3 perlunicode - Unicode support in Perl
7 =head2 Important Caveats
9 Unicode support is an extensive requirement. While Perl does not
10 implement the Unicode standard or the accompanying technical reports
11 from cover to cover, Perl does support many Unicode features.
13 People who want to learn to use Unicode in Perl, should probably read
14 the L<Perl Unicode tutorial, perlunitut|perlunitut>, before reading
15 this reference document.
17 Also, the use of Unicode may present security issues that aren't obvious.
18 Read L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
22 =item Input and Output Layers
24 Perl knows when a filehandle uses Perl's internal Unicode encodings
25 (UTF-8, or UTF-EBCDIC if in EBCDIC) if the filehandle is opened with
26 the ":encoding(utf8)" layer. Other encodings can be converted to Perl's
27 encoding on input or from Perl's encoding on output by use of the
28 ":encoding(...)" layer. See L<open>.
30 To indicate that Perl source itself is in UTF-8, use C<use utf8;>.
32 =item Regular Expressions
34 The regular expression compiler produces polymorphic opcodes. That is,
35 the pattern adapts to the data and automatically switches to the Unicode
36 character scheme when presented with data that is internally encoded in
37 UTF-8, or instead uses a traditional byte scheme when presented with
40 =item C<use utf8> still needed to enable UTF-8/UTF-EBCDIC in scripts
42 As a compatibility measure, the C<use utf8> pragma must be explicitly
43 included to enable recognition of UTF-8 in the Perl scripts themselves
44 (in string or regular expression literals, or in identifier names) on
45 ASCII-based machines or to recognize UTF-EBCDIC on EBCDIC-based
46 machines. B<These are the only times when an explicit C<use utf8>
47 is needed.> See L<utf8>.
49 =item BOM-marked scripts and UTF-16 scripts autodetected
51 If a Perl script begins marked with the Unicode BOM (UTF-16LE, UTF16-BE,
52 or UTF-8), or if the script looks like non-BOM-marked UTF-16 of either
53 endianness, Perl will correctly read in the script as Unicode.
54 (BOMless UTF-8 cannot be effectively recognized or differentiated from
55 ISO 8859-1 or other eight-bit encodings.)
57 =item C<use encoding> needed to upgrade non-Latin-1 byte strings
59 By default, there is a fundamental asymmetry in Perl's Unicode model:
60 implicit upgrading from byte strings to Unicode strings assumes that
61 they were encoded in I<ISO 8859-1 (Latin-1)>, but Unicode strings are
62 downgraded with UTF-8 encoding. This happens because the first 256
63 codepoints in Unicode happens to agree with Latin-1.
65 See L</"Byte and Character Semantics"> for more details.
69 =head2 Byte and Character Semantics
71 Beginning with version 5.6, Perl uses logically-wide characters to
72 represent strings internally.
74 In future, Perl-level operations will be expected to work with
75 characters rather than bytes.
77 However, as an interim compatibility measure, Perl aims to
78 provide a safe migration path from byte semantics to character
79 semantics for programs. For operations where Perl can unambiguously
80 decide that the input data are characters, Perl switches to
81 character semantics. For operations where this determination cannot
82 be made without additional information from the user, Perl decides in
83 favor of compatibility and chooses to use byte semantics.
85 Under byte semantics, when C<use locale> is in effect, Perl uses the
86 semantics associated with the current locale. Absent a C<use locale>, and
87 absent a C<use feature 'unicode_strings'> pragma, Perl currently uses US-ASCII
88 (or Basic Latin in Unicode terminology) byte semantics, meaning that characters
89 whose ordinal numbers are in the range 128 - 255 are undefined except for their
90 ordinal numbers. This means that none have case (upper and lower), nor are any
91 a member of character classes, like C<[:alpha:]> or C<\w>. (But all do belong
92 to the C<\W> class or the Perl regular expression extension C<[:^alpha:]>.)
94 This behavior preserves compatibility with earlier versions of Perl,
95 which allowed byte semantics in Perl operations only if
96 none of the program's inputs were marked as being a source of Unicode
97 character data. Such data may come from filehandles, from calls to
98 external programs, from information provided by the system (such as %ENV),
99 or from literals and constants in the source text.
101 The C<bytes> pragma will always, regardless of platform, force byte
102 semantics in a particular lexical scope. See L<bytes>.
104 The C<use feature 'unicode_strings'> pragma is intended always,
105 regardless of platform, to force character (Unicode) semantics in a
106 particular lexical scope.
107 See L</The "Unicode Bug"> below.
109 The C<utf8> pragma is primarily a compatibility device that enables
110 recognition of UTF-(8|EBCDIC) in literals encountered by the parser.
111 Note that this pragma is only required while Perl defaults to byte
112 semantics; when character semantics become the default, this pragma
113 may become a no-op. See L<utf8>.
115 Unless explicitly stated, Perl operators use character semantics
116 for Unicode data and byte semantics for non-Unicode data.
117 The decision to use character semantics is made transparently. If
118 input data comes from a Unicode source--for example, if a character
119 encoding layer is added to a filehandle or a literal Unicode
120 string constant appears in a program--character semantics apply.
121 Otherwise, byte semantics are in effect. The C<bytes> pragma should
122 be used to force byte semantics on Unicode data, and the C<use feature
123 'unicode_strings'> pragma to force Unicode semantics on byte data (though in
124 5.12 it isn't fully implemented).
126 If strings operating under byte semantics and strings with Unicode
127 character data are concatenated, the new string will have
128 character semantics. This can cause surprises: See L</BUGS>, below.
129 You can choose to be warned when this happens. See L<encoding::warnings>.
131 Under character semantics, many operations that formerly operated on
132 bytes now operate on characters. A character in Perl is
133 logically just a number ranging from 0 to 2**31 or so. Larger
134 characters may encode into longer sequences of bytes internally, but
135 this internal detail is mostly hidden for Perl code.
136 See L<perluniintro> for more.
138 =head2 Effects of Character Semantics
140 Character semantics have the following effects:
146 Strings--including hash keys--and regular expression patterns may
147 contain characters that have an ordinal value larger than 255.
149 If you use a Unicode editor to edit your program, Unicode characters may
150 occur directly within the literal strings in UTF-8 encoding, or UTF-16.
151 (The former requires a BOM or C<use utf8>, the latter requires a BOM.)
153 Unicode characters can also be added to a string by using the C<\N{U+...}>
154 notation. The Unicode code for the desired character, in hexadecimal,
155 should be placed in the braces, after the C<U>. For instance, a smiley face is
158 Alternatively, you can use the C<\x{...}> notation for characters 0x100 and
159 above. For characters below 0x100 you may get byte semantics instead of
160 character semantics; see L</The "Unicode Bug">. On EBCDIC machines there is
161 the additional problem that the value for such characters gives the EBCDIC
162 character rather than the Unicode one.
166 use charnames ':full';
168 you can use the C<\N{...}> notation and put the official Unicode
169 character name within the braces, such as C<\N{WHITE SMILING FACE}>.
174 If an appropriate L<encoding> is specified, identifiers within the
175 Perl script may contain Unicode alphanumeric characters, including
176 ideographs. Perl does not currently attempt to canonicalize variable
181 Regular expressions match characters instead of bytes. "." matches
182 a character instead of a byte.
186 Bracketed character classes in regular expressions match characters instead of
187 bytes and match against the character properties specified in the
188 Unicode properties database. C<\w> can be used to match a Japanese
189 ideograph, for instance.
193 Named Unicode properties, scripts, and block ranges may be used (like bracketed
194 character classes) by using the C<\p{}> "matches property" construct and
195 the C<\P{}> negation, "doesn't match property".
196 See L</"Unicode Character Properties"> for more details.
198 You can define your own character properties and use them
199 in the regular expression with the C<\p{}> or C<\P{}> construct.
200 See L</"User-Defined Character Properties"> for more details.
204 The special pattern C<\X> matches a logical character, an "extended grapheme
205 cluster" in Standardese. In Unicode what appears to the user to be a single
206 character, for example an accented C<G>, may in fact be composed of a sequence
207 of characters, in this case a C<G> followed by an accent character. C<\X>
208 will match the entire sequence.
212 The C<tr///> operator translates characters instead of bytes. Note
213 that the C<tr///CU> functionality has been removed. For similar
214 functionality see pack('U0', ...) and pack('C0', ...).
218 Case translation operators use the Unicode case translation tables
219 when character input is provided. Note that C<uc()>, or C<\U> in
220 interpolated strings, translates to uppercase, while C<ucfirst>,
221 or C<\u> in interpolated strings, translates to titlecase in languages
222 that make the distinction (which is equivalent to uppercase in languages
223 without the distinction).
227 Most operators that deal with positions or lengths in a string will
228 automatically switch to using character positions, including
229 C<chop()>, C<chomp()>, C<substr()>, C<pos()>, C<index()>, C<rindex()>,
230 C<sprintf()>, C<write()>, and C<length()>. An operator that
231 specifically does not switch is C<vec()>. Operators that really don't
232 care include operators that treat strings as a bucket of bits such as
233 C<sort()>, and operators dealing with filenames.
237 The C<pack()>/C<unpack()> letter C<C> does I<not> change, since it is often
238 used for byte-oriented formats. Again, think C<char> in the C language.
240 There is a new C<U> specifier that converts between Unicode characters
241 and code points. There is also a C<W> specifier that is the equivalent of
242 C<chr>/C<ord> and properly handles character values even if they are above 255.
246 The C<chr()> and C<ord()> functions work on characters, similar to
247 C<pack("W")> and C<unpack("W")>, I<not> C<pack("C")> and
248 C<unpack("C")>. C<pack("C")> and C<unpack("C")> are methods for
249 emulating byte-oriented C<chr()> and C<ord()> on Unicode strings.
250 While these methods reveal the internal encoding of Unicode strings,
251 that is not something one normally needs to care about at all.
255 The bit string operators, C<& | ^ ~>, can operate on character data.
256 However, for backward compatibility, such as when using bit string
257 operations when characters are all less than 256 in ordinal value, one
258 should not use C<~> (the bit complement) with characters of both
259 values less than 256 and values greater than 256. Most importantly,
260 DeMorgan's laws (C<~($x|$y) eq ~$x&~$y> and C<~($x&$y) eq ~$x|~$y>)
261 will not hold. The reason for this mathematical I<faux pas> is that
262 the complement cannot return B<both> the 8-bit (byte-wide) bit
263 complement B<and> the full character-wide bit complement.
267 You can define your own mappings to be used in C<lc()>,
268 C<lcfirst()>, C<uc()>, and C<ucfirst()> (or their double-quoted string inlined
269 versions such as C<\U>). See
270 L<User-Defined Case-Mappings|/"User-Defined Case Mappings (for serious hackers only)">
279 And finally, C<scalar reverse()> reverses by character rather than by byte.
283 =head2 Unicode Character Properties
285 Most Unicode character properties are accessible by using regular expressions.
286 They are used (like bracketed character classes) by using the C<\p{}> "matches
287 property" construct and the C<\P{}> negation, "doesn't match property".
289 Note that the only time that Perl considers a sequence of individual code
290 points as a single logical character is in the C<\X> construct, already
291 mentioned above. Therefore "character" in this discussion means a single
294 For instance, C<\p{Uppercase}> matches any single character with the Unicode
295 "Uppercase" property, while C<\p{L}> matches any character with a
296 General_Category of "L" (letter) property. Brackets are not
297 required for single letter property names, so C<\p{L}> is equivalent to C<\pL>.
299 More formally, C<\p{Uppercase}> matches any single character whose Unicode
300 Uppercase property value is True, and C<\P{Uppercase}> matches any character
301 whose Uppercase property value is False, and they could have been written as
302 C<\p{Uppercase=True}> and C<\p{Uppercase=False}>, respectively.
304 This formality is needed when properties are not binary, that is if they can
305 take on more values than just True and False. For example, the Bidi_Class (see
306 L</"Bidirectional Character Types"> below), can take on a number of different
307 values, such as Left, Right, Whitespace, and others. To match these, one needs
308 to specify the property name (Bidi_Class), and the value being matched against
309 (Left, Right, etc.). This is done, as in the examples above, by having the
310 two components separated by an equal sign (or interchangeably, a colon), like
311 C<\p{Bidi_Class: Left}>.
313 All Unicode-defined character properties may be written in these compound forms
314 of C<\p{property=value}> or C<\p{property:value}>, but Perl provides some
315 additional properties that are written only in the single form, as well as
316 single-form short-cuts for all binary properties and certain others described
317 below, in which you may omit the property name and the equals or colon
320 Most Unicode character properties have at least two synonyms (or aliases if you
321 prefer), a short one that is easier to type, and a longer one which is more
322 descriptive and hence it is easier to understand what it means. Thus the "L"
323 and "Letter" above are equivalent and can be used interchangeably. Likewise,
324 "Upper" is a synonym for "Uppercase", and we could have written
325 C<\p{Uppercase}> equivalently as C<\p{Upper}>. Also, there are typically
326 various synonyms for the values the property can be. For binary properties,
327 "True" has 3 synonyms: "T", "Yes", and "Y"; and "False has correspondingly "F",
328 "No", and "N". But be careful. A short form of a value for one property may
329 not mean the same thing as the same short form for another. Thus, for the
330 General_Category property, "L" means "Letter", but for the Bidi_Class property,
331 "L" means "Left". A complete list of properties and synonyms is in
334 Upper/lower case differences in the property names and values are irrelevant,
335 thus C<\p{Upper}> means the same thing as C<\p{upper}> or even C<\p{UpPeR}>.
336 Similarly, you can add or subtract underscores anywhere in the middle of a
337 word, so that these are also equivalent to C<\p{U_p_p_e_r}>. And white space
338 is irrelevant adjacent to non-word characters, such as the braces and the equals
339 or colon separators so C<\p{ Upper }> and C<\p{ Upper_case : Y }> are
340 equivalent to these as well. In fact, in most cases, white space and even
341 hyphens can be added or deleted anywhere. So even C<\p{ Up-per case = Yes}> is
342 equivalent. All this is called "loose-matching" by Unicode. The few places
343 where stricter matching is employed is in the middle of numbers, and the Perl
344 extension properties that begin or end with an underscore. Stricter matching
345 cares about white space (except adjacent to the non-word characters) and
346 hyphens, and non-interior underscores.
348 You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret
349 (^) between the first brace and the property name: C<\p{^Tamil}> is
350 equal to C<\P{Tamil}>.
352 Almost all properties are immune to case-insensitive matching. That is,
353 adding a C</i> regular expression modifier does not change what they
354 match. There are two sets that are affected.
358 and C<Titlecase_Letter>,
359 all of which match C<Cased_Letter> under C</i> matching.
360 And the second set is
364 all of which match C<Cased> under C</i> matching.
365 This set also includes its subsets C<PosixUpper> and C<PosixLower> both
366 of which under C</i> matching match C<PosixAlpha>.
367 (The difference between these sets is that some things, such as Roman
368 Numerals come in both upper and lower case so they are C<Cased>, but aren't considered to be
369 letters, so they aren't C<Cased_Letter>s.)
370 L<perluniprops> includes a notation for all forms that have C</i>
373 =head3 B<General_Category>
375 Every Unicode character is assigned a general category, which is the "most
376 usual categorization of a character" (from
377 L<http://www.unicode.org/reports/tr44>).
379 The compound way of writing these is like C<\p{General_Category=Number}>
380 (short, C<\p{gc:n}>). But Perl furnishes shortcuts in which everything up
381 through the equal or colon separator is omitted. So you can instead just write
384 Here are the short and long forms of the General Category properties:
389 LC, L& Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
402 Nd Decimal_Number (also Digit)
406 P Punctuation (also Punct)
407 Pc Connector_Punctuation
411 Pi Initial_Punctuation
412 (may behave like Ps or Pe depending on usage)
414 (may behave like Ps or Pe depending on usage)
426 Zp Paragraph_Separator
429 Cc Control (also Cntrl)
435 Single-letter properties match all characters in any of the
436 two-letter sub-properties starting with the same letter.
437 C<LC> and C<L&> are special cases, which are both aliases for the set consisting of everything matched by C<Ll>, C<Lu>, and C<Lt>.
439 =head3 B<Bidirectional Character Types>
441 Because scripts differ in their directionality (Hebrew is
442 written right to left, for example) Unicode supplies these properties in
443 the Bidi_Class class:
448 LRE Left-to-Right Embedding
449 LRO Left-to-Right Override
452 RLE Right-to-Left Embedding
453 RLO Right-to-Left Override
454 PDF Pop Directional Format
456 ES European Separator
457 ET European Terminator
462 B Paragraph Separator
467 This property is always written in the compound form.
468 For example, C<\p{Bidi_Class:R}> matches characters that are normally
469 written right to left.
473 The world's languages are written in a number of scripts. This sentence
474 (unless you're reading it in translation) is written in Latin, while Russian is
475 written in Cyrillic, and Greek is written in, well, Greek; Japanese mainly in
476 Hiragana or Katakana. There are many more.
478 The Unicode Script property gives what script a given character is in,
479 and the property can be specified with the compound form like
480 C<\p{Script=Hebrew}> (short: C<\p{sc=hebr}>). Perl furnishes shortcuts for all
481 script names. You can omit everything up through the equals (or colon), and
482 simply write C<\p{Latin}> or C<\P{Cyrillic}>.
484 A complete list of scripts and their shortcuts is in L<perluniprops>.
486 =head3 B<Use of "Is" Prefix>
488 For backward compatibility (with Perl 5.6), all properties mentioned
489 so far may have C<Is> or C<Is_> prepended to their name, so C<\P{Is_Lu}>, for
490 example, is equal to C<\P{Lu}>, and C<\p{IsScript:Arabic}> is equal to
495 In addition to B<scripts>, Unicode also defines B<blocks> of
496 characters. The difference between scripts and blocks is that the
497 concept of scripts is closer to natural languages, while the concept
498 of blocks is more of an artificial grouping based on groups of Unicode
499 characters with consecutive ordinal values. For example, the "Basic Latin"
500 block is all characters whose ordinals are between 0 and 127, inclusive, in
501 other words, the ASCII characters. The "Latin" script contains some letters
502 from this block as well as several more, like "Latin-1 Supplement",
503 "Latin Extended-A", etc., but it does not contain all the characters from
504 those blocks. It does not, for example, contain the digits 0-9, because
505 those digits are shared across many scripts. The digits 0-9 and similar groups,
506 like punctuation, are in the script called C<Common>. There is also a
507 script called C<Inherited> for characters that modify other characters,
508 and inherit the script value of the controlling character. (Note that
509 there are a number of different sets of digits in Unicode that are
510 equivalent to 0-9 and are matchable by C<\d> in a regular expression.
511 If they are used in a single language only, they are in that language's
512 script. Only the sets that are used across languages are in the
515 For more about scripts versus blocks, see UAX#24 "Unicode Script Property":
516 L<http://www.unicode.org/reports/tr24>
518 The Script property is likely to be the one you want to use when processing
519 natural language; the Block property may be useful in working with the nuts and
522 Block names are matched in the compound form, like C<\p{Block: Arrows}> or
523 C<\p{Blk=Hebrew}>. Unlike most other properties only a few block names have a
524 Unicode-defined short name. But Perl does provide a (slight) shortcut: You
525 can say, for example C<\p{In_Arrows}> or C<\p{In_Hebrew}>. For backwards
526 compatibility, the C<In> prefix may be omitted if there is no naming conflict
527 with a script or any other property, and you can even use an C<Is> prefix
528 instead in those cases. But it is not a good idea to do this, for a couple
535 It is confusing. There are many naming conflicts, and you may forget some.
536 For example, C<\p{Hebrew}> means the I<script> Hebrew, and NOT the I<block>
537 Hebrew. But would you remember that 6 months from now?
541 It is unstable. A new version of Unicode may pre-empt the current meaning by
542 creating a property with the same name. There was a time in very early Unicode
543 releases when C<\p{Hebrew}> would have matched the I<block> Hebrew; now it
548 Some people just prefer to always use C<\p{Block: foo}> and C<\p{Script: bar}>
549 instead of the shortcuts, for clarity, and because they can't remember the
550 difference between 'In' and 'Is' anyway (or aren't confident that those who
551 eventually will read their code will know).
553 A complete list of blocks and their shortcuts is in L<perluniprops>.
555 =head3 B<Other Properties>
557 There are many more properties than the very basic ones described here.
558 A complete list is in L<perluniprops>.
560 Unicode defines all its properties in the compound form, so all single-form
561 properties are Perl extensions. A number of these are just synonyms for the
562 Unicode ones, but some are genunine extensions, including a couple that are in
563 the compound form. And quite a few of these are actually recommended by Unicode
564 (in L<http://www.unicode.org/reports/tr18>).
566 This section gives some details on all the extensions that aren't synonyms for
567 compound-form Unicode properties (for those, you'll have to refer to the
568 L<Unicode Standard|http://www.unicode.org/reports/tr44>.
574 This matches any of the 1_114_112 Unicode code points. It is a synonym for
577 =item B<C<\p{Alnum}>>
579 This matches any C<\p{Alphabetic}> or C<\p{Decimal_Number}> character.
583 This matches any of the 1_114_112 Unicode code points. It is a synonym for
586 =item B<C<\p{Assigned}>>
588 This matches any assigned code point; that is, any code point whose general
589 category is not Unassigned (or equivalently, not Cn).
591 =item B<C<\p{Blank}>>
593 This is the same as C<\h> and C<\p{HorizSpace}>: A character that changes the
594 spacing horizontally.
596 =item B<C<\p{Decomposition_Type: Non_Canonical}>> (Short: C<\p{Dt=NonCanon}>)
598 Matches a character that has a non-canonical decomposition.
600 To understand the use of this rarely used property=value combination, it is
601 necessary to know some basics about decomposition.
602 Consider a character, say H. It could appear with various marks around it,
603 such as an acute accent, or a circumflex, or various hooks, circles, arrows,
604 I<etc.>, above, below, to one side and/or the other, etc. There are many
605 possibilities among the world's languages. The number of combinations is
606 astronomical, and if there were a character for each combination, it would
607 soon exhaust Unicode's more than a million possible characters. So Unicode
608 took a different approach: there is a character for the base H, and a
609 character for each of the possible marks, and they can be combined variously
610 to get a final logical character. So a logical character--what appears to be a
611 single character--can be a sequence of more than one individual characters.
612 This is called an "extended grapheme cluster". (Perl furnishes the C<\X>
613 construct to match such sequences.)
615 But Unicode's intent is to unify the existing character set standards and
616 practices, and a number of pre-existing standards have single characters that
617 mean the same thing as some of these combinations. An example is ISO-8859-1,
618 which has quite a few of these in the Latin-1 range, an example being "LATIN
619 CAPITAL LETTER E WITH ACUTE". Because this character was in this pre-existing
620 standard, Unicode added it to its repertoire. But this character is considered
621 by Unicode to be equivalent to the sequence consisting of first the character
622 "LATIN CAPITAL LETTER E", then the character "COMBINING ACUTE ACCENT".
624 "LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed" character, and
625 the equivalence with the sequence is called canonical equivalence. All
626 pre-composed characters are said to have a decomposition (into the equivalent
627 sequence) and the decomposition type is also called canonical.
629 However, many more characters have a different type of decomposition, a
630 "compatible" or "non-canonical" decomposition. The sequences that form these
631 decompositions are not considered canonically equivalent to the pre-composed
632 character. An example, again in the Latin-1 range, is the "SUPERSCRIPT ONE".
633 It is kind of like a regular digit 1, but not exactly; its decomposition
634 into the digit 1 is called a "compatible" decomposition, specifically a
635 "super" decomposition. There are several such compatibility
636 decompositions (see L<http://www.unicode.org/reports/tr44>), including one
637 called "compat" which means some miscellaneous type of decomposition
638 that doesn't fit into the decomposition categories that Unicode has chosen.
640 Note that most Unicode characters don't have a decomposition, so their
641 decomposition type is "None".
643 Perl has added the C<Non_Canonical> type, for your convenience, to mean any of
644 the compatibility decompositions.
646 =item B<C<\p{Graph}>>
648 Matches any character that is graphic. Theoretically, this means a character
649 that on a printer would cause ink to be used.
651 =item B<C<\p{HorizSpace}>>
653 This is the same as C<\h> and C<\p{Blank}>: A character that changes the
654 spacing horizontally.
658 This is a synonym for C<\p{Present_In=*}>
660 =item B<C<\p{PerlSpace}>>
662 This is the same as C<\s>, restricted to ASCII, namely C<S<[ \f\n\r\t]>>.
664 Mnemonic: Perl's (original) space
666 =item B<C<\p{PerlWord}>>
668 This is the same as C<\w>, restricted to ASCII, namely C<[A-Za-z0-9_]>
670 Mnemonic: Perl's (original) word.
672 =item B<C<\p{PosixAlnum}>>
674 This matches any alphanumeric character in the ASCII range, namely
677 =item B<C<\p{PosixAlpha}>>
679 This matches any alphabetic character in the ASCII range, namely C<[A-Za-z]>.
681 =item B<C<\p{PosixBlank}>>
683 This matches any blank character in the ASCII range, namely C<S<[ \t]>>.
685 =item B<C<\p{PosixCntrl}>>
687 This matches any control character in the ASCII range, namely C<[\x00-\x1F\x7F]>
689 =item B<C<\p{PosixDigit}>>
691 This matches any digit character in the ASCII range, namely C<[0-9]>.
693 =item B<C<\p{PosixGraph}>>
695 This matches any graphical character in the ASCII range, namely C<[\x21-\x7E]>.
697 =item B<C<\p{PosixLower}>>
699 This matches any lowercase character in the ASCII range, namely C<[a-z]>.
701 =item B<C<\p{PosixPrint}>>
703 This matches any printable character in the ASCII range, namely C<[\x20-\x7E]>.
704 These are the graphical characters plus SPACE.
706 =item B<C<\p{PosixPunct}>>
708 This matches any punctuation character in the ASCII range, namely
709 C<[\x21-\x2F\x3A-\x40\x5B-\x60\x7B-\x7E]>. These are the
710 graphical characters that aren't word characters. Note that the Posix standard
711 includes in its definition of punctuation, those characters that Unicode calls
714 =item B<C<\p{PosixSpace}>>
716 This matches any space character in the ASCII range, namely
717 C<S<[ \f\n\r\t\x0B]>> (the last being a vertical tab).
719 =item B<C<\p{PosixUpper}>>
721 This matches any uppercase character in the ASCII range, namely C<[A-Z]>.
723 =item B<C<\p{Present_In: *}>> (Short: C<\p{In=*}>)
725 This property is used when you need to know in what Unicode version(s) a
728 The "*" above stands for some two digit Unicode version number, such as
729 C<1.1> or C<4.0>; or the "*" can also be C<Unassigned>. This property will
730 match the code points whose final disposition has been settled as of the
731 Unicode release given by the version number; C<\p{Present_In: Unassigned}>
732 will match those code points whose meaning has yet to be assigned.
734 For example, C<U+0041> "LATIN CAPITAL LETTER A" was present in the very first
735 Unicode release available, which is C<1.1>, so this property is true for all
736 valid "*" versions. On the other hand, C<U+1EFF> was not assigned until version
737 5.1 when it became "LATIN SMALL LETTER Y WITH LOOP", so the only "*" that
738 would match it are 5.1, 5.2, and later.
740 Unicode furnishes the C<Age> property from which this is derived. The problem
741 with Age is that a strict interpretation of it (which Perl takes) has it
742 matching the precise release a code point's meaning is introduced in. Thus
743 C<U+0041> would match only 1.1; and C<U+1EFF> only 5.1. This is not usually what
746 Some non-Perl implementations of the Age property may change its meaning to be
747 the same as the Perl Present_In property; just be aware of that.
749 Another confusion with both these properties is that the definition is not
750 that the code point has been assigned, but that the meaning of the code point
751 has been determined. This is because 66 code points will always be
752 unassigned, and, so the Age for them is the Unicode version the decision to
753 make them so was made in. For example, C<U+FDD0> is to be permanently
754 unassigned to a character, and the decision to do that was made in version 3.1,
755 so C<\p{Age=3.1}> matches this character and C<\p{Present_In: 3.1}> and up
758 =item B<C<\p{Print}>>
760 This matches any character that is graphical or blank, except controls.
762 =item B<C<\p{SpacePerl}>>
764 This is the same as C<\s>, including beyond ASCII.
766 Mnemonic: Space, as modified by Perl. (It doesn't include the vertical tab
767 which both the Posix standard and Unicode consider to be space.)
769 =item B<C<\p{VertSpace}>>
771 This is the same as C<\v>: A character that changes the spacing vertically.
775 This is the same as C<\w>, including beyond ASCII.
779 =head2 User-Defined Character Properties
781 You can define your own binary character properties by defining subroutines
782 whose names begin with "In" or "Is". The subroutines can be defined in any
783 package. The user-defined properties can be used in the regular expression
784 C<\p> and C<\P> constructs; if you are using a user-defined property from a
785 package other than the one you are in, you must specify its package in the
786 C<\p> or C<\P> construct.
788 # assuming property Is_Foreign defined in Lang::
789 package main; # property package name required
790 if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
792 package Lang; # property package name not required
793 if ($txt =~ /\p{IsForeign}+/) { ... }
796 Note that the effect is compile-time and immutable once defined.
797 However the subroutines are passed a single parameter which is 0 if
798 case-sensitive matching is in effect, and non-zero if caseless matching
799 is in effect. The subroutine may return different values depending on
800 the value of the flag, and one set of values will immutably be in effect
801 for all case-sensitive matches; the other set for all case-insensitive
804 The subroutines must return a specially-formatted string, with one
805 or more newline-separated lines. Each line must be one of the following:
811 A single hexadecimal number denoting a Unicode code point to include.
815 Two hexadecimal numbers separated by horizontal whitespace (space or
816 tabular characters) denoting a range of Unicode code points to include.
820 Something to include, prefixed by "+": a built-in character
821 property (prefixed by "utf8::") or a user-defined character property,
822 to represent all the characters in that property; two hexadecimal code
823 points for a range; or a single hexadecimal code point.
827 Something to exclude, prefixed by "-": an existing character
828 property (prefixed by "utf8::") or a user-defined character property,
829 to represent all the characters in that property; two hexadecimal code
830 points for a range; or a single hexadecimal code point.
834 Something to negate, prefixed "!": an existing character
835 property (prefixed by "utf8::") or a user-defined character property,
836 to represent all the characters in that property; two hexadecimal code
837 points for a range; or a single hexadecimal code point.
841 Something to intersect with, prefixed by "&": an existing character
842 property (prefixed by "utf8::") or a user-defined character property,
843 for all the characters except the characters in the property; two
844 hexadecimal code points for a range; or a single hexadecimal code point.
848 For example, to define a property that covers both the Japanese
849 syllabaries (hiragana and katakana), you can define
858 Imagine that the here-doc end marker is at the beginning of the line.
859 Now you can use C<\p{InKana}> and C<\P{InKana}>.
861 You could also have used the existing block property names:
870 Suppose you wanted to match only the allocated characters,
871 not the raw block ranges: in other words, you want to remove
882 The negation is useful for defining (surprise!) negated classes.
892 Intersection is useful for getting the common characters matched by
893 two (or more) classes.
902 It's important to remember not to use "&" for the first set; that
903 would be intersecting with nothing (resulting in an empty set).
905 =head2 User-Defined Case Mappings (for serious hackers only)
907 You can also define your own mappings to be used in C<lc()>,
908 C<lcfirst()>, C<uc()>, and C<ucfirst()> (or their string-inlined versions,
909 C<\L>, C<\l>, C<\U>, and C<\u>). The mappings are currently only valid
910 on strings encoded in UTF-8, but see below for a partial workaround for
913 The principle is similar to that of user-defined character
914 properties: define subroutines that do the mappings.
915 C<ToLower> is used for C<lc()>, C<\L>, C<lcfirst()>, and C<\l>; C<ToTitle> for
916 C<ucfirst()> and C<\u>; and C<ToUpper> for C<uc()> and C<\U>.
918 C<ToUpper()> should look something like this:
927 This sample C<ToUpper()> has the effect of mapping "a-z" to "A-Z", 0x101
928 to 0x100, and all other characters map to themselves. The first
929 returned line means to map the code point at 0x61 ("a") to 0x41 ("A"),
930 the code point at 0x62 ("b") to 0x42 ("B"), ..., 0x7A ("z") to 0x5A
931 ("Z"). The second line maps just the code point 0x101 to 0x100. Since
932 there are no other mappings defined, all other code points map to
935 This mechanism is not well behaved as far as affecting other packages
936 and scopes. All non-threaded programs have exactly one uppercasing
937 behavior, one lowercasing behavior, and one titlecasing behavior in
938 effect for utf8-encoded strings for the duration of the program. Each
939 of these behaviors is irrevocably determined the first time the
940 corresponding function is called to change a utf8-encoded string's case.
941 If a corresponding C<To-> function has been defined in the package that
942 makes that first call, the mapping defined by that function will be the
943 mapping used for the duration of the program's execution across all
944 packages and scopes. If no corresponding C<To-> function has been
945 defined in that package, the standard official mapping will be used for
946 all packages and scopes, and any corresponding C<To-> function anywhere
947 will be ignored. Threaded programs have similar behavior. If the
948 program's casing behavior has been decided at the time of a thread's
949 creation, the thread will inherit that behavior. But, if the behavior
950 hasn't been decided, the thread gets to decide for itself, and its
951 decision does not affect other threads nor its creator.
953 As shown by the example above, you have to furnish a complete mapping;
954 you can't just override a couple of characters and leave the rest
955 unchanged. You can find all the official mappings in the directory
956 C<$Config{privlib}>F</unicore/To/>. The mapping data is returned as the
957 here-document. The C<utf8::ToSpecI<Foo>> hashes in those files are special
958 exception mappings derived from
959 C<$Config{privlib}>F</unicore/SpecialCasing.txt>. (The "Digit" and
960 "Fold" mappings that one can see in the directory are not directly
961 user-accessible, one can use either the L<Unicode::UCD> module, or just match
962 case-insensitively, which is what uses the "Fold" mapping. Neither are user
965 If you have many mappings to change, you can take the official mapping data,
966 change by hand the affected code points, and place the whole thing into your
967 subroutine. But this will only be valid on Perls that use the same Unicode
968 version. Another option would be to have your subroutine read the official
969 mapping file(s) and overwrite the affected code points.
971 If you have only a few mappings to change you can use the
972 following trick (but see below for a big caveat), here illustrated for
976 use charnames ":full";
979 my $official = do "$Config{privlib}/unicore/To/Upper.pl";
980 $utf8::ToSpecUpper{'i'} =
981 "\N{LATIN CAPITAL LETTER I WITH DOT ABOVE}";
985 This takes the official mappings and overrides just one, for "LATIN SMALL
986 LETTER I". Each hash key must be the string of bytes that form the UTF-8
987 (on EBCDIC platforms, UTF-EBCDIC) of the character, as illustrated by
988 the inverse function.
991 my $official = do $lower;
992 $utf8::ToSpecLower{"\xc4\xb0"} = "i";
996 This example is for an ASCII platform, and C<\xc4\xb0> is the string of
997 bytes that together form the UTF-8 that represents C<\N{LATIN CAPITAL
998 LETTER I WITH DOT ABOVE}>, C<U+0130>. You can avoid having to figure out
999 these bytes, and at the same time make it work on all platforms by
1003 my $official = do $lower;
1004 my $sequence = "\N{LATIN CAPITAL LETTER I WITH DOT ABOVE}";
1005 utf8::encode($sequence);
1006 $utf8::ToSpecLower{$sequence} = "i";
1010 This works because C<utf8::encode()> takes the single character and
1011 converts it to the sequence of bytes that constitute it. Note that we took
1012 advantage of the fact that C<"i"> is the same in UTF-8 or UTF_EBCIDIC as not;
1013 otherwise we would have had to write
1015 $utf8::ToSpecLower{$sequence} = "\N{LATIN SMALL LETTER I}";
1017 in the ToLower example, and in the ToUpper example, use
1019 my $sequence = "\N{LATIN SMALL LETTER I}";
1020 utf8::encode($sequence);
1022 A big caveat to the above trick, and to this whole mechanism in general,
1023 is that they work only on strings encoded in UTF-8. You can partially
1024 get around this by using C<use subs>. For example:
1026 use subs qw(uc ucfirst lc lcfirst);
1030 utf8::upgrade($string);
1031 return CORE::uc($string);
1036 utf8::upgrade($string);
1038 # Unless an I is before a dot_above, it turns into a dotless i.
1039 # (The character class with the combining classes matches non-above
1040 # marks following the I. Any number of these may be between the 'I' and
1041 # the dot_above, and the dot_above will still apply to the 'I'.
1042 use charnames ":full";
1045 (?! [^\p{ccc=0}\p{ccc=Above}]* \N{COMBINING DOT ABOVE} )
1046 /\N{LATIN SMALL LETTER DOTLESS I}/gx;
1048 # But when the I is followed by a dot_above, remove the
1049 # dot_above so the end result will be i.
1051 ([^\p{ccc=0}\p{ccc=Above}]* )
1052 \N{COMBINING DOT ABOVE}
1054 return CORE::lc($string);
1057 These examples (also for Turkish) make sure the input is in UTF-8, and then
1058 call the corresponding official function, which will use the C<ToUpper()> and
1059 C<ToLower()> functions you have defined.
1060 (For Turkish, there are other required functions: C<ucfirst>, C<lcfirst>,
1061 and C<ToTitle>. These are very similar to the ones given above.)
1063 The reason this is a partial work-around is that it doesn't affect the C<\l>,
1064 C<\L>, C<\u>, and C<\U> case change operations, which still require the source
1065 to be encoded in utf8 (see L</The "Unicode Bug">).
1067 The C<lc()> example shows how you can add context-dependent casing. Note
1068 that context-dependent casing suffers from the problem that the string
1069 passed to the casing function may not have sufficient context to make
1070 the proper choice. And, it will not be called for C<\l>, C<\L>, C<\u>,
1073 =head2 Character Encodings for Input and Output
1077 =head2 Unicode Regular Expression Support Level
1079 The following list of Unicode support for regular expressions describes
1080 all the features currently supported. The references to "Level N"
1081 and the section numbers refer to the Unicode Technical Standard #18,
1082 "Unicode Regular Expressions", version 11, in May 2005.
1088 Level 1 - Basic Unicode Support
1090 RL1.1 Hex Notation - done [1]
1091 RL1.2 Properties - done [2][3]
1092 RL1.2a Compatibility Properties - done [4]
1093 RL1.3 Subtraction and Intersection - MISSING [5]
1094 RL1.4 Simple Word Boundaries - done [6]
1095 RL1.5 Simple Loose Matches - done [7]
1096 RL1.6 Line Boundaries - MISSING [8]
1097 RL1.7 Supplementary Code Points - done [9]
1101 [3] supports not only minimal list, but all Unicode character
1102 properties (see L</Unicode Character Properties>)
1103 [4] \d \D \s \S \w \W \X [:prop:] [:^prop:]
1104 [5] can use regular expression look-ahead [a] or
1105 user-defined character properties [b] to emulate set
1108 [7] note that Perl does Full case-folding in matching (but with
1109 bugs), not Simple: for example U+1F88 is equivalent to
1110 U+1F00 U+03B9, not with 1F80. This difference matters
1111 mainly for certain Greek capital letters with certain
1112 modifiers: the Full case-folding decomposes the letter,
1113 while the Simple case-folding would map it to a single
1115 [8] should do ^ and $ also on U+000B (\v in C), FF (\f), CR
1116 (\r), CRLF (\r\n), NEL (U+0085), LS (U+2028), and PS
1117 (U+2029); should also affect <>, $., and script line
1118 numbers; should not split lines within CRLF [c] (i.e. there
1119 is no empty line between \r and \n)
1120 [9] UTF-8/UTF-EBDDIC used in perl allows not only U+10000 to
1121 U+10FFFF but also beyond U+10FFFF [d]
1123 [a] You can mimic class subtraction using lookahead.
1124 For example, what UTS#18 might write as
1126 [{Greek}-[{UNASSIGNED}]]
1128 in Perl can be written as:
1130 (?!\p{Unassigned})\p{InGreekAndCoptic}
1131 (?=\p{Assigned})\p{InGreekAndCoptic}
1133 But in this particular example, you probably really want
1137 which will match assigned characters known to be part of the Greek script.
1139 Also see the Unicode::Regex::Set module, it does implement the full
1140 UTS#18 grouping, intersection, union, and removal (subtraction) syntax.
1142 [b] '+' for union, '-' for removal (set-difference), '&' for intersection
1143 (see L</"User-Defined Character Properties">)
1145 [c] Try the C<:crlf> layer (see L<PerlIO>).
1147 [d] U+FFFF will currently generate a warning message if 'utf8' warnings are
1152 Level 2 - Extended Unicode Support
1154 RL2.1 Canonical Equivalents - MISSING [10][11]
1155 RL2.2 Default Grapheme Clusters - MISSING [12]
1156 RL2.3 Default Word Boundaries - MISSING [14]
1157 RL2.4 Default Loose Matches - MISSING [15]
1158 RL2.5 Name Properties - MISSING [16]
1159 RL2.6 Wildcard Properties - MISSING
1161 [10] see UAX#15 "Unicode Normalization Forms"
1162 [11] have Unicode::Normalize but not integrated to regexes
1163 [12] have \X but we don't have a "Grapheme Cluster Mode"
1164 [14] see UAX#29, Word Boundaries
1165 [15] see UAX#21 "Case Mappings"
1166 [16] missing loose match [e]
1168 [e] C<\N{...}> allows namespaces (see L<charnames>).
1172 Level 3 - Tailored Support
1174 RL3.1 Tailored Punctuation - MISSING
1175 RL3.2 Tailored Grapheme Clusters - MISSING [17][18]
1176 RL3.3 Tailored Word Boundaries - MISSING
1177 RL3.4 Tailored Loose Matches - MISSING
1178 RL3.5 Tailored Ranges - MISSING
1179 RL3.6 Context Matching - MISSING [19]
1180 RL3.7 Incremental Matches - MISSING
1181 ( RL3.8 Unicode Set Sharing )
1182 RL3.9 Possible Match Sets - MISSING
1183 RL3.10 Folded Matching - MISSING [20]
1184 RL3.11 Submatchers - MISSING
1186 [17] see UAX#10 "Unicode Collation Algorithms"
1187 [18] have Unicode::Collate but not integrated to regexes
1188 [19] have (?<=x) and (?=x), but look-aheads or look-behinds
1189 should see outside of the target substring
1190 [20] need insensitive matching for linguistic features other
1191 than case; for example, hiragana to katakana, wide and
1192 narrow, simplified Han to traditional Han (see UTR#30
1193 "Character Foldings")
1197 =head2 Unicode Encodings
1199 Unicode characters are assigned to I<code points>, which are abstract
1200 numbers. To use these numbers, various encodings are needed.
1208 UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
1209 encoding. For ASCII (and we really do mean 7-bit ASCII, not another
1210 8-bit encoding), UTF-8 is transparent.
1212 The following table is from Unicode 3.2.
1214 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
1216 U+0000..U+007F 00..7F
1217 U+0080..U+07FF * C2..DF 80..BF
1218 U+0800..U+0FFF E0 * A0..BF 80..BF
1219 U+1000..U+CFFF E1..EC 80..BF 80..BF
1220 U+D000..U+D7FF ED 80..9F 80..BF
1221 U+D800..U+DFFF +++++++ utf16 surrogates, not legal utf8 +++++++
1222 U+E000..U+FFFF EE..EF 80..BF 80..BF
1223 U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF
1224 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
1225 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
1227 Note the gaps before several of the byte entries above marked by '*'. These are
1228 caused by legal UTF-8 avoiding non-shortest encodings: it is technically
1229 possible to UTF-8-encode a single code point in different ways, but that is
1230 explicitly forbidden, and the shortest possible encoding should always be used
1231 (and that is what Perl does).
1233 Another way to look at it is via bits:
1235 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
1238 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
1239 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
1240 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
1242 As you can see, the continuation bytes all begin with "10", and the
1243 leading bits of the start byte tell how many bytes there are in the
1246 The original UTF-8 specification allowed up to 6 bytes, to allow
1247 encoding of numbers up to 0x7FFF_FFFF. Perl continues to allow those,
1248 and has extended that up to 13 bytes to encode code points up to what
1249 can fit in a 64-bit word. However, Perl will warn if you output any of
1250 these, as being non-portable; and under strict UTF-8 input protocols,
1253 The Unicode non-character code points are also disallowed in UTF-8 in
1254 "open interchange". See L</Non-character code points>.
1260 Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
1264 UTF-16, UTF-16BE, UTF-16LE, Surrogates, and BOMs (Byte Order Marks)
1266 The followings items are mostly for reference and general Unicode
1267 knowledge, Perl doesn't use these constructs internally.
1269 UTF-16 is a 2 or 4 byte encoding. The Unicode code points
1270 C<U+0000..U+FFFF> are stored in a single 16-bit unit, and the code
1271 points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is
1272 using I<surrogates>, the first 16-bit unit being the I<high
1273 surrogate>, and the second being the I<low surrogate>.
1275 Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
1276 range of Unicode code points in pairs of 16-bit units. The I<high
1277 surrogates> are the range C<U+D800..U+DBFF> and the I<low surrogates>
1278 are the range C<U+DC00..U+DFFF>. The surrogate encoding is
1280 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1281 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
1285 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
1287 Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
1288 itself can be used for in-memory computations, but if storage or
1289 transfer is required either UTF-16BE (big-endian) or UTF-16LE
1290 (little-endian) encodings must be chosen.
1292 This introduces another problem: what if you just know that your data
1293 is UTF-16, but you don't know which endianness? Byte Order Marks, or
1294 BOMs, are a solution to this. A special character has been reserved
1295 in Unicode to function as a byte order marker: the character with the
1296 code point C<U+FEFF> is the BOM.
1298 The trick is that if you read a BOM, you will know the byte order,
1299 since if it was written on a big-endian platform, you will read the
1300 bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
1301 you will read the bytes C<0xFF 0xFE>. (And if the originating platform
1302 was writing in UTF-8, you will read the bytes C<0xEF 0xBB 0xBF>.)
1304 The way this trick works is that the character with the code point
1305 C<U+FFFE> is not supposed to be in input streams, so the
1306 sequence of bytes C<0xFF 0xFE> is unambiguously "BOM, represented in
1307 little-endian format" and cannot be C<U+FFFE>, represented in big-endian
1310 Surrogates have no meaning in Unicode outside their use in pairs to
1311 represent other code points. However, Perl allows them to be
1312 represented individually internally, for example by saying
1313 C<chr(0xD801)>, so that the all code points, not just Unicode ones, are
1314 representable. Unicode does define semantics for them, such as their
1315 General Category is "Cs". But because their use is somewhat dangerous,
1316 Perl will warn (using the warning category UTF8) if an attempt is made
1317 to do things like take the lower case of one, or match
1318 case-insensitively, or to output them. (But don't try this on Perls
1323 UTF-32, UTF-32BE, UTF-32LE
1325 The UTF-32 family is pretty much like the UTF-16 family, expect that
1326 the units are 32-bit, and therefore the surrogate scheme is not
1327 needed. The BOM signatures will be C<0x00 0x00 0xFE 0xFF> for BE and
1328 C<0xFF 0xFE 0x00 0x00> for LE.
1334 Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
1335 encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
1336 because it does not use surrogates. UCS-4 is a 32-bit encoding,
1337 functionally identical to UTF-32.
1343 A seven-bit safe (non-eight-bit) encoding, which is useful if the
1344 transport or storage is not eight-bit safe. Defined by RFC 2152.
1348 =head2 Non-character code points
1350 66 code points are set aside in Unicode as "non-character code points".
1351 These all have the Unassigned (Cn) General Category, and they never will
1352 be assigned. These are never supposed to be in legal Unicode input
1353 streams, so that code can use them as sentinels that can be mixed in
1354 with character data, and they always will be distinguishable from that data.
1355 To keep them out of Perl input streams, strict UTF-8 should be
1356 specified, such as by using the layer C<:encoding('UTF-8')>. The
1357 non-character code points are the 32 between U+FDD0 and U+FDEF, and the
1358 34 code points U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, ... U+10FFFE, U+10FFFF.
1359 Some people are under the mistaken impression that these are "illegal",
1360 but that is not true. An application or cooperating set of applications
1361 can legally use them at will internally; but these code points are
1362 "illegal for open interchange".
1364 =head2 Security Implications of Unicode
1366 Read L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
1367 Also, note the following:
1375 Unfortunately, the specification of UTF-8 leaves some room for
1376 interpretation of how many bytes of encoded output one should generate
1377 from one input Unicode character. Strictly speaking, the shortest
1378 possible sequence of UTF-8 bytes should be generated,
1379 because otherwise there is potential for an input buffer overflow at
1380 the receiving end of a UTF-8 connection. Perl always generates the
1381 shortest length UTF-8, and with warnings on, Perl will warn about
1382 non-shortest length UTF-8 along with other malformations, such as the
1383 surrogates, which are not real Unicode code points.
1387 Regular expression pattern matching may surprise you if you're not
1388 accustomed to Unicode. Starting in Perl 5.14, there are a number of
1389 modifiers available that control this. For convenience, they will be
1390 referred to in this section using the notation, e.g., C<"/a"> even
1391 though in 5.14, they are not usable in a postfix form after the
1392 (typical) trailing slash of a regular expression. (In 5.14, they are
1393 usable only infix, for example by C</(?a:foo)/>, or by setting them to
1394 apply across a scope by, e.g., C<use re '/a';>. It is planned to lift
1395 this restriction in 5.16.)
1397 The C<"/l"> modifier says that the regular expression should match based
1398 on whatever locale is in effect at execution time. For example, C<\w>
1399 will match the "word" characters of that locale, and C<"/i">
1400 case-insensitive matching will match according to the locale's case
1401 folding rules. See L<perllocale>). C<\d> will likely match just 10
1402 digit characters. This modifier is automatically selected within the
1403 scope of either C<use locale> or C<use re '/l'>.
1405 The C<"/u"> modifier says that the regular expression should match based
1406 on Unicode semantics. C<\w> will match any of the more than 100_000
1407 word characters in Unicode. Unlike most locales, which are specific to
1408 a language and country pair, Unicode classifies all the characters that
1409 are letters I<somewhere> as C<\w>. For example, your locale might not
1410 think that "LATIN SMALL LETTER ETH" is a letter (unless you happen to
1411 speak Icelandic), but Unicode does. Similarly, all the characters that
1412 are decimal digits somewhere in the world will match C<\d>; this is
1413 hundreds, not 10, possible matches. (And some of those digits look like
1414 some of the 10 ASCII digits, but mean a different number, so a human
1415 could easily think a number is a different quantity than it really is.)
1416 Also, case-insensitive matching works on the full set of Unicode
1417 characters. The "KELVIN SIGN", for example matches the letters "k" and
1418 "K"; and "LATIN SMALL LETTER LONG S" (which looks very much like an "f",
1419 and was common in the 18th century but is now obsolete), matches "s" and
1420 "S". This modifier is automatically selected within the scope of either
1421 C<use re '/u'> or C<use feature 'unicode_strings'> (which in turn is
1422 selected by C<use 5.012>.
1424 The C<"/a"> modifier is like the C<"/u"> modifier, except that it
1425 restricts certain constructs to match only in the ASCII range. C<\w>
1426 will match only the 63 characters "[A-Za-z0-9_]"; C<\d>, only the 10
1427 digits 0-9; C<\s>, only the five characters "[ \f\n\r\t]"; and the
1428 C<"[[:posix:]]"> classes only the appropriate ASCII characters. (See
1429 L<perlrecharclass>.) This modifier is like the C<"/u"> modifier in that
1430 things like "KELVIN SIGN" match the letters "k" and "K"; and non-ASCII
1431 characters continue to have Unicode semantics. This modifier is
1432 recommended for people who only incidentally use Unicode. One can write
1433 C<\d> with confidence that it will only match ASCII characters, and
1434 should the need arise to match beyond ASCII, you can use C<\p{Digit}> or
1435 C<\p{Word}>. (See L<perlrecharclass> for how to extend C<\s>, and the
1436 Posix classes beyond ASCII under this modifier.) This modifier is
1437 automatically selected within the scope of C<use re '/a'>.
1439 The C<"/d"> modifier gives the regular expression behavior that Perl has
1440 had between 5.6 and 5.12. For backwards compatibility it is selected
1441 by default, but it leads to a number of issues, as outlined in
1442 L</The "Unicode Bug">. When this modifier is in effect, regular
1443 expression matching uses the semantics of what is called the "C" or
1444 "Posix" locale, unless the pattern or target string of the match is
1445 encoded in UTF-8, in which case it uses Unicode semantics. That is, it
1446 uses what this document calls "byte" semantics unless there is some
1447 UTF-8-ness involved, in which case it uses "character" semantics. Note
1448 that byte semantics are not the same as C<"/a"> matching, as the former
1449 doesn't know about the characters that are in the Latin-1 range which
1450 aren't ASCII (such as "LATIN SMALL LETTER ETH), but C<"/a"> does.
1452 As discussed elsewhere, Perl has one foot (two hooves?) planted in
1453 each of two worlds: the old world of bytes and the new world of
1454 characters, upgrading from bytes to characters when necessary.
1455 If your legacy code does not explicitly use Unicode, no automatic
1456 switch-over to characters should happen. Characters shouldn't get
1457 downgraded to bytes, either. It is possible to accidentally mix bytes
1458 and characters, however (see L<perluniintro>), in which case C<\w> in
1459 regular expressions might start behaving differently. Review your
1460 code. Use warnings and the C<strict> pragma.
1462 There are some additional rules as to which of these modifiers is in
1463 effect if there are contradictory rules present. First, an explicit
1464 modifier in a regular expression always overrides any pragmas. And a
1465 modifier in an inner cluster or capture group overrides one in an outer
1466 group (for that inner group only). If both C<use locale> and C<use
1467 feature 'unicode_strings> are in effect, the C<"/l"> modifier is
1468 selected. And finally, a C<use re> that specifies a modifier has
1469 precedence over both those pragmas.
1473 =head2 Unicode in Perl on EBCDIC
1475 The way Unicode is handled on EBCDIC platforms is still
1476 experimental. On such platforms, references to UTF-8 encoding in this
1477 document and elsewhere should be read as meaning the UTF-EBCDIC
1478 specified in Unicode Technical Report 16, unless ASCII vs. EBCDIC issues
1479 are specifically discussed. There is no C<utfebcdic> pragma or
1480 ":utfebcdic" layer; rather, "utf8" and ":utf8" are reused to mean
1481 the platform's "natural" 8-bit encoding of Unicode. See L<perlebcdic>
1482 for more discussion of the issues.
1486 Usually locale settings and Unicode do not affect each other, but
1487 there are exceptions:
1493 You can enable automatic UTF-8-ification of your standard file
1494 handles, default C<open()> layer, and C<@ARGV> by using either
1495 the C<-C> command line switch or the C<PERL_UNICODE> environment
1496 variable, see L<perlrun> for the documentation of the C<-C> switch.
1500 Perl tries really hard to work both with Unicode and the old
1501 byte-oriented world. Most often this is nice, but sometimes Perl's
1502 straddling of the proverbial fence causes problems. Here's an example
1503 of how things can go wrong. A locale can define a code point to be
1504 anything it wants. It could make 'A' into a control character, for example.
1505 But strings encoded in utf8 always have Unicode semantics, so an 'A' in
1506 such a string is always an uppercase letter, never a control, no matter
1507 what the locale says it should be.
1511 =head2 When Unicode Does Not Happen
1513 While Perl does have extensive ways to input and output in Unicode,
1514 and few other 'entry points' like the @ARGV which can be interpreted
1515 as Unicode (UTF-8), there still are many places where Unicode (in some
1516 encoding or another) could be given as arguments or received as
1517 results, or both, but it is not.
1519 The following are such interfaces. Also, see L</The "Unicode Bug">.
1520 For all of these interfaces Perl
1521 currently (as of 5.8.3) simply assumes byte strings both as arguments
1522 and results, or UTF-8 strings if the C<encoding> pragma has been used.
1524 One reason why Perl does not attempt to resolve the role of Unicode in
1525 these cases is that the answers are highly dependent on the operating
1526 system and the file system(s). For example, whether filenames can be
1527 in Unicode, and in exactly what kind of encoding, is not exactly a
1528 portable concept. Similarly for the qx and system: how well will the
1529 'command line interface' (and which of them?) handle Unicode?
1535 chdir, chmod, chown, chroot, exec, link, lstat, mkdir,
1536 rename, rmdir, stat, symlink, truncate, unlink, utime, -X
1548 open, opendir, sysopen
1552 qx (aka the backtick operator), system
1560 =head2 The "Unicode Bug"
1562 The term, the "Unicode bug" has been applied to an inconsistency with the
1563 Unicode characters whose ordinals are in the Latin-1 Supplement block, that
1564 is, between 128 and 255. Without a locale specified, unlike all other
1565 characters or code points, these characters have very different semantics in
1566 byte semantics versus character semantics, unless
1567 C<use feature 'unicode_strings'> is specified.
1569 In character semantics they are interpreted as Unicode code points, which means
1570 they have the same semantics as Latin-1 (ISO-8859-1).
1572 In byte semantics, they are considered to be unassigned characters, meaning
1573 that the only semantics they have is their ordinal numbers, and that they are
1574 not members of various character classes. None are considered to match C<\w>
1575 for example, but all match C<\W>. (On EBCDIC platforms, the behavior may
1576 be different from this, depending on the underlying C language library
1579 The behavior is known to have effects on these areas:
1585 Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
1586 and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in regular expression
1591 Using caseless (C</i>) regular expression matching
1595 Matching a number of properties in regular expressions, namely C<\b>,
1596 C<\B>, C<\s>, C<\S>, C<\w>, C<\W>, and all the Posix character classes
1597 I<except> C<[[:ascii:]]>.
1601 User-defined case change mappings. You can create a C<ToUpper()> function, for
1602 example, which overrides Perl's built-in case mappings. The scalar must be
1603 encoded in utf8 for your function to actually be invoked.
1607 This behavior can lead to unexpected results in which a string's semantics
1608 suddenly change if a code point above 255 is appended to or removed from it,
1609 which changes the string's semantics from byte to character or vice versa. As
1610 an example, consider the following program and its output:
1615 for ($s1, $s2, $s1.$s2) {
1623 If there's no C<\w> in C<s1> or in C<s2>, why does their concatenation have one?
1625 This anomaly stems from Perl's attempt to not disturb older programs that
1626 didn't use Unicode, and hence had no semantics for characters outside of the
1627 ASCII range (except in a locale), along with Perl's desire to add Unicode
1628 support seamlessly. The result wasn't seamless: these characters were
1631 Starting in Perl 5.14, C<use feature 'unicode_strings'> can be used to
1632 cause Perl to use Unicode semantics on all string operations within the
1633 scope of the feature subpragma. Regular expressions compiled in its
1634 scope retain that behavior even when executed or compiled into larger
1635 regular expressions outside the scope. (The pragma does not, however,
1636 affect user-defined case changing operations. These still require a
1637 UTF-8 encoded string to operate.)
1639 In Perl 5.12, the subpragma affected casing changes, but not regular
1640 expressions. See L<perlfunc/lc> for details on how this pragma works in
1641 combination with various others for casing.
1643 For earlier Perls, or when a string is passed to a function outside the
1644 subpragma's scope, a workaround is to always call C<utf8::upgrade($string)>,
1645 or to use the standard module L<Encode>. Also, a scalar that has any characters
1646 whose ordinal is above 0x100, or which were specified using either of the
1647 C<\N{...}> notations will automatically have character semantics.
1649 =head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
1651 Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
1652 there are situations where you simply need to force a byte
1653 string into UTF-8, or vice versa. The low-level calls
1654 utf8::upgrade($bytestring) and utf8::downgrade($utf8string[, FAIL_OK]) are
1657 Note that utf8::downgrade() can fail if the string contains characters
1658 that don't fit into a byte.
1660 Calling either function on a string that already is in the desired state is a
1663 =head2 Using Unicode in XS
1665 If you want to handle Perl Unicode in XS extensions, you may find the
1666 following C APIs useful. See also L<perlguts/"Unicode Support"> for an
1667 explanation about Unicode at the XS level, and L<perlapi> for the API
1674 C<DO_UTF8(sv)> returns true if the C<UTF8> flag is on and the bytes
1675 pragma is not in effect. C<SvUTF8(sv)> returns true if the C<UTF8>
1676 flag is on; the bytes pragma is ignored. The C<UTF8> flag being on
1677 does B<not> mean that there are any characters of code points greater
1678 than 255 (or 127) in the scalar or that there are even any characters
1679 in the scalar. What the C<UTF8> flag means is that the sequence of
1680 octets in the representation of the scalar is the sequence of UTF-8
1681 encoded code points of the characters of a string. The C<UTF8> flag
1682 being off means that each octet in this representation encodes a
1683 single character with code point 0..255 within the string. Perl's
1684 Unicode model is not to use UTF-8 until it is absolutely necessary.
1688 C<uvchr_to_utf8(buf, chr)> writes a Unicode character code point into
1689 a buffer encoding the code point as UTF-8, and returns a pointer
1690 pointing after the UTF-8 bytes. It works appropriately on EBCDIC machines.
1694 C<utf8_to_uvchr(buf, lenp)> reads UTF-8 encoded bytes from a buffer and
1695 returns the Unicode character code point and, optionally, the length of
1696 the UTF-8 byte sequence. It works appropriately on EBCDIC machines.
1700 C<utf8_length(start, end)> returns the length of the UTF-8 encoded buffer
1701 in characters. C<sv_len_utf8(sv)> returns the length of the UTF-8 encoded
1706 C<sv_utf8_upgrade(sv)> converts the string of the scalar to its UTF-8
1707 encoded form. C<sv_utf8_downgrade(sv)> does the opposite, if
1708 possible. C<sv_utf8_encode(sv)> is like sv_utf8_upgrade except that
1709 it does not set the C<UTF8> flag. C<sv_utf8_decode()> does the
1710 opposite of C<sv_utf8_encode()>. Note that none of these are to be
1711 used as general-purpose encoding or decoding interfaces: C<use Encode>
1712 for that. C<sv_utf8_upgrade()> is affected by the encoding pragma
1713 but C<sv_utf8_downgrade()> is not (since the encoding pragma is
1714 designed to be a one-way street).
1718 C<is_utf8_char(s)> returns true if the pointer points to a valid UTF-8
1723 C<is_utf8_string(buf, len)> returns true if C<len> bytes of the buffer
1728 C<UTF8SKIP(buf)> will return the number of bytes in the UTF-8 encoded
1729 character in the buffer. C<UNISKIP(chr)> will return the number of bytes
1730 required to UTF-8-encode the Unicode character code point. C<UTF8SKIP()>
1731 is useful for example for iterating over the characters of a UTF-8
1732 encoded buffer; C<UNISKIP()> is useful, for example, in computing
1733 the size required for a UTF-8 encoded buffer.
1737 C<utf8_distance(a, b)> will tell the distance in characters between the
1738 two pointers pointing to the same UTF-8 encoded buffer.
1742 C<utf8_hop(s, off)> will return a pointer to a UTF-8 encoded buffer
1743 that is C<off> (positive or negative) Unicode characters displaced
1744 from the UTF-8 buffer C<s>. Be careful not to overstep the buffer:
1745 C<utf8_hop()> will merrily run off the end or the beginning of the
1746 buffer if told to do so.
1750 C<pv_uni_display(dsv, spv, len, pvlim, flags)> and
1751 C<sv_uni_display(dsv, ssv, pvlim, flags)> are useful for debugging the
1752 output of Unicode strings and scalars. By default they are useful
1753 only for debugging--they display B<all> characters as hexadecimal code
1754 points--but with the flags C<UNI_DISPLAY_ISPRINT>,
1755 C<UNI_DISPLAY_BACKSLASH>, and C<UNI_DISPLAY_QQ> you can make the
1756 output more readable.
1760 C<foldEQ_utf8(s1, pe1, l1, u1, s2, pe2, l2, u2)> can be used to
1761 compare two strings case-insensitively in Unicode. For case-sensitive
1762 comparisons you can just use C<memEQ()> and C<memNE()> as usual, except
1763 if one string is in utf8 and the other isn't.
1767 For more information, see L<perlapi>, and F<utf8.c> and F<utf8.h>
1768 in the Perl source code distribution.
1770 =head2 Hacking Perl to work on earlier Unicode versions (for very serious hackers only)
1772 Perl by default comes with the latest supported Unicode version built in, but
1773 you can change to use any earlier one.
1775 Download the files in the version of Unicode that you want from the Unicode web
1776 site L<http://www.unicode.org>). These should replace the existing files in
1777 C<\$Config{privlib}>/F<unicore>. (C<\%Config> is available from the Config
1778 module.) Follow the instructions in F<README.perl> in that directory to change
1779 some of their names, and then run F<make>.
1781 It is even possible to download them to a different directory, and then change
1782 F<utf8_heavy.pl> in the directory C<\$Config{privlib}> to point to the new
1783 directory, or maybe make a copy of that directory before making the change, and
1784 using C<@INC> or the C<-I> run-time flag to switch between versions at will
1785 (but because of caching, not in the middle of a process), but all this is
1786 beyond the scope of these instructions.
1790 =head2 Interaction with Locales
1792 Use of locales with Unicode data may lead to odd results. Currently,
1793 Perl attempts to attach 8-bit locale info to characters in the range
1794 0..255, but this technique is demonstrably incorrect for locales that
1795 use characters above that range when mapped into Unicode. Perl's
1796 Unicode support will also tend to run slower. Use of locales with
1797 Unicode is discouraged.
1799 =head2 Problems with characters in the Latin-1 Supplement range
1801 See L</The "Unicode Bug">
1803 =head2 Problems with case-insensitive regular expression matching
1805 There are problems with case-insensitive matches, including those involving
1806 character classes (enclosed in [square brackets]), characters whose fold
1807 is to multiple characters (such as the single character LATIN SMALL LIGATURE
1808 FFL matches case-insensitively with the 3-character string C<ffl>), and
1809 characters in the Latin-1 Supplement.
1811 =head2 Interaction with Extensions
1813 When Perl exchanges data with an extension, the extension should be
1814 able to understand the UTF8 flag and act accordingly. If the
1815 extension doesn't know about the flag, it's likely that the extension
1816 will return incorrectly-flagged data.
1818 So if you're working with Unicode data, consult the documentation of
1819 every module you're using if there are any issues with Unicode data
1820 exchange. If the documentation does not talk about Unicode at all,
1821 suspect the worst and probably look at the source to learn how the
1822 module is implemented. Modules written completely in Perl shouldn't
1823 cause problems. Modules that directly or indirectly access code written
1824 in other programming languages are at risk.
1826 For affected functions, the simple strategy to avoid data corruption is
1827 to always make the encoding of the exchanged data explicit. Choose an
1828 encoding that you know the extension can handle. Convert arguments passed
1829 to the extensions to that encoding and convert results back from that
1830 encoding. Write wrapper functions that do the conversions for you, so
1831 you can later change the functions when the extension catches up.
1833 To provide an example, let's say the popular Foo::Bar::escape_html
1834 function doesn't deal with Unicode data yet. The wrapper function
1835 would convert the argument to raw UTF-8 and convert the result back to
1836 Perl's internal representation like so:
1838 sub my_escape_html ($) {
1840 return unless defined $what;
1841 Encode::decode_utf8(Foo::Bar::escape_html(
1842 Encode::encode_utf8($what)));
1845 Sometimes, when the extension does not convert data but just stores
1846 and retrieves them, you will be in a position to use the otherwise
1847 dangerous Encode::_utf8_on() function. Let's say the popular
1848 C<Foo::Bar> extension, written in C, provides a C<param> method that
1849 lets you store and retrieve data according to these prototypes:
1851 $self->param($name, $value); # set a scalar
1852 $value = $self->param($name); # retrieve a scalar
1854 If it does not yet provide support for any encoding, one could write a
1855 derived class with such a C<param> method:
1858 my($self,$name,$value) = @_;
1859 utf8::upgrade($name); # make sure it is UTF-8 encoded
1860 if (defined $value) {
1861 utf8::upgrade($value); # make sure it is UTF-8 encoded
1862 return $self->SUPER::param($name,$value);
1864 my $ret = $self->SUPER::param($name);
1865 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
1870 Some extensions provide filters on data entry/exit points, such as
1871 DB_File::filter_store_key and family. Look out for such filters in
1872 the documentation of your extensions, they can make the transition to
1873 Unicode data much easier.
1877 Some functions are slower when working on UTF-8 encoded strings than
1878 on byte encoded strings. All functions that need to hop over
1879 characters such as length(), substr() or index(), or matching regular
1880 expressions can work B<much> faster when the underlying data are
1883 In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
1884 a caching scheme was introduced which will hopefully make the slowness
1885 somewhat less spectacular, at least for some operations. In general,
1886 operations with UTF-8 encoded strings are still slower. As an example,
1887 the Unicode properties (character classes) like C<\p{Nd}> are known to
1888 be quite a bit slower (5-20 times) than their simpler counterparts
1889 like C<\d> (then again, there 268 Unicode characters matching C<Nd>
1890 compared with the 10 ASCII characters matching C<d>).
1892 =head2 Problems on EBCDIC platforms
1894 There are a number of known problems with Perl on EBCDIC platforms. If you
1895 want to use Perl there, send email to perlbug@perl.org.
1897 In earlier versions, when byte and character data were concatenated,
1898 the new string was sometimes created by
1899 decoding the byte strings as I<ISO 8859-1 (Latin-1)>, even if the
1900 old Unicode string used EBCDIC.
1902 If you find any of these, please report them as bugs.
1904 =head2 Porting code from perl-5.6.X
1906 Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer
1907 was required to use the C<utf8> pragma to declare that a given scope
1908 expected to deal with Unicode data and had to make sure that only
1909 Unicode data were reaching that scope. If you have code that is
1910 working with 5.6, you will need some of the following adjustments to
1911 your code. The examples are written such that the code will continue
1912 to work under 5.6, so you should be safe to try them out.
1918 A filehandle that should read or write UTF-8
1921 binmode $fh, ":encoding(utf8)";
1926 A scalar that is going to be passed to some extension
1928 Be it Compress::Zlib, Apache::Request or any extension that has no
1929 mention of Unicode in the manpage, you need to make sure that the
1930 UTF8 flag is stripped off. Note that at the time of this writing
1931 (October 2002) the mentioned modules are not UTF-8-aware. Please
1932 check the documentation to verify if this is still true.
1936 $val = Encode::encode_utf8($val); # make octets
1941 A scalar we got back from an extension
1943 If you believe the scalar comes back as UTF-8, you will most likely
1944 want the UTF8 flag restored:
1948 $val = Encode::decode_utf8($val);
1953 Same thing, if you are really sure it is UTF-8
1957 Encode::_utf8_on($val);
1962 A wrapper for fetchrow_array and fetchrow_hashref
1964 When the database contains only UTF-8, a wrapper function or method is
1965 a convenient way to replace all your fetchrow_array and
1966 fetchrow_hashref calls. A wrapper function will also make it easier to
1967 adapt to future enhancements in your database driver. Note that at the
1968 time of this writing (October 2002), the DBI has no standardized way
1969 to deal with UTF-8 data. Please check the documentation to verify if
1973 # $what is one of fetchrow_{array,hashref}
1974 my($self, $sth, $what) = @_;
1980 my @arr = $sth->$what;
1982 defined && /[^\000-\177]/ && Encode::_utf8_on($_);
1986 my $ret = $sth->$what;
1988 for my $k (keys %$ret) {
1991 && Encode::_utf8_on($_) for $ret->{$k};
1995 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
2005 A large scalar that you know can only contain ASCII
2007 Scalars that contain only ASCII and are marked as UTF-8 are sometimes
2008 a drag to your program. If you recognize such a situation, just remove
2011 utf8::downgrade($val) if $] > 5.007;
2017 L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>,
2018 L<perlretut>, L<perlvar/"${^UNICODE}">
2019 L<http://www.unicode.org/reports/tr44>).