used in just a few
586 scripts, so are in "Inherited" in "Script", but not in
587 "Script_Extensions".)
588 It is worth stressing that there are several different sets of digits
590 in Unicode that are equivalent to 0-9 and are matchable by "\d" in a
591 regular expression. If they are used in a single language only, they
592 are in that language's "Script" and "Script_Extensions". If they are
593 used in more than one script, they will be in "sc=Common", but only if
594 they are used in many scripts should they be in "scx=Common".
595 The explanation above has omitted some detail; refer to UAX#24 "Unicode
597 Script Property": <http://www.unicode.org/reports/tr24>.
598 A complete list of scripts and their shortcuts is in perluniprops.
600 Use of the "Is" Prefix
602 For backward compatibility (with ancient Perl 5.6), all properties
604 writable without using the compound form mentioned so far may have "Is"
605 or "Is_" prepended to their name, so "\P{Is_Lu}", for example, is equal
606 to "\P{Lu}", and "\p{IsScript:Arabic}" is equal to "\p{Arabic}".
607 Blocks
609 In addition to scripts, Unicode also defines blocks of characters. The
611 difference between scripts and blocks is that the concept of scripts is
612 closer to natural languages, while the concept of blocks is more of an
613 artificial grouping based on groups of Unicode characters with
614 consecutive ordinal values. For example, the "Basic Latin" block is all
615 the characters whose ordinals are between 0 and 127, inclusive; in
616 other words, the ASCII characters. The "Latin" script contains some
617 letters from this as well as several other blocks, like "Latin-1
618 Supplement", "Latin Extended-A", etc., but it does not contain all the
619 characters from those blocks. It does not, for example, contain the
620 digits 0-9, because those digits are shared across many scripts, and
621 hence are in the "Common" script.
622 For more about scripts versus blocks, see UAX#24 "Unicode Script
624 Property": <http://www.unicode.org/reports/tr24>
625 The "Script_Extensions" or "Script" properties are likely to be the
627 ones you want to use when processing natural language; the "Block"
628 property may occasionally be useful in working with the nuts and bolts
629 of Unicode.
630 Block names are matched in the compound form, like "\p{Block: Arrows}"
632 or "\p{Blk=Hebrew}". Unlike most other properties, only a few block
633 names have a Unicode-defined short name.
634 Perl also defines single form synonyms for the block property in cases
636 where these do not conflict with something else. But don't use any of
637 these, because they are unstable. Since these are Perl extensions,
638 they are subordinate to official Unicode property names; Unicode
639 doesn't know nor care about Perl's extensions. It may happen that a
640 name that currently means the Perl extension will later be changed
641 without warning to mean a different Unicode property in a future
642 version of the perl interpreter that uses a later Unicode release, and
643 your code would no longer work. The extensions are mentioned here for
644 completeness: Take the block name and prefix it with one of: "In" (for
645 example "\p{Blk=Arrows}" can currently be written as "\p{In_Arrows}");
646 or sometimes "Is" (like "\p{Is_Arrows}"); or sometimes no prefix at all
647 ("\p{Arrows}"). As of this writing (Unicode 9.0) there are no
648 conflicts with using the "In_" prefix, but there are plenty with the
649 other two forms. For example, "\p{Is_Hebrew}" and "\p{Hebrew}" mean
650 "\p{Script_Extensions=Hebrew}" which is NOT the same thing as
651 "\p{Blk=Hebrew}". Our advice used to be to use the "In_" prefix as a
652 single form way of specifying a block. But Unicode 8.0 added
653 properties whose names begin with "In", and it's now clear that it's
654 only luck that's so far prevented a conflict. Using "In" is only
655 marginally less typing than "Blk:", and the latter's meaning is clearer
656 anyway, and guaranteed to never conflict. So don't take chances. Use
657 "\p{Blk=foo}" for new code. And be sure that block is what you really
658 really want to do. In most cases scripts are what you want instead.
659 A complete list of blocks is in perluniprops.
661 Other Properties
663 There are many more properties than the very basic ones described here.
665 A complete list is in perluniprops.
666 Unicode defines all its properties in the compound form, so all single-
668 form properties are Perl extensions. Most of these are just synonyms
669 for the Unicode ones, but some are genuine extensions, including
670 several that are in the compound form. And quite a few of these are
671 actually recommended by Unicode (in
672 <http://www.unicode.org/reports/tr18>).
673 This section gives some details on all extensions that aren't just
675 synonyms for compound-form Unicode properties (for those properties,
676 you'll have to refer to the Unicode Standard
677 <http://www.unicode.org/reports/tr44>.
678 "\p{All}"
680 This matches every possible code point. It is equivalent to
681 "qr/./s". Unlike all the other non-user-defined "\p{}" property
682 matches, no warning is ever generated if this is property is
683 matched against a non-Unicode code point (see "Beyond Unicode code
684 points" below).
685 "\p{Alnum}"
687 This matches any "\p{Alphabetic}" or "\p{Decimal_Number}"
688 character.
689 "\p{Any}"
691 This matches any of the 1_114_112 Unicode code points. It is a
692 synonym for "\p{Unicode}".
693 "\p{ASCII}"
695 This matches any of the 128 characters in the US-ASCII character
696 set, which is a subset of Unicode.
697 "\p{Assigned}"
699 This matches any assigned code point; that is, any code point whose
700 general category is not "Unassigned" (or equivalently, not "Cn").
701 "\p{Blank}"
703 This is the same as "\h" and "\p{HorizSpace}": A character that
704 changes the spacing horizontally.
705 "\p{Decomposition_Type: Non_Canonical}" (Short: "\p{Dt=NonCanon}")
707 Matches a character that has a non-canonical decomposition.
708 The "Extended Grapheme Clusters (Logical characters)" section above
710 talked about canonical decompositions. However, many more
711 characters have a different type of decomposition, a "compatible"
712 or "non-canonical" decomposition. The sequences that form these
713 decompositions are not considered canonically equivalent to the
714 pre-composed character. An example is the "SUPERSCRIPT ONE". It
715 is somewhat like a regular digit 1, but not exactly; its
716 decomposition into the digit 1 is called a "compatible"
717 decomposition, specifically a "super" decomposition. There are
718 several such compatibility decompositions (see
719 <http://www.unicode.org/reports/tr44>), including one called
720 "compat", which means some miscellaneous type of decomposition that
721 doesn't fit into the other decomposition categories that Unicode
722 has chosen.
723 Note that most Unicode characters don't have a decomposition, so
725 their decomposition type is "None".
726 For your convenience, Perl has added the "Non_Canonical"
728 decomposition type to mean any of the several compatibility
729 decompositions.
730 "\p{Graph}"
732 Matches any character that is graphic. Theoretically, this means a
733 character that on a printer would cause ink to be used.
734 "\p{HorizSpace}"
736 This is the same as "\h" and "\p{Blank}": a character that changes
737 the spacing horizontally.
738 "\p{In=*}"
740 This is a synonym for "\p{Present_In=*}"
741 "\p{PerlSpace}"
743 This is the same as "\s", restricted to ASCII, namely "[ \f\n\r\t]"
744 and starting in Perl v5.18, a vertical tab.
745 Mnemonic: Perl's (original) space
747 "\p{PerlWord}"
749 This is the same as "\w", restricted to ASCII, namely
750 "[A-Za-z0-9_]"
751 Mnemonic: Perl's (original) word.
753 "\p{Posix...}"
755 There are several of these, which are equivalents, using the "\p{}"
756 notation, for Posix classes and are described in "POSIX Character
757 Classes" in perlrecharclass.
758 "\p{Present_In: *}" (Short: "\p{In=*}")
760 This property is used when you need to know in what Unicode
761 version(s) a character is.
762 The "*" above stands for some two digit Unicode version number,
764 such as 1.1 or 4.0; or the "*" can also be "Unassigned". This
765 property will match the code points whose final disposition has
766 been settled as of the Unicode release given by the version number;
767 "\p{Present_In: Unassigned}" will match those code points whose
768 meaning has yet to be assigned.
769 For example, "U+0041" "LATIN CAPITAL LETTER A" was present in the
771 very first Unicode release available, which is 1.1, so this
772 property is true for all valid "*" versions. On the other hand,
773 "U+1EFF" was not assigned until version 5.1 when it became "LATIN
774 SMALL LETTER Y WITH LOOP", so the only "*" that would match it are
775 5.1, 5.2, and later.
776 Unicode furnishes the "Age" property from which this is derived.
778 The problem with Age is that a strict interpretation of it (which
779 Perl takes) has it matching the precise release a code point's
780 meaning is introduced in. Thus "U+0041" would match only 1.1; and
781 "U+1EFF" only 5.1. This is not usually what you want.
782 Some non-Perl implementations of the Age property may change its
784 meaning to be the same as the Perl "Present_In" property; just be
785 aware of that.
786 Another confusion with both these properties is that the definition
788 is not that the code point has been assigned, but that the meaning
789 of the code point has been determined. This is because 66 code
790 points will always be unassigned, and so the "Age" for them is the
791 Unicode version in which the decision to make them so was made.
792 For example, "U+FDD0" is to be permanently unassigned to a
793 character, and the decision to do that was made in version 3.1, so
794 "\p{Age=3.1}" matches this character, as also does "\p{Present_In:
795 3.1}" and up.
796 "\p{Print}"
798 This matches any character that is graphical or blank, except
799 controls.
800 "\p{SpacePerl}"
802 This is the same as "\s", including beyond ASCII.
803 Mnemonic: Space, as modified by Perl. (It doesn't include the
805 vertical tab until v5.18, which both the Posix standard and Unicode
806 consider white space.)
807 "\p{Title}" and "\p{Titlecase}"
809 Under case-sensitive matching, these both match the same code
810 points as "\p{General Category=Titlecase_Letter}" ("\p{gc=lt}").
811 The difference is that under "/i" caseless matching, these match
812 the same as "\p{Cased}", whereas "\p{gc=lt}" matches
813 "\p{Cased_Letter").
814 "\p{Unicode}"
816 This matches any of the 1_114_112 Unicode code points. "\p{Any}".
817 "\p{VertSpace}"
819 This is the same as "\v": A character that changes the spacing
820 vertically.
821 "\p{Word}"
823 This is the same as "\w", including over 100_000 characters beyond
824 ASCII.
825 "\p{XPosix...}"
827 There are several of these, which are the standard Posix classes
828 extended to the full Unicode range. They are described in "POSIX
829 Character Classes" in perlrecharclass.
830 User-Defined Character Properties
832 You can define your own binary character properties by defining
833 subroutines whose names begin with "In" or "Is". (The experimental
834 feature "(?[ ])" in perlre provides an alternative which allows more
835 complex definitions.) The subroutines can be defined in any package.
836 The user-defined properties can be used in the regular expression
837 "\p{}" and "\P{}" constructs; if you are using a user-defined property
838 from a package other than the one you are in, you must specify its
839 package in the "\p{}" or "\P{}" construct.
840 # assuming property Is_Foreign defined in Lang::
842 package main; # property package name required
843 if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
844 package Lang; # property package name not required
846 if ($txt =~ /\p{IsForeign}+/) { ... }
847 Note that the effect is compile-time and immutable once defined.
849 However, the subroutines are passed a single parameter, which is 0 if
850 case-sensitive matching is in effect and non-zero if caseless matching
851 is in effect. The subroutine may return different values depending on
852 the value of the flag, and one set of values will immutably be in
853 effect for all case-sensitive matches, and the other set for all case-
854 insensitive matches.
855 Note that if the regular expression is tainted, then Perl will die
857 rather than calling the subroutine when the name of the subroutine is
858 determined by the tainted data.
859 The subroutines must return a specially-formatted string, with one or
861 more newline-separated lines. Each line must be one of the following:
862 · A single hexadecimal number denoting a code point to include.
864 · Two hexadecimal numbers separated by horizontal whitespace (space
866 or tabular characters) denoting a range of code points to include.
867 · Something to include, prefixed by "+": a built-in character
869 property (prefixed by "utf8::") or a fully qualified (including
870 package name) user-defined character property, to represent all the
871 characters in that property; two hexadecimal code points for a
872 range; or a single hexadecimal code point.
873 · Something to exclude, prefixed by "-": an existing character
875 property (prefixed by "utf8::") or a fully qualified (including
876 package name) user-defined character property, to represent all the
877 characters in that property; two hexadecimal code points for a
878 range; or a single hexadecimal code point.
879 · Something to negate, prefixed "!": an existing character property
881 (prefixed by "utf8::") or a fully qualified (including package
882 name) user-defined character property, to represent all the
883 characters in that property; two hexadecimal code points for a
884 range; or a single hexadecimal code point.
885 · Something to intersect with, prefixed by "&": an existing character
887 property (prefixed by "utf8::") or a fully qualified (including
888 package name) user-defined character property, for all the
889 characters except the characters in the property; two hexadecimal
890 code points for a range; or a single hexadecimal code point.
891 For example, to define a property that covers both the Japanese
893 syllabaries (hiragana and katakana), you can define
894 sub InKana {
896 return <<END;
897 3040\t309F
898 30A0\t30FF
899 END
900 }
901 Imagine that the here-doc end marker is at the beginning of the line.
903 Now you can use "\p{InKana}" and "\P{InKana}".
904 You could also have used the existing block property names:
906 sub InKana {
908 return <<'END';
909 +utf8::InHiragana
910 +utf8::InKatakana
911 END
912 }
913 Suppose you wanted to match only the allocated characters, not the raw
915 block ranges: in other words, you want to remove the unassigned
916 characters:
917 sub InKana {
919 return <<'END';
920 +utf8::InHiragana
921 +utf8::InKatakana
922 -utf8::IsCn
923 END
924 }
925 The negation is useful for defining (surprise!) negated classes.
927 sub InNotKana {
929 return <<'END';
930 !utf8::InHiragana
931 -utf8::InKatakana
932 +utf8::IsCn
933 END
934 }
935 This will match all non-Unicode code points, since every one of them is
937 not in Kana. You can use intersection to exclude these, if desired, as
938 this modified example shows:
939 sub InNotKana {
941 return <<'END';
942 !utf8::InHiragana
943 -utf8::InKatakana
944 +utf8::IsCn
945 &utf8::Any
946 END
947 }
948 &utf8::Any must be the last line in the definition.
950 Intersection is used generally for getting the common characters
952 matched by two (or more) classes. It's important to remember not to
953 use "&" for the first set; that would be intersecting with nothing,
954 resulting in an empty set.
955 Unlike non-user-defined "\p{}" property matches, no warning is ever
957 generated if these properties are matched against a non-Unicode code
958 point (see "Beyond Unicode code points" below).
959 User-Defined Case Mappings (for serious hackers only)
961 This feature has been removed as of Perl 5.16. The CPAN module
962 "Unicode::Casing" provides better functionality without the drawbacks
963 that this feature had. If you are using a Perl earlier than 5.16, this
964 feature was most fully documented in the 5.14 version of this pod:
965 <http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>
966 Character Encodings for Input and Output
968 See Encode.
969 Unicode Regular Expression Support Level
971 The following list of Unicode supported features for regular
972 expressions describes all features currently directly supported by core
973 Perl. The references to "Level N" and the section numbers refer to
974 UTS#18 "Unicode Regular Expressions"
975 <http://www.unicode.org/reports/tr18>, version 13, November 2013.
976 Level 1 - Basic Unicode Support
978 RL1.1 Hex Notation - Done [1]
980 RL1.2 Properties - Done [2]
981 RL1.2a Compatibility Properties - Done [3]
982 RL1.3 Subtraction and Intersection - Experimental [4]
983 RL1.4 Simple Word Boundaries - Done [5]
984 RL1.5 Simple Loose Matches - Done [6]
985 RL1.6 Line Boundaries - Partial [7]
986 RL1.7 Supplementary Code Points - Done [8]
987 [1] "\N{U+...}" and "\x{...}"
989 [2] "\p{...}" "\P{...}". This requirement is for a minimal list of
990 properties. Perl supports these and all other Unicode character
991 properties, as R2.7 asks (see "Unicode Character Properties" above).
992 [3] Perl has "\d" "\D" "\s" "\S" "\w" "\W" "\X" "[:prop:]" "[:^prop:]",
993 plus all the properties specified by
994 <http://www.unicode.org/reports/tr18/#Compatibility_Properties>. These
995 are described above in "Other Properties"
996 [4] The experimental feature "(?[...])" starting in v5.18 accomplishes
997 this.
998 See "(?[ ])" in perlre. If you don't want to use an experimental
1000 feature, you can use one of the following:
1001 · Regular expression lookahead
1003 You can mimic class subtraction using lookahead. For example,
1005 what UTS#18 might write as
1006 [{Block=Greek}-[{UNASSIGNED}]]
1008 in Perl can be written as:
1010 (?!\p{Unassigned})\p{Block=Greek}
1012 (?=\p{Assigned})\p{Block=Greek}
1013 But in this particular example, you probably really want
1015 \p{Greek}
1017 which will match assigned characters known to be part of the
1019 Greek script.
1020 · CPAN module "Unicode::Regex::Set"
1022 It does implement the full UTS#18 grouping, intersection,
1024 union, and removal (subtraction) syntax.
1025 · "User-Defined Character Properties"
1027 "+" for union, "-" for removal (set-difference), "&" for
1029 intersection
1030 [5] "\b" "\B" meet most, but not all, the details of this requirement,
1032 but "\b{wb}" and "\B{wb}" do, as well as the stricter R2.3.
1033 [6] Note that Perl does Full case-folding in matching, not Simple:
1034 For example "U+1F88" is equivalent to "U+1F00 U+03B9", instead of
1036 just "U+1F80". This difference matters mainly for certain Greek
1037 capital letters with certain modifiers: the Full case-folding
1038 decomposes the letter, while the Simple case-folding would map it
1039 to a single character.
1040 [7] The reason this is considered to be only partially implemented is
1042 that Perl has "qr/\b{lb}/" and "Unicode::LineBreak" that are
1043 conformant with UAX#14 "Unicode Line Breaking Algorithm"
1044 <http://www.unicode.org/reports/tr14>. The regular expression
1045 construct provides default behavior, while the heavier-weight
1046 module provides customizable line breaking.
1047 But Perl treats "\n" as the start- and end-line delimiter, whereas
1049 Unicode specifies more characters that should be so-interpreted.
1050 These are:
1052 VT U+000B (\v in C)
1054 FF U+000C (\f)
1055 CR U+000D (\r)
1056 NEL U+0085
1057 LS U+2028
1058 PS U+2029
1059 "^" and "$" in regular expression patterns are supposed to match
1061 all these, but don't. These characters also don't, but should,
1062 affect "<>" $., and script line numbers.
1063 Also, lines should not be split within "CRLF" (i.e. there is no
1065 empty line between "\r" and "\n"). For "CRLF", try the ":crlf"
1066 layer (see PerlIO).
1067 [8] UTF-8/UTF-EBDDIC used in Perl allows not only "U+10000" to
1069 "U+10FFFF" but also beyond "U+10FFFF"
1070 Level 2 - Extended Unicode Support
1072 RL2.1 Canonical Equivalents - Retracted [9]
1074 by Unicode
1075 RL2.2 Extended Grapheme Clusters - Partial [10]
1076 RL2.3 Default Word Boundaries - Done [11]
1077 RL2.4 Default Case Conversion - Done
1078 RL2.5 Name Properties - Done
1079 RL2.6 Wildcard Properties - Missing
1080 RL2.7 Full Properties - Done
1081 [9] Unicode has rewritten this portion of UTS#18 to say that getting
1083 canonical equivalence (see UAX#15 "Unicode Normalization Forms"
1084 <http://www.unicode.org/reports/tr15>) is basically to be done at the
1085 programmer level. Use NFD to write both your regular expressions and
1086 text to match them against (you can use Unicode::Normalize).
1087 [10] Perl has "\X" and "\b{gcb}" but we don't have a "Grapheme Cluster
1088 Mode".
1089 [11] see UAX#29 "Unicode Text Segmentation"
1090 <http://www.unicode.org/reports/tr29>,
1091 Level 3 - Tailored Support
1093 RL3.1 Tailored Punctuation - Missing
1095 RL3.2 Tailored Grapheme Clusters - Missing [12]
1096 RL3.3 Tailored Word Boundaries - Missing
1097 RL3.4 Tailored Loose Matches - Retracted by Unicode
1098 RL3.5 Tailored Ranges - Retracted by Unicode
1099 RL3.6 Context Matching - Missing [13]
1100 RL3.7 Incremental Matches - Missing
1101 RL3.8 Unicode Set Sharing - Unicode is proposing
1102 to retract this
1103 RL3.9 Possible Match Sets - Missing
1104 RL3.10 Folded Matching - Retracted by Unicode
1105 RL3.11 Submatchers - Missing
1106 [12] Perl has Unicode::Collate, but it isn't integrated with regular
1108 expressions. See UTS#10 "Unicode Collation Algorithms"
1109 <http://www.unicode.org/reports/tr10>.
1110 [13] Perl has "(?<=x)" and "(?=x)", but lookaheads or lookbehinds
1111 should see outside of the target substring
1112 Unicode Encodings
1114 Unicode characters are assigned to code points, which are abstract
1115 numbers. To use these numbers, various encodings are needed.
1116 · UTF-8
1118 UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
1120 encoding. In most of Perl's documentation, including elsewhere in
1121 this document, the term "UTF-8" means also "UTF-EBCDIC". But in
1122 this section, "UTF-8" refers only to the encoding used on ASCII
1123 platforms. It is a superset of 7-bit US-ASCII, so anything encoded
1124 in ASCII has the identical representation when encoded in UTF-8.
1125 The following table is from Unicode 3.2.
1127 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
1129 U+0000..U+007F 00..7F
1131 U+0080..U+07FF * C2..DF 80..BF
1132 U+0800..U+0FFF E0 * A0..BF 80..BF
1133 U+1000..U+CFFF E1..EC 80..BF 80..BF
1134 U+D000..U+D7FF ED 80..9F 80..BF
1135 U+D800..U+DFFF +++++ utf16 surrogates, not legal utf8 +++++
1136 U+E000..U+FFFF EE..EF 80..BF 80..BF
1137 U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF
1138 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
1139 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
1140 Note the gaps marked by "*" before several of the byte entries
1142 above. These are caused by legal UTF-8 avoiding non-shortest
1143 encodings: it is technically possible to UTF-8-encode a single code
1144 point in different ways, but that is explicitly forbidden, and the
1145 shortest possible encoding should always be used (and that is what
1146 Perl does).
1147 Another way to look at it is via bits:
1149 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
1151 0aaaaaaa 0aaaaaaa
1153 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
1154 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
1155 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
1156 As you can see, the continuation bytes all begin with "10", and the
1158 leading bits of the start byte tell how many bytes there are in the
1159 encoded character.
1160 The original UTF-8 specification allowed up to 6 bytes, to allow
1162 encoding of numbers up to "0x7FFF_FFFF". Perl continues to allow
1163 those, and has extended that up to 13 bytes to encode code points
1164 up to what can fit in a 64-bit word. However, Perl will warn if
1165 you output any of these as being non-portable; and under strict
1166 UTF-8 input protocols, they are forbidden. In addition, it is
1167 deprecated to use a code point larger than what a signed integer
1168 variable on your system can hold. On 32-bit ASCII systems, this
1169 means "0x7FFF_FFFF" is the legal maximum going forward (much higher
1170 on 64-bit systems).
1171 · UTF-EBCDIC
1173 Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
1175 This means that all the basic characters (which includes all those
1176 that have ASCII equivalents (like "A", "0", "%", etc.) are the
1177 same in both EBCDIC and UTF-EBCDIC.)
1178 UTF-EBCDIC is used on EBCDIC platforms. It generally requires more
1180 bytes to represent a given code point than UTF-8 does; the largest
1181 Unicode code points take 5 bytes to represent (instead of 4 in
1182 UTF-8), and, extended for 64-bit words, it uses 14 bytes instead of
1183 13 bytes in UTF-8.
1184 · UTF-16, UTF-16BE, UTF-16LE, Surrogates, and "BOM"'s (Byte Order
1186 Marks)
1187 The followings items are mostly for reference and general Unicode
1189 knowledge, Perl doesn't use these constructs internally.
1190 Like UTF-8, UTF-16 is a variable-width encoding, but where UTF-8
1192 uses 8-bit code units, UTF-16 uses 16-bit code units. All code
1193 points occupy either 2 or 4 bytes in UTF-16: code points
1194 "U+0000..U+FFFF" are stored in a single 16-bit unit, and code
1195 points "U+10000..U+10FFFF" in two 16-bit units. The latter case is
1196 using surrogates, the first 16-bit unit being the high surrogate,
1197 and the second being the low surrogate.
1198 Surrogates are code points set aside to encode the
1200 "U+10000..U+10FFFF" range of Unicode code points in pairs of 16-bit
1201 units. The high surrogates are the range "U+D800..U+DBFF" and the
1202 low surrogates are the range "U+DC00..U+DFFF". The surrogate
1203 encoding is
1204 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1206 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
1207 and the decoding is
1209 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
1211 Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
1213 itself can be used for in-memory computations, but if storage or
1214 transfer is required either UTF-16BE (big-endian) or UTF-16LE
1215 (little-endian) encodings must be chosen.
1216 This introduces another problem: what if you just know that your
1218 data is UTF-16, but you don't know which endianness? Byte Order
1219 Marks, or "BOM"'s, are a solution to this. A special character has
1220 been reserved in Unicode to function as a byte order marker: the
1221 character with the code point "U+FEFF" is the "BOM".
1222 The trick is that if you read a "BOM", you will know the byte
1224 order, since if it was written on a big-endian platform, you will
1225 read the bytes "0xFE 0xFF", but if it was written on a little-
1226 endian platform, you will read the bytes "0xFF 0xFE". (And if the
1227 originating platform was writing in ASCII platform UTF-8, you will
1228 read the bytes "0xEF 0xBB 0xBF".)
1229 The way this trick works is that the character with the code point
1231 "U+FFFE" is not supposed to be in input streams, so the sequence of
1232 bytes "0xFF 0xFE" is unambiguously ""BOM", represented in little-
1233 endian format" and cannot be "U+FFFE", represented in big-endian
1234 format".
1235 Surrogates have no meaning in Unicode outside their use in pairs to
1237 represent other code points. However, Perl allows them to be
1238 represented individually internally, for example by saying
1239 "chr(0xD801)", so that all code points, not just those valid for
1240 open interchange, are representable. Unicode does define semantics
1241 for them, such as their "General_Category" is "Cs". But because
1242 their use is somewhat dangerous, Perl will warn (using the warning
1243 category "surrogate", which is a sub-category of "utf8") if an
1244 attempt is made to do things like take the lower case of one, or
1245 match case-insensitively, or to output them. (But don't try this
1246 on Perls before 5.14.)
1247 · UTF-32, UTF-32BE, UTF-32LE
1249 The UTF-32 family is pretty much like the UTF-16 family, except
1251 that the units are 32-bit, and therefore the surrogate scheme is
1252 not needed. UTF-32 is a fixed-width encoding. The "BOM"
1253 signatures are "0x00 0x00 0xFE 0xFF" for BE and "0xFF 0xFE 0x00
1254 0x00" for LE.
1255 · UCS-2, UCS-4
1257 Legacy, fixed-width encodings defined by the ISO 10646 standard.
1259 UCS-2 is a 16-bit encoding. Unlike UTF-16, UCS-2 is not extensible
1260 beyond "U+FFFF", because it does not use surrogates. UCS-4 is a
1261 32-bit encoding, functionally identical to UTF-32 (the difference
1262 being that UCS-4 forbids neither surrogates nor code points larger
1263 than "0x10_FFFF").
1264 · UTF-7
1266 A seven-bit safe (non-eight-bit) encoding, which is useful if the
1268 transport or storage is not eight-bit safe. Defined by RFC 2152.
1269 Noncharacter code points
1271 66 code points are set aside in Unicode as "noncharacter code points".
1272 These all have the "Unassigned" ("Cn") "General_Category", and no
1273 character will ever be assigned to any of them. They are the 32 code
1274 points between "U+FDD0" and "U+FDEF" inclusive, and the 34 code points:
1275 U+FFFE U+FFFF
1277 U+1FFFE U+1FFFF
1278 U+2FFFE U+2FFFF
1279 ...
1280 U+EFFFE U+EFFFF
1281 U+FFFFE U+FFFFF
1282 U+10FFFE U+10FFFF
1283 Until Unicode 7.0, the noncharacters were "forbidden for use in open
1285 interchange of Unicode text data", so that code that processed those
1286 streams could use these code points as sentinels that could be mixed in
1287 with character data, and would always be distinguishable from that
1288 data. (Emphasis above and in the next paragraph are added in this
1289 document.)
1290 Unicode 7.0 changed the wording so that they are "not recommended for
1292 use in open interchange of Unicode text data". The 7.0 Standard goes
1293 on to say:
1294 "If a noncharacter is received in open interchange, an application
1296 is not required to interpret it in any way. It is good practice,
1297 however, to recognize it as a noncharacter and to take appropriate
1298 action, such as replacing it with "U+FFFD" replacement character,
1299 to indicate the problem in the text. It is not recommended to
1300 simply delete noncharacter code points from such text, because of
1301 the potential security issues caused by deleting uninterpreted
1302 characters. (See conformance clause C7 in Section 3.2, Conformance
1303 Requirements, and Unicode Technical Report #36, "Unicode Security
1304 Considerations"
1305 <http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
1306 This change was made because it was found that various commercial tools
1308 like editors, or for things like source code control, had been written
1309 so that they would not handle program files that used these code
1310 points, effectively precluding their use almost entirely! And that was
1311 never the intent. They've always been meant to be usable within an
1312 application, or cooperating set of applications, at will.
1313 If you're writing code, such as an editor, that is supposed to be able
1315 to handle any Unicode text data, then you shouldn't be using these code
1316 points yourself, and instead allow them in the input. If you need
1317 sentinels, they should instead be something that isn't legal Unicode.
1318 For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
1319 they never appear in well-formed UTF-8. (There are equivalents for
1320 UTF-EBCDIC). You can also store your Unicode code points in integer
1321 variables and use negative values as sentinels.
1322 If you're not writing such a tool, then whether you accept
1324 noncharacters as input is up to you (though the Standard recommends
1325 that you not). If you do strict input stream checking with Perl, these
1326 code points continue to be forbidden. This is to maintain backward
1327 compatibility (otherwise potential security holes could open up, as an
1328 unsuspecting application that was written assuming the noncharacters
1329 would be filtered out before getting to it, could now, without warning,
1330 start getting them). To do strict checking, you can use the layer
1331 ":encoding('UTF-8')".
1332 Perl continues to warn (using the warning category "nonchar", which is
1334 a sub-category of "utf8") if an attempt is made to output
1335 noncharacters.
1336 Beyond Unicode code points
1338 The maximum Unicode code point is "U+10FFFF", and Unicode only defines
1339 operations on code points up through that. But Perl works on code
1340 points up to the maximum permissible unsigned number available on the
1341 platform. However, Perl will not accept these from input streams
1342 unless lax rules are being used, and will warn (using the warning
1343 category "non_unicode", which is a sub-category of "utf8") if any are
1344 output.
1345 Since Unicode rules are not defined on these code points, if a Unicode-
1347 defined operation is done on them, Perl uses what we believe are
1348 sensible rules, while generally warning, using the "non_unicode"
1349 category. For example, "uc("\x{11_0000}")" will generate such a
1350 warning, returning the input parameter as its result, since Perl
1351 defines the uppercase of every non-Unicode code point to be the code
1352 point itself. (All the case changing operations, not just uppercasing,
1353 work this way.)
1354 The situation with matching Unicode properties in regular expressions,
1356 the "\p{}" and "\P{}" constructs, against these code points is not as
1357 clear cut, and how these are handled has changed as we've gained
1358 experience.
1359 One possibility is to treat any match against these code points as
1361 undefined. But since Perl doesn't have the concept of a match being
1362 undefined, it converts this to failing or "FALSE". This is almost, but
1363 not quite, what Perl did from v5.14 (when use of these code points
1364 became generally reliable) through v5.18. The difference is that Perl
1365 treated all "\p{}" matches as failing, but all "\P{}" matches as
1366 succeeding.
1367 One problem with this is that it leads to unexpected, and confusing
1369 results in some cases:
1370 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Failed on <= v5.18
1372 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Failed! on <= v5.18
1373 That is, it treated both matches as undefined, and converted that to
1375 false (raising a warning on each). The first case is the expected
1376 result, but the second is likely counterintuitive: "How could both be
1377 false when they are complements?" Another problem was that the
1378 implementation optimized many Unicode property matches down to already
1379 existing simpler, faster operations, which don't raise the warning. We
1380 chose to not forgo those optimizations, which help the vast majority of
1381 matches, just to generate a warning for the unlikely event that an
1382 above-Unicode code point is being matched against.
1383 As a result of these problems, starting in v5.20, what Perl does is to
1385 treat non-Unicode code points as just typical unassigned Unicode
1386 characters, and matches accordingly. (Note: Unicode has atypical
1387 unassigned code points. For example, it has noncharacter code points,
1388 and ones that, when they do get assigned, are destined to be written
1389 Right-to-left, as Arabic and Hebrew are. Perl assumes that no non-
1390 Unicode code point has any atypical properties.)
1391 Perl, in most cases, will raise a warning when matching an above-
1393 Unicode code point against a Unicode property when the result is "TRUE"
1394 for "\p{}", and "FALSE" for "\P{}". For example:
1395 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails, no warning
1397 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Succeeds, with warning
1398 In both these examples, the character being matched is non-Unicode, so
1400 Unicode doesn't define how it should match. It clearly isn't an ASCII
1401 hex digit, so the first example clearly should fail, and so it does,
1402 with no warning. But it is arguable that the second example should
1403 have an undefined, hence "FALSE", result. So a warning is raised for
1404 it.
1405 Thus the warning is raised for many fewer cases than in earlier Perls,
1407 and only when what the result is could be arguable. It turns out that
1408 none of the optimizations made by Perl (or are ever likely to be made)
1409 cause the warning to be skipped, so it solves both problems of Perl's
1410 earlier approach. The most commonly used property that is affected by
1411 this change is "\p{Unassigned}" which is a short form for
1412 "\p{General_Category=Unassigned}". Starting in v5.20, all non-Unicode
1413 code points are considered "Unassigned". In earlier releases the
1414 matches failed because the result was considered undefined.
1415 The only place where the warning is not raised when it might ought to
1417 have been is if optimizations cause the whole pattern match to not even
1418 be attempted. For example, Perl may figure out that for a string to
1419 match a certain regular expression pattern, the string has to contain
1420 the substring "foobar". Before attempting the match, Perl may look for
1421 that substring, and if not found, immediately fail the match without
1422 actually trying it; so no warning gets generated even if the string
1423 contains an above-Unicode code point.
1424 This behavior is more "Do what I mean" than in earlier Perls for most
1426 applications. But it catches fewer issues for code that needs to be
1427 strictly Unicode compliant. Therefore there is an additional mode of
1428 operation available to accommodate such code. This mode is enabled if
1429 a regular expression pattern is compiled within the lexical scope where
1430 the "non_unicode" warning class has been made fatal, say by:
1431 use warnings FATAL => "non_unicode"
1433 (see warnings). In this mode of operation, Perl will raise the warning
1435 for all matches against a non-Unicode code point (not just the arguable
1436 ones), and it skips the optimizations that might cause the warning to
1437 not be output. (It currently still won't warn if the match isn't even
1438 attempted, like in the "foobar" example above.)
1439 In summary, Perl now normally treats non-Unicode code points as typical
1441 Unicode unassigned code points for regular expression matches, raising
1442 a warning only when it is arguable what the result should be. However,
1443 if this warning has been made fatal, it isn't skipped.
1444 There is one exception to all this. "\p{All}" looks like a Unicode
1446 property, but it is a Perl extension that is defined to be true for all
1447 possible code points, Unicode or not, so no warning is ever generated
1448 when matching this against a non-Unicode code point. (Prior to v5.20,
1449 it was an exact synonym for "\p{Any}", matching code points 0 through
1450 0x10FFFF.)
1451 Security Implications of Unicode
1453 First, read Unicode Security Considerations
1454 <http://www.unicode.org/reports/tr36>.
1455 Also, note the following:
1457 · Malformed UTF-8
1459 UTF-8 is very structured, so many combinations of bytes are
1461 invalid. In the past, Perl tried to soldier on and make some sense
1462 of invalid combinations, but this can lead to security holes, so
1463 now, if the Perl core needs to process an invalid combination, it
1464 will either raise a fatal error, or will replace those bytes by the
1465 sequence that forms the Unicode REPLACEMENT CHARACTER, for which
1466 purpose Unicode created it.
1467 Every code point can be represented by more than one possible
1469 syntactically valid UTF-8 sequence. Early on, both Unicode and
1470 Perl considered any of these to be valid, but now, all sequences
1471 longer than the shortest possible one are considered to be
1472 malformed.
1473 Unicode considers many code points to be illegal, or to be avoided.
1475 Perl generally accepts them, once they have passed through any
1476 input filters that may try to exclude them. These have been
1477 discussed above (see "Surrogates" under UTF-16 in "Unicode
1478 Encodings", "Noncharacter code points", and "Beyond Unicode code
1479 points").
1480 · Regular expression pattern matching may surprise you if you're not
1482 accustomed to Unicode. Starting in Perl 5.14, several pattern
1483 modifiers are available to control this, called the character set
1484 modifiers. Details are given in "Character set modifiers" in
1485 perlre.
1486 As discussed elsewhere, Perl has one foot (two hooves?) planted in each
1488 of two worlds: the old world of ASCII and single-byte locales, and the
1489 new world of Unicode, upgrading when necessary. If your legacy code
1490 does not explicitly use Unicode, no automatic switch-over to Unicode
1491 should happen.
1492 Unicode in Perl on EBCDIC
1494 Unicode is supported on EBCDIC platforms. See perlebcdic.
1495 Unless ASCII vs. EBCDIC issues are specifically being discussed,
1497 references to UTF-8 encoding in this document and elsewhere should be
1498 read as meaning UTF-EBCDIC on EBCDIC platforms. See "Unicode and UTF"
1499 in perlebcdic.
1500 Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
1502 hidden from you; "use utf8" (and NOT something like "use utfebcdic")
1503 declares the script is in the platform's "native" 8-bit encoding of
1504 Unicode. (Similarly for the ":utf8" layer.)
1505 Locales
1507 See "Unicode and UTF-8" in perllocale
1508 When Unicode Does Not Happen
1510 There are still many places where Unicode (in some encoding or another)
1511 could be given as arguments or received as results, or both in Perl,
1512 but it is not, in spite of Perl having extensive ways to input and
1513 output in Unicode, and a few other "entry points" like the @ARGV array
1514 (which can sometimes be interpreted as UTF-8).
1515 The following are such interfaces. Also, see "The "Unicode Bug"". For
1517 all of these interfaces Perl currently (as of v5.16.0) simply assumes
1518 byte strings both as arguments and results, or UTF-8 strings if the
1519 (deprecated) "encoding" pragma has been used.
1520 One reason that Perl does not attempt to resolve the role of Unicode in
1522 these situations is that the answers are highly dependent on the
1523 operating system and the file system(s). For example, whether
1524 filenames can be in Unicode and in exactly what kind of encoding, is
1525 not exactly a portable concept. Similarly for "qx" and "system": how
1526 well will the "command-line interface" (and which of them?) handle
1527 Unicode?
1528 · "chdir", "chmod", "chown", "chroot", "exec", "link", "lstat",
1530 "mkdir", "rename", "rmdir", "stat", "symlink", "truncate",
1531 "unlink", "utime", "-X"
1532 · %ENV
1534 · "glob" (aka the "<*>")
1536 · "open", "opendir", "sysopen"
1538 · "qx" (aka the backtick operator), "system"
1540 · "readdir", "readlink"
1542 The "Unicode Bug"
1544 The term, "Unicode bug" has been applied to an inconsistency with the
1545 code points in the "Latin-1 Supplement" block, that is, between 128 and
1546 255. Without a locale specified, unlike all other characters or code
1547 points, these characters can have very different semantics depending on
1548 the rules in effect. (Characters whose code points are above 255 force
1549 Unicode rules; whereas the rules for ASCII characters are the same
1550 under both ASCII and Unicode rules.)
1551 Under Unicode rules, these upper-Latin1 characters are interpreted as
1553 Unicode code points, which means they have the same semantics as
1554 Latin-1 (ISO-8859-1) and C1 controls.
1555 As explained in "ASCII Rules versus Unicode Rules", under ASCII rules,
1557 they are considered to be unassigned characters.
1558 This can lead to unexpected results. For example, a string's semantics
1560 can suddenly change if a code point above 255 is appended to it, which
1561 changes the rules from ASCII to Unicode. As an example, consider the
1562 following program and its output:
1563 $ perl -le'
1565 no feature "unicode_strings";
1566 $s1 = "\xC2";
1567 $s2 = "\x{2660}";
1568 for ($s1, $s2, $s1.$s2) {
1569 print /\w/ || 0;
1570 }
1571 '
1572 0
1573 0
1574 1
1575 If there's no "\w" in "s1" nor in "s2", why does their concatenation
1577 have one?
1578 This anomaly stems from Perl's attempt to not disturb older programs
1580 that didn't use Unicode, along with Perl's desire to add Unicode
1581 support seamlessly. But the result turned out to not be seamless. (By
1582 the way, you can choose to be warned when things like this happen. See
1583 "encoding::warnings".)
1584 "use feature 'unicode_strings'" was added, starting in Perl v5.12, to
1586 address this problem. It affects these things:
1587 · Changing the case of a scalar, that is, using "uc()", "ucfirst()",
1589 "lc()", and "lcfirst()", or "\L", "\U", "\u" and "\l" in double-
1590 quotish contexts, such as regular expression substitutions.
1591 Under "unicode_strings" starting in Perl 5.12.0, Unicode rules are
1593 generally used. See "lc" in perlfunc for details on how this works
1594 in combination with various other pragmas.
1595 · Using caseless ("/i") regular expression matching.
1597 Starting in Perl 5.14.0, regular expressions compiled within the
1599 scope of "unicode_strings" use Unicode rules even when executed or
1600 compiled into larger regular expressions outside the scope.
1601 · Matching any of several properties in regular expressions.
1603 These properties are "\b" (without braces), "\B" (without braces),
1605 "\s", "\S", "\w", "\W", and all the Posix character classes except
1606 "[[:ascii:]]".
1607 Starting in Perl 5.14.0, regular expressions compiled within the
1609 scope of "unicode_strings" use Unicode rules even when executed or
1610 compiled into larger regular expressions outside the scope.
1611 · In "quotemeta" or its inline equivalent "\Q".
1613 Starting in Perl 5.16.0, consistent quoting rules are used within
1615 the scope of "unicode_strings", as described in "quotemeta" in
1616 perlfunc. Prior to that, or outside its scope, no code points
1617 above 127 are quoted in UTF-8 encoded strings, but in byte encoded
1618 strings, code points between 128-255 are always quoted.
1619 · In the ".." or range operator.
1621 Starting in Perl 5.26.0, the range operator on strings treats their
1623 lengths consistently within the scope of "unicode_strings". Prior
1624 to that, or outside its scope, it could produce strings whose
1625 length in characters exceeded that of the right-hand side, where
1626 the right-hand side took up more bytes than the correct range
1627 endpoint.
1628 · In "split"'s special-case whitespace splitting.
1630 Starting in Perl 5.28.0, the "split" function with a pattern
1632 specified as a string containing a single space handles whitespace
1633 characters consistently within the scope of of "unicode_strings".
1634 Prior to that, or outside its scope, characters that are whitespace
1635 according to Unicode rules but not according to ASCII rules were
1636 treated as field contents rather than field separators when they
1637 appear in byte-encoded strings.
1638 You can see from the above that the effect of "unicode_strings"
1640 increased over several Perl releases. (And Perl's support for Unicode
1641 continues to improve; it's best to use the latest available release in
1642 order to get the most complete and accurate results possible.) Note
1643 that "unicode_strings" is automatically chosen if you "use 5.012" or
1644 higher.
1645 For Perls earlier than those described above, or when a string is
1647 passed to a function outside the scope of "unicode_strings", see the
1648 next section.
1649 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
1651 Sometimes (see "When Unicode Does Not Happen" or "The "Unicode Bug"")
1652 there are situations where you simply need to force a byte string into
1653 UTF-8, or vice versa. The standard module Encode can be used for this,
1654 or the low-level calls "utf8::upgrade($bytestring)" and
1655 "utf8::downgrade($utf8string[, FAIL_OK])".
1656 Note that "utf8::downgrade()" can fail if the string contains
1658 characters that don't fit into a byte.
1659 Calling either function on a string that already is in the desired
1661 state is a no-op.
1662 "ASCII Rules versus Unicode Rules" gives all the ways that a string is
1664 made to use Unicode rules.
1665 Using Unicode in XS
1667 See "Unicode Support" in perlguts for an introduction to Unicode at the
1668 XS level, and "Unicode Support" in perlapi for the API details.
1669 Hacking Perl to work on earlier Unicode versions (for very serious hackers
1671 only)
1672 Perl by default comes with the latest supported Unicode version built-
1673 in, but the goal is to allow you to change to use any earlier one. In
1674 Perls v5.20 and v5.22, however, the earliest usable version is Unicode
1675 5.1. Perl v5.18 and v5.24 are able to handle all earlier versions.
1676 Download the files in the desired version of Unicode from the Unicode
1678 web site <http://www.unicode.org>). These should replace the existing
1679 files in lib/unicore in the Perl source tree. Follow the instructions
1680 in README.perl in that directory to change some of their names, and
1681 then build perl (see INSTALL).
1682 Porting code from perl-5.6.X
1684 Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6
1685 the programmer was required to use the "utf8" pragma to declare that a
1686 given scope expected to deal with Unicode data and had to make sure
1687 that only Unicode data were reaching that scope. If you have code that
1688 is working with 5.6, you will need some of the following adjustments to
1689 your code. The examples are written such that the code will continue to
1690 work under 5.6, so you should be safe to try them out.
1691 · A filehandle that should read or write UTF-8
1693 if ($] > 5.008) {
1695 binmode $fh, ":encoding(UTF-8)";
1696 }
1697 · A scalar that is going to be passed to some extension
1699 Be it "Compress::Zlib", "Apache::Request" or any extension that has
1701 no mention of Unicode in the manpage, you need to make sure that the
1702 UTF8 flag is stripped off. Note that at the time of this writing
1703 (January 2012) the mentioned modules are not UTF-8-aware. Please
1704 check the documentation to verify if this is still true.
1705 if ($] > 5.008) {
1707 require Encode;
1708 $val = Encode::encode("UTF-8", $val); # make octets
1709 }
1710 · A scalar we got back from an extension
1712 If you believe the scalar comes back as UTF-8, you will most likely
1714 want the UTF8 flag restored:
1715 if ($] > 5.008) {
1717 require Encode;
1718 $val = Encode::decode("UTF-8", $val);
1719 }
1720 · Same thing, if you are really sure it is UTF-8
1722 if ($] > 5.008) {
1724 require Encode;
1725 Encode::_utf8_on($val);
1726 }
1727 · A wrapper for DBI "fetchrow_array" and "fetchrow_hashref"
1729 When the database contains only UTF-8, a wrapper function or method
1731 is a convenient way to replace all your "fetchrow_array" and
1732 "fetchrow_hashref" calls. A wrapper function will also make it
1733 easier to adapt to future enhancements in your database driver. Note
1734 that at the time of this writing (January 2012), the DBI has no
1735 standardized way to deal with UTF-8 data. Please check the DBI
1736 documentation to verify if that is still true.
1737 sub fetchrow {
1739 # $what is one of fetchrow_{array,hashref}
1740 my($self, $sth, $what) = @_;
1741 if ($] < 5.008) {
1742 return $sth->$what;
1743 } else {
1744 require Encode;
1745 if (wantarray) {
1746 my @arr = $sth->$what;
1747 for (@arr) {
1748 defined && /[^\000-\177]/ && Encode::_utf8_on($_);
1749 }
1750 return @arr;
1751 } else {
1752 my $ret = $sth->$what;
1753 if (ref $ret) {
1754 for my $k (keys %$ret) {
1755 defined
1756 && /[^\000-\177]/
1757 && Encode::_utf8_on($_) for $ret->{$k};
1758 }
1759 return $ret;
1760 } else {
1761 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
1762 return $ret;
1763 }
1764 }
1765 }
1766 }
1767 · A large scalar that you know can only contain ASCII
1769 Scalars that contain only ASCII and are marked as UTF-8 are
1771 sometimes a drag to your program. If you recognize such a situation,
1772 just remove the UTF8 flag:
1773 utf8::downgrade($val) if $] > 5.008;
1775
1777 See also "The "Unicode Bug"" above.
1778 Interaction with Extensions
1780 When Perl exchanges data with an extension, the extension should be
1781 able to understand the UTF8 flag and act accordingly. If the extension
1782 doesn't recognize that flag, it's likely that the extension will return
1783 incorrectly-flagged data.
1784 So if you're working with Unicode data, consult the documentation of
1786 every module you're using if there are any issues with Unicode data
1787 exchange. If the documentation does not talk about Unicode at all,
1788 suspect the worst and probably look at the source to learn how the
1789 module is implemented. Modules written completely in Perl shouldn't
1790 cause problems. Modules that directly or indirectly access code written
1791 in other programming languages are at risk.
1792 For affected functions, the simple strategy to avoid data corruption is
1794 to always make the encoding of the exchanged data explicit. Choose an
1795 encoding that you know the extension can handle. Convert arguments
1796 passed to the extensions to that encoding and convert results back from
1797 that encoding. Write wrapper functions that do the conversions for you,
1798 so you can later change the functions when the extension catches up.
1799 To provide an example, let's say the popular "Foo::Bar::escape_html"
1801 function doesn't deal with Unicode data yet. The wrapper function would
1802 convert the argument to raw UTF-8 and convert the result back to Perl's
1803 internal representation like so:
1804 sub my_escape_html ($) {
1806 my($what) = shift;
1807 return unless defined $what;
1808 Encode::decode("UTF-8", Foo::Bar::escape_html(
1809 Encode::encode("UTF-8", $what)));
1810 }
1811 Sometimes, when the extension does not convert data but just stores and
1813 retrieves it, you will be able to use the otherwise dangerous
1814 "Encode::_utf8_on()" function. Let's say the popular "Foo::Bar"
1815 extension, written in C, provides a "param" method that lets you store
1816 and retrieve data according to these prototypes:
1817 $self->param($name, $value); # set a scalar
1819 $value = $self->param($name); # retrieve a scalar
1820 If it does not yet provide support for any encoding, one could write a
1822 derived class with such a "param" method:
1823 sub param {
1825 my($self,$name,$value) = @_;
1826 utf8::upgrade($name); # make sure it is UTF-8 encoded
1827 if (defined $value) {
1828 utf8::upgrade($value); # make sure it is UTF-8 encoded
1829 return $self->SUPER::param($name,$value);
1830 } else {
1831 my $ret = $self->SUPER::param($name);
1832 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
1833 return $ret;
1834 }
1835 }
1836 Some extensions provide filters on data entry/exit points, such as
1838 "DB_File::filter_store_key" and family. Look out for such filters in
1839 the documentation of your extensions; they can make the transition to
1840 Unicode data much easier.
1841 Speed
1843 Some functions are slower when working on UTF-8 encoded strings than on
1844 byte encoded strings. All functions that need to hop over characters
1845 such as "length()", "substr()" or "index()", or matching regular
1846 expressions can work much faster when the underlying data are byte-
1847 encoded.
1848 In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a
1850 caching scheme was introduced which improved the situation. In
1851 general, operations with UTF-8 encoded strings are still slower. As an
1852 example, the Unicode properties (character classes) like "\p{Nd}" are
1853 known to be quite a bit slower (5-20 times) than their simpler
1854 counterparts like "[0-9]" (then again, there are hundreds of Unicode
1855 characters matching "Nd" compared with the 10 ASCII characters matching
1856 "[0-9]").
1857
1859 perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes,
1860 perlretut, "${^UNICODE}" in perlvar,
1861 <http://www.unicode.org/reports/tr44>).
1862perl v5.28.2 2018-11-01 PERLUNICODE(1)