1PERLUNICODE(1)         Perl Programmers Reference Guide         PERLUNICODE(1)
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NAME

6       perlunicode - Unicode support in Perl
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DESCRIPTION

9       If you haven't already, before reading this document, you should become
10       familiar with both perlunitut and perluniintro.
11
12       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
22       And it worked; nowadays, those legacy standards are rarely used.  Most
23       everyone uses Unicode.
24
25       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       <https://www.unicode.org>.
29
30   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
35       Unicode support is an extensive requirement. While Perl does not
36       implement the Unicode standard or the accompanying technical reports
37       from cover to cover, Perl does support many Unicode features.
38
39       Also, the use of Unicode may present security issues that aren't
40       obvious, see "Security Implications of Unicode" below.
41
42       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
50           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
55       Input and Output Layers
56           Use the ":encoding(...)" layer  to read from and write to
57           filehandles using the specified encoding.  (See open.)
58
59       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
63       "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
70           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
74       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
80   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
87       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
94       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
99       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
103       There are various things to note:
104
105       ·   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
113           The exceptions are:
114
115           ·   the bit-oriented "vec"
116
117
118
119           ·   the byte-oriented "pack"/"unpack" "C" format
120
121               However, the "W" specifier does operate on whole characters, as
122               does the "U" specifier.
123
124           ·   some operators that interact with the platform's operating
125               system
126
127               Operators dealing with filenames are examples.
128
129           ·   when the functions are called from within the scope of the
130               "use bytes" pragma
131
132               Likely, you should use this only for debugging anyway.
133
134       ·   Strings--including hash keys--and regular expression patterns may
135           contain characters that have ordinal values larger than 255.
136
137           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
142           "Creating Unicode" in perluniintro gives other ways to place non-
143           ASCII characters in your strings.
144
145       ·   The "chr()" and "ord()" functions work on whole characters.
146
147       ·   Regular expressions match whole characters.  For example, "."
148           matches a whole character instead of only a single byte.
149
150       ·   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
155       ·   "scalar reverse()" reverses by character rather than by byte.
156
157       ·   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
166       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
170   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
179       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
185       Here are the ways that Perl knows that a string should be treated as
186       Unicode:
187
188       ·   Within the scope of "use utf8"
189
190           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
194       ·   Within the scope of "use feature 'unicode_strings'"
195
196           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
201       ·   Within the scope of "use 5.012" or higher
202
203           This implicitly turns on "use feature 'unicode_strings'".
204
205       ·   Within the scope of "use locale 'not_characters'", or "use locale"
206           and the current locale is a UTF-8 locale.
207
208           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
212       ·   When the string contains a Unicode-only code point
213
214           Perl has never accepted code points above 255 without them being
215           Unicode, so their use implies Unicode for the whole string.
216
217       ·   When the string contains a Unicode named code point "\N{...}"
218
219           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
223       ·   When the string has come from an external source marked as Unicode
224
225           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
229       ·   When the string has been upgraded to UTF-8
230
231           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
235       ·   There are additional methods for regular expression patterns
236
237           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
242       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
245       Note also that some interactions with the platform's operating system
246       never use Unicode rules.
247
248       When Unicode rules are in effect:
249
250       ·   Case translation operators use the Unicode case translation tables.
251
252           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
258           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
266       ·   Character classes in regular expressions match based on the
267           character properties specified in the Unicode properties database.
268
269           "\w" can be used to match a Japanese ideograph, for instance; and
270           "[[:digit:]]" a Bengali number.
271
272       ·   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
277           See "Unicode Character Properties" for more details.
278
279           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
283   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
300       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
310       "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
323       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
328       For more detailed information, see <http://unicode.org/reports/tr15/>.
329
330   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
336       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
340       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
346       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
352       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
362       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
369       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 short form spelled the same for another.  Thus, for
380       the "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
384       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 generally irrelevant adjacent to
389       non-word characters, such as the braces and the equals or colon
390       separators, so "\p{   Upper  }" and "\p{ Upper_case : Y }" are
391       equivalent to these as well.  In fact, white space and even hyphens can
392       usually be added or deleted anywhere.  So even "\p{ Up-per case = Yes}"
393       is equivalent.  All this is called "loose-matching" by Unicode.  The
394       "name" property has some restrictions on this due to a few outlier
395       names.  Full details are given in
396       <https://www.unicode.org/reports/tr44/tr44-24.html#UAX44-LM2>.
397
398       The few places where stricter matching is used is in the middle of
399       numbers, the "name" property, and in the Perl extension properties that
400       begin or end with an underscore.  Stricter matching cares about white
401       space (except adjacent to non-word characters), hyphens, and non-
402       interior underscores.
403
404       You can also use negation in both "\p{}" and "\P{}" by introducing a
405       caret ("^") between the first brace and the property name: "\p{^Tamil}"
406       is equal to "\P{Tamil}".
407
408       Almost all properties are immune to case-insensitive matching.  That
409       is, adding a "/i" regular expression modifier does not change what they
410       match.  There are two sets that are affected.  The first set is
411       "Uppercase_Letter", "Lowercase_Letter", and "Titlecase_Letter", all of
412       which match "Cased_Letter" under "/i" matching.  And the second set is
413       "Uppercase", "Lowercase", and "Titlecase", all of which match "Cased"
414       under "/i" matching.  This set also includes its subsets "PosixUpper"
415       and "PosixLower" both of which under "/i" match "PosixAlpha".  (The
416       difference between these sets is that some things, such as Roman
417       numerals, come in both upper and lower case so they are "Cased", but
418       aren't considered letters, so they aren't "Cased_Letter"'s.)
419
420       See "Beyond Unicode code points" for special considerations when
421       matching Unicode properties against non-Unicode code points.
422
423       General_Category
424
425       Every Unicode character is assigned a general category, which is the
426       "most usual categorization of a character" (from
427       <https://www.unicode.org/reports/tr44>).
428
429       The compound way of writing these is like "\p{General_Category=Number}"
430       (short: "\p{gc:n}").  But Perl furnishes shortcuts in which everything
431       up through the equal or colon separator is omitted.  So you can instead
432       just write "\pN".
433
434       Here are the short and long forms of the values the "General Category"
435       property can have:
436
437           Short       Long
438
439           L           Letter
440           LC, L&      Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
441           Lu          Uppercase_Letter
442           Ll          Lowercase_Letter
443           Lt          Titlecase_Letter
444           Lm          Modifier_Letter
445           Lo          Other_Letter
446
447           M           Mark
448           Mn          Nonspacing_Mark
449           Mc          Spacing_Mark
450           Me          Enclosing_Mark
451
452           N           Number
453           Nd          Decimal_Number (also Digit)
454           Nl          Letter_Number
455           No          Other_Number
456
457           P           Punctuation (also Punct)
458           Pc          Connector_Punctuation
459           Pd          Dash_Punctuation
460           Ps          Open_Punctuation
461           Pe          Close_Punctuation
462           Pi          Initial_Punctuation
463                       (may behave like Ps or Pe depending on usage)
464           Pf          Final_Punctuation
465                       (may behave like Ps or Pe depending on usage)
466           Po          Other_Punctuation
467
468           S           Symbol
469           Sm          Math_Symbol
470           Sc          Currency_Symbol
471           Sk          Modifier_Symbol
472           So          Other_Symbol
473
474           Z           Separator
475           Zs          Space_Separator
476           Zl          Line_Separator
477           Zp          Paragraph_Separator
478
479           C           Other
480           Cc          Control (also Cntrl)
481           Cf          Format
482           Cs          Surrogate
483           Co          Private_Use
484           Cn          Unassigned
485
486       Single-letter properties match all characters in any of the two-letter
487       sub-properties starting with the same letter.  "LC" and "L&" are
488       special: both are aliases for the set consisting of everything matched
489       by "Ll", "Lu", and "Lt".
490
491       Bidirectional Character Types
492
493       Because scripts differ in their directionality (Hebrew and Arabic are
494       written right to left, for example) Unicode supplies a "Bidi_Class"
495       property.  Some of the values this property can have are:
496
497           Value       Meaning
498
499           L           Left-to-Right
500           LRE         Left-to-Right Embedding
501           LRO         Left-to-Right Override
502           R           Right-to-Left
503           AL          Arabic Letter
504           RLE         Right-to-Left Embedding
505           RLO         Right-to-Left Override
506           PDF         Pop Directional Format
507           EN          European Number
508           ES          European Separator
509           ET          European Terminator
510           AN          Arabic Number
511           CS          Common Separator
512           NSM         Non-Spacing Mark
513           BN          Boundary Neutral
514           B           Paragraph Separator
515           S           Segment Separator
516           WS          Whitespace
517           ON          Other Neutrals
518
519       This property is always written in the compound form.  For example,
520       "\p{Bidi_Class:R}" matches characters that are normally written right
521       to left.  Unlike the "General_Category" property, this property can
522       have more values added in a future Unicode release.  Those listed above
523       comprised the complete set for many Unicode releases, but others were
524       added in Unicode 6.3; you can always find what the current ones are in
525       perluniprops.  And <https://www.unicode.org/reports/tr9/> describes how
526       to use them.
527
528       Scripts
529
530       The world's languages are written in many different scripts.  This
531       sentence (unless you're reading it in translation) is written in Latin,
532       while Russian is written in Cyrillic, and Greek is written in, well,
533       Greek; Japanese mainly in Hiragana or Katakana.  There are many more.
534
535       The Unicode "Script" and "Script_Extensions" properties give what
536       script a given character is in.  The "Script_Extensions" property is an
537       improved version of "Script", as demonstrated below.  Either property
538       can be specified with the compound form like "\p{Script=Hebrew}"
539       (short: "\p{sc=hebr}"), or "\p{Script_Extensions=Javanese}" (short:
540       "\p{scx=java}").  In addition, Perl furnishes shortcuts for all
541       "Script_Extensions" property names.  You can omit everything up through
542       the equals (or colon), and simply write "\p{Latin}" or "\P{Cyrillic}".
543       (This is not true for "Script", which is required to be written in the
544       compound form.  Prior to Perl v5.26, the single form returned the plain
545       old "Script" version, but was changed because "Script_Extensions" gives
546       better results.)
547
548       The difference between these two properties involves characters that
549       are used in multiple scripts.  For example the digits '0' through '9'
550       are used in many parts of the world.  These are placed in a script
551       named "Common".  Other characters are used in just a few scripts.  For
552       example, the "KATAKANA-HIRAGANA DOUBLE HYPHEN" is used in both Japanese
553       scripts, Katakana and Hiragana, but nowhere else.  The "Script"
554       property places all characters that are used in multiple scripts in the
555       "Common" script, while the "Script_Extensions" property places those
556       that are used in only a few scripts into each of those scripts; while
557       still using "Common" for those used in many scripts.  Thus both these
558       match:
559
560        "0" =~ /\p{sc=Common}/     # Matches
561        "0" =~ /\p{scx=Common}/    # Matches
562
563       and only the first of these match:
564
565        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common}  # Matches
566        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match
567
568       And only the last two of these match:
569
570        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana}  # No match
571        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana}  # No match
572        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
573        "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches
574
575       "Script_Extensions" is thus an improved "Script", in which there are
576       fewer characters in the "Common" script, and correspondingly more in
577       other scripts.  It is new in Unicode version 6.0, and its data are
578       likely to change significantly in later releases, as things get sorted
579       out.  New code should probably be using "Script_Extensions" and not
580       plain "Script".  If you compile perl with a Unicode release that
581       doesn't have "Script_Extensions", the single form Perl extensions will
582       instead refer to the plain "Script" property.  If you compile with a
583       version of Unicode that doesn't have the "Script" property, these
584       extensions will not be defined at all.
585
586       (Actually, besides "Common", the "Inherited" script, contains
587       characters that are used in multiple scripts.  These are modifier
588       characters which inherit the script value of the controlling character.
589       Some of these are used in many scripts, and so go into "Inherited" in
590       both "Script" and "Script_Extensions".  Others are used in just a few
591       scripts, so are in "Inherited" in "Script", but not in
592       "Script_Extensions".)
593
594       It is worth stressing that there are several different sets of digits
595       in Unicode that are equivalent to 0-9 and are matchable by "\d" in a
596       regular expression.  If they are used in a single language only, they
597       are in that language's "Script" and "Script_Extensions".  If they are
598       used in more than one script, they will be in "sc=Common", but only if
599       they are used in many scripts should they be in "scx=Common".
600
601       The explanation above has omitted some detail; refer to UAX#24 "Unicode
602       Script Property": <https://www.unicode.org/reports/tr24>.
603
604       A complete list of scripts and their shortcuts is in perluniprops.
605
606       Use of the "Is" Prefix
607
608       For backward compatibility (with ancient Perl 5.6), all properties
609       writable without using the compound form mentioned so far may have "Is"
610       or "Is_" prepended to their name, so "\P{Is_Lu}", for example, is equal
611       to "\P{Lu}", and "\p{IsScript:Arabic}" is equal to "\p{Arabic}".
612
613       Blocks
614
615       In addition to scripts, Unicode also defines blocks of characters.  The
616       difference between scripts and blocks is that the concept of scripts is
617       closer to natural languages, while the concept of blocks is more of an
618       artificial grouping based on groups of Unicode characters with
619       consecutive ordinal values. For example, the "Basic Latin" block is all
620       the characters whose ordinals are between 0 and 127, inclusive; in
621       other words, the ASCII characters.  The "Latin" script contains some
622       letters from this as well as several other blocks, like "Latin-1
623       Supplement", "Latin Extended-A", etc., but it does not contain all the
624       characters from those blocks. It does not, for example, contain the
625       digits 0-9, because those digits are shared across many scripts, and
626       hence are in the "Common" script.
627
628       For more about scripts versus blocks, see UAX#24 "Unicode Script
629       Property": <https://www.unicode.org/reports/tr24>
630
631       The "Script_Extensions" or "Script" properties are likely to be the
632       ones you want to use when processing natural language; the "Block"
633       property may occasionally be useful in working with the nuts and bolts
634       of Unicode.
635
636       Block names are matched in the compound form, like "\p{Block: Arrows}"
637       or "\p{Blk=Hebrew}".  Unlike most other properties, only a few block
638       names have a Unicode-defined short name.
639
640       Perl also defines single form synonyms for the block property in cases
641       where these do not conflict with something else.  But don't use any of
642       these, because they are unstable.  Since these are Perl extensions,
643       they are subordinate to official Unicode property names; Unicode
644       doesn't know nor care about Perl's extensions.  It may happen that a
645       name that currently means the Perl extension will later be changed
646       without warning to mean a different Unicode property in a future
647       version of the perl interpreter that uses a later Unicode release, and
648       your code would no longer work.  The extensions are mentioned here for
649       completeness:  Take the block name and prefix it with one of: "In" (for
650       example "\p{Blk=Arrows}" can currently be written as "\p{In_Arrows}");
651       or sometimes "Is" (like "\p{Is_Arrows}"); or sometimes no prefix at all
652       ("\p{Arrows}").  As of this writing (Unicode 9.0) there are no
653       conflicts with using the "In_" prefix, but there are plenty with the
654       other two forms.  For example, "\p{Is_Hebrew}" and "\p{Hebrew}" mean
655       "\p{Script_Extensions=Hebrew}" which is NOT the same thing as
656       "\p{Blk=Hebrew}".  Our advice used to be to use the "In_" prefix as a
657       single form way of specifying a block.  But Unicode 8.0 added
658       properties whose names begin with "In", and it's now clear that it's
659       only luck that's so far prevented a conflict.  Using "In" is only
660       marginally less typing than "Blk:", and the latter's meaning is clearer
661       anyway, and guaranteed to never conflict.  So don't take chances.  Use
662       "\p{Blk=foo}" for new code.  And be sure that block is what you really
663       really want to do.  In most cases scripts are what you want instead.
664
665       A complete list of blocks is in perluniprops.
666
667       Other Properties
668
669       There are many more properties than the very basic ones described here.
670       A complete list is in perluniprops.
671
672       Unicode defines all its properties in the compound form, so all single-
673       form properties are Perl extensions.  Most of these are just synonyms
674       for the Unicode ones, but some are genuine extensions, including
675       several that are in the compound form.  And quite a few of these are
676       actually recommended by Unicode (in
677       <https://www.unicode.org/reports/tr18>).
678
679       This section gives some details on all extensions that aren't just
680       synonyms for compound-form Unicode properties (for those properties,
681       you'll have to refer to the Unicode Standard
682       <https://www.unicode.org/reports/tr44>.
683
684       "\p{All}"
685           This matches every possible code point.  It is equivalent to
686           "qr/./s".  Unlike all the other non-user-defined "\p{}" property
687           matches, no warning is ever generated if this is property is
688           matched against a non-Unicode code point (see "Beyond Unicode code
689           points" below).
690
691       "\p{Alnum}"
692           This matches any "\p{Alphabetic}" or "\p{Decimal_Number}"
693           character.
694
695       "\p{Any}"
696           This matches any of the 1_114_112 Unicode code points.  It is a
697           synonym for "\p{Unicode}".
698
699       "\p{ASCII}"
700           This matches any of the 128 characters in the US-ASCII character
701           set, which is a subset of Unicode.
702
703       "\p{Assigned}"
704           This matches any assigned code point; that is, any code point whose
705           general category is not "Unassigned" (or equivalently, not "Cn").
706
707       "\p{Blank}"
708           This is the same as "\h" and "\p{HorizSpace}":  A character that
709           changes the spacing horizontally.
710
711       "\p{Decomposition_Type: Non_Canonical}"    (Short: "\p{Dt=NonCanon}")
712           Matches a character that has a non-canonical decomposition.
713
714           The "Extended Grapheme Clusters (Logical characters)" section above
715           talked about canonical decompositions.  However, many more
716           characters have a different type of decomposition, a "compatible"
717           or "non-canonical" decomposition.  The sequences that form these
718           decompositions are not considered canonically equivalent to the
719           pre-composed character.  An example is the "SUPERSCRIPT ONE".  It
720           is somewhat like a regular digit 1, but not exactly; its
721           decomposition into the digit 1 is called a "compatible"
722           decomposition, specifically a "super" decomposition.  There are
723           several such compatibility decompositions (see
724           <https://www.unicode.org/reports/tr44>), including one called
725           "compat", which means some miscellaneous type of decomposition that
726           doesn't fit into the other decomposition categories that Unicode
727           has chosen.
728
729           Note that most Unicode characters don't have a decomposition, so
730           their decomposition type is "None".
731
732           For your convenience, Perl has added the "Non_Canonical"
733           decomposition type to mean any of the several compatibility
734           decompositions.
735
736       "\p{Graph}"
737           Matches any character that is graphic.  Theoretically, this means a
738           character that on a printer would cause ink to be used.
739
740       "\p{HorizSpace}"
741           This is the same as "\h" and "\p{Blank}":  a character that changes
742           the spacing horizontally.
743
744       "\p{In=*}"
745           This is a synonym for "\p{Present_In=*}"
746
747       "\p{PerlSpace}"
748           This is the same as "\s", restricted to ASCII, namely "[ \f\n\r\t]"
749           and starting in Perl v5.18, a vertical tab.
750
751           Mnemonic: Perl's (original) space
752
753       "\p{PerlWord}"
754           This is the same as "\w", restricted to ASCII, namely
755           "[A-Za-z0-9_]"
756
757           Mnemonic: Perl's (original) word.
758
759       "\p{Posix...}"
760           There are several of these, which are equivalents, using the "\p{}"
761           notation, for Posix classes and are described in "POSIX Character
762           Classes" in perlrecharclass.
763
764       "\p{Present_In: *}"    (Short: "\p{In=*}")
765           This property is used when you need to know in what Unicode
766           version(s) a character is.
767
768           The "*" above stands for some Unicode version number, such as 1.1
769           or 12.0; or the "*" can also be "Unassigned".  This property will
770           match the code points whose final disposition has been settled as
771           of the Unicode release given by the version number; "\p{Present_In:
772           Unassigned}" will match those code points whose meaning has yet to
773           be assigned.
774
775           For example, "U+0041" "LATIN CAPITAL LETTER A" was present in the
776           very first Unicode release available, which is 1.1, so this
777           property is true for all valid "*" versions.  On the other hand,
778           "U+1EFF" was not assigned until version 5.1 when it became "LATIN
779           SMALL LETTER Y WITH LOOP", so the only "*" that would match it are
780           5.1, 5.2, and later.
781
782           Unicode furnishes the "Age" property from which this is derived.
783           The problem with Age is that a strict interpretation of it (which
784           Perl takes) has it matching the precise release a code point's
785           meaning is introduced in.  Thus "U+0041" would match only 1.1; and
786           "U+1EFF" only 5.1.  This is not usually what you want.
787
788           Some non-Perl implementations of the Age property may change its
789           meaning to be the same as the Perl "Present_In" property; just be
790           aware of that.
791
792           Another confusion with both these properties is that the definition
793           is not that the code point has been assigned, but that the meaning
794           of the code point has been determined.  This is because 66 code
795           points will always be unassigned, and so the "Age" for them is the
796           Unicode version in which the decision to make them so was made.
797           For example, "U+FDD0" is to be permanently unassigned to a
798           character, and the decision to do that was made in version 3.1, so
799           "\p{Age=3.1}" matches this character, as also does "\p{Present_In:
800           3.1}" and up.
801
802       "\p{Print}"
803           This matches any character that is graphical or blank, except
804           controls.
805
806       "\p{SpacePerl}"
807           This is the same as "\s", including beyond ASCII.
808
809           Mnemonic: Space, as modified by Perl.  (It doesn't include the
810           vertical tab until v5.18, which both the Posix standard and Unicode
811           consider white space.)
812
813       "\p{Title}" and  "\p{Titlecase}"
814           Under case-sensitive matching, these both match the same code
815           points as "\p{General Category=Titlecase_Letter}" ("\p{gc=lt}").
816           The difference is that under "/i" caseless matching, these match
817           the same as "\p{Cased}", whereas "\p{gc=lt}" matches
818           "\p{Cased_Letter").
819
820       "\p{Unicode}"
821           This matches any of the 1_114_112 Unicode code points.  "\p{Any}".
822
823       "\p{VertSpace}"
824           This is the same as "\v":  A character that changes the spacing
825           vertically.
826
827       "\p{Word}"
828           This is the same as "\w", including over 100_000 characters beyond
829           ASCII.
830
831       "\p{XPosix...}"
832           There are several of these, which are the standard Posix classes
833           extended to the full Unicode range.  They are described in "POSIX
834           Character Classes" in perlrecharclass.
835
836   Comparison of "\N{...}" and "\p{name=...}"
837       Starting in Perl 5.32, you can specify a character by its name in
838       regular expression patterns using "\p{name=...}".  This is in addition
839       to the longstanding method of using "\N{...}".  The following
840       summarizes the differences between these two:
841
842                              \N{...}       \p{Name=...}
843        can interpolate    only with eval       yes            [1]
844        custom names            yes             no             [2]
845        name aliases            yes             yes            [3]
846        named sequences         yes             yes            [4]
847        name value parsing     exact       Unicode loose       [5]
848
849       [1] The ability to interpolate means you can do something like
850
851            qr/\p{na=latin capital letter $which}/
852
853           and specify $which elsewhere.
854
855       [2] You can create your own names for characters, and override official
856           ones when using "\N{...}".  See "CUSTOM ALIASES" in charnames.
857
858       [3] Some characters have multiple names (synonyms).
859
860       [4] Some particular sequences of characters are given a single name, in
861           addition to their individual ones.
862
863       [5] Exact name value matching means you have to specify case, hyphens,
864           underscores, and spaces precisely in the name you want.  Loose
865           matching follows the Unicode rules
866           <https://www.unicode.org/reports/tr44/tr44-24.html#UAX44-LM2>,
867           where these are mostly irrelevant.  Except for a few outlier
868           character names, these are the same rules as are already used for
869           any other "\p{...}" property.
870
871   Wildcards in Property Values
872       Starting in Perl 5.30, it is possible to do something like this:
873
874        qr!\p{numeric_value=/\A[0-5]\z/}!
875
876       or, by abbreviating and adding "/x",
877
878        qr! \p{nv= /(?x) \A [0-5] \z / }!
879
880       This matches all code points whose numeric value is one of 0, 1, 2, 3,
881       4, or 5.  This particular example could instead have been written as
882
883        qr! \A [ \p{nv=0}\p{nv=1}\p{nv=2}\p{nv=3}\p{nv=4}\p{nv=5} ] \z !xx
884
885       in earlier perls, so in this case this feature just makes things easier
886       and shorter to write.  If we hadn't included the "\A" and "\z", these
887       would have matched things like "1/2" because that contains a 1 (as well
888       as a 2).  As written, it matches things like subscripts that have these
889       numeric values.  If we only wanted the decimal digits with those
890       numeric values, we could say,
891
892        qr! (?[ \d & \p{nv=/[0-5]/ ]) }!x
893
894       The "\d" gets rid of needing to anchor the pattern, since it forces the
895       result to only match "[0-9]", and the "[0-5]" further restricts it.
896
897       The text in the above examples enclosed between the "/" characters can
898       be just about any regular expression.  It is independent of the main
899       pattern, so doesn't share any capturing groups, etc.  The delimiters
900       for it must be ASCII punctuation, but it may NOT be delimited by "{",
901       nor "}" nor contain a literal "}", as that delimits the end of the
902       enclosing "\p{}".  Like any pattern, certain other delimiters are
903       terminated by their mirror images.  These are "(", ""["", and "<".  If
904       the delimiter is any of "-", "_", "+", or "\", or is the same delimiter
905       as is used for the enclosing pattern, it must be preceded by a
906       backslash escape, both fore and aft.
907
908       Beware of using "$" to indicate to match the end of the string.  It can
909       too easily be interpreted as being a punctuation variable, like $/.
910
911       No modifiers may follow the final delimiter.  Instead, use
912       "(?adlupimnsx-imnsx)" in perlre and/or "(?adluimnsx-imnsx:pattern)" in
913       perlre to specify modifiers.  However, certain modifiers are illegal in
914       your wildcard subpattern.  The only character set modifier specifiable
915       is "/aa"; any other character set, and "-m", and "p", and "s" are all
916       illegal.  Specifying modifiers like "qr/.../gc" that aren't legal in
917       the "(?...)" notation normally raise a warning, but with wildcard
918       subpatterns, their use is an error.  The "m" modifier is ineffective;
919       everything that matches will be a single line.
920
921       By default, your pattern is matched case-insensitively, as if "/i" had
922       been specified.  You can change this by saying "(?-i)" in your pattern.
923
924       There are also certain operations that are illegal.  You can't nest
925       "\p{...}" and "\P{...}" calls within a wildcard subpattern, and "\G"
926       doesn't make sense, so is also prohibited.
927
928       And the "*" quantifier (or its equivalent "(0,}") is illegal.
929
930       This feature is not available when the left-hand side is prefixed by
931       "Is_", nor for any form that is marked as "Discouraged" in
932       "Discouraged" in perluniprops.
933
934       This experimental feature has been added to begin to implement
935       <https://www.unicode.org/reports/tr18/#Wildcard_Properties>.  Using it
936       will raise a (default-on) warning in the
937       "experimental::uniprop_wildcards" category.  We reserve the right to
938       change its operation as we gain experience.
939
940       Your subpattern can be just about anything, but for it to have some
941       utility, it should match when called with either or both of a) the full
942       name of the property value with underscores (and/or spaces in the Block
943       property) and some things uppercase; or b) the property value in all
944       lowercase with spaces and underscores squeezed out.  For example,
945
946        qr!\p{Blk=/Old I.*/}!
947        qr!\p{Blk=/oldi.*/}!
948
949       would match the same things.
950
951       Another example that shows that within "\p{...}", "/x" isn't needed to
952       have spaces:
953
954        qr!\p{scx= /Hebrew|Greek/ }!
955
956       To be safe, we should have anchored the above example, to prevent
957       matches for something like "Hebrew_Braille", but there aren't any
958       script names like that, so far.  A warning is issued if none of the
959       legal values for a property are matched by your pattern.  It's likely
960       that a future release will raise a warning if your pattern ends up
961       causing every possible code point to match.
962
963       Starting in 5.32, the Name, Name Aliases, and Named Sequences
964       properties are allowed to be matched.  They are considered to be a
965       single combination property, just as has long been the case for "\N{}".
966       Loose matching doesn't work in exactly the same way for these as it
967       does for the values of other properties.  The rules are given in
968       <https://www.unicode.org/reports/tr44/tr44-24.html#UAX44-LM2>.  As a
969       result, Perl doesn't try loose matching for you, like it does in other
970       properties.  All letters in names are uppercase, but you can add "(?i)"
971       to your subpattern to ignore case.  If you're uncertain where a blank
972       is, you can use " ?" in your subpattern.  No character name contains an
973       underscore, so don't bother trying to match one.  The use of hyphens is
974       particularly problematic; refer to the above link.  But note that, as
975       of Unicode 13.0, the only script in modern usage which has weirdnesses
976       with these is Tibetan; also the two Korean characters U+116C HANGUL
977       JUNGSEONG OE and U+1180 HANGUL JUNGSEONG O-E.  Unicode makes no
978       promises to not add hyphen-problematic names in the future.
979
980       Using wildcards on these is resource intensive, given the hundreds of
981       thousands of legal names that must be checked against.
982
983       An example of using Name property wildcards is
984
985        qr!\p{name=/(SMILING|GRINNING) FACE/}!
986
987       Another is
988
989        qr/(?[ \p{name=\/CJK\/} - \p{ideographic} ])/
990
991       which is the 200-ish (as of Unicode 13.0) CJK characters that aren't
992       ideographs.
993
994       There are certain properties that wildcard subpatterns don't currently
995       work with.  These are:
996
997        Bidi Mirroring Glyph
998        Bidi Paired Bracket
999        Case Folding
1000        Decomposition Mapping
1001        Equivalent Unified Ideograph
1002        Lowercase Mapping
1003        NFKC Case Fold
1004        Titlecase Mapping
1005        Uppercase Mapping
1006
1007       Nor is the "@unicode_property@" form implemented.
1008
1009       Here's a complete example of matching IPV4 internet protocol addresses
1010       in any (single) script
1011
1012        no warnings 'experimental::regex_sets';
1013        no warnings 'experimental::uniprop_wildcards';
1014
1015        # Can match a substring, so this intermediate regex needs to have
1016        # context or anchoring in its final use.  Using nt=de yields decimal
1017        # digits.  When specifying a subset of these, we must include \d to
1018        # prevent things like U+00B2 SUPERSCRIPT TWO from matching
1019        my $zero_through_255 =
1020         qr/ \b (*sr:                                  # All from same sript
1021                   (?[ \p{nv=0} & \d ])*               # Optional leading zeros
1022               (                                       # Then one of:
1023                                         \d{1,2}       #   0 - 99
1024                   | (?[ \p{nv=1} & \d ])  \d{2}       #   100 - 199
1025                   | (?[ \p{nv=2} & \d ])
1026                      (  (?[ \p{nv=:[0-4]:} & \d ]) \d #   200 - 249
1027                       | (?[ \p{nv=5}     & \d ])
1028                         (?[ \p{nv=:[0-5]:} & \d ])    #   250 - 255
1029                      )
1030               )
1031             )
1032           \b
1033         /x;
1034
1035        my $ipv4 = qr/ \A (*sr:         $zero_through_255
1036                                (?: [.] $zero_through_255 ) {3}
1037                          )
1038                       \z
1039                   /x;
1040
1041   User-Defined Character Properties
1042       You can define your own binary character properties by defining
1043       subroutines whose names begin with "In" or "Is".  (The experimental
1044       feature "(?[ ])" in perlre provides an alternative which allows more
1045       complex definitions.)  The subroutines can be defined in any package.
1046       They override any Unicode properties expressed as the same names.  The
1047       user-defined properties can be used in the regular expression "\p{}"
1048       and "\P{}" constructs; if you are using a user-defined property from a
1049       package other than the one you are in, you must specify its package in
1050       the "\p{}" or "\P{}" construct.
1051
1052           # assuming property Is_Foreign defined in Lang::
1053           package main;  # property package name required
1054           if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
1055
1056           package Lang;  # property package name not required
1057           if ($txt =~ /\p{IsForeign}+/) { ... }
1058
1059       Note that the effect is compile-time and immutable once defined.
1060       However, the subroutines are passed a single parameter, which is 0 if
1061       case-sensitive matching is in effect and non-zero if caseless matching
1062       is in effect.  The subroutine may return different values depending on
1063       the value of the flag, and one set of values will immutably be in
1064       effect for all case-sensitive matches, and the other set for all case-
1065       insensitive matches.
1066
1067       Note that if the regular expression is tainted, then Perl will die
1068       rather than calling the subroutine when the name of the subroutine is
1069       determined by the tainted data.
1070
1071       The subroutines must return a specially-formatted string, with one or
1072       more newline-separated lines.  Each line must be one of the following:
1073
1074       ·   A single hexadecimal number denoting a code point to include.
1075
1076       ·   Two hexadecimal numbers separated by horizontal whitespace (space
1077           or tabular characters) denoting a range of code points to include.
1078           The second number must not be smaller than the first.
1079
1080       ·   Something to include, prefixed by "+": a built-in character
1081           property (prefixed by "utf8::") or a fully qualified (including
1082           package name) user-defined character property, to represent all the
1083           characters in that property; two hexadecimal code points for a
1084           range; or a single hexadecimal code point.
1085
1086       ·   Something to exclude, prefixed by "-": an existing character
1087           property (prefixed by "utf8::") or a fully qualified (including
1088           package name) user-defined character property, to represent all the
1089           characters in that property; two hexadecimal code points for a
1090           range; or a single hexadecimal code point.
1091
1092       ·   Something to negate, prefixed "!": an existing character property
1093           (prefixed by "utf8::") or a fully qualified (including package
1094           name) user-defined character property, to represent all the
1095           characters in that property; two hexadecimal code points for a
1096           range; or a single hexadecimal code point.
1097
1098       ·   Something to intersect with, prefixed by "&": an existing character
1099           property (prefixed by "utf8::") or a fully qualified (including
1100           package name) user-defined character property, for all the
1101           characters except the characters in the property; two hexadecimal
1102           code points for a range; or a single hexadecimal code point.
1103
1104       For example, to define a property that covers both the Japanese
1105       syllabaries (hiragana and katakana), you can define
1106
1107           sub InKana {
1108               return <<END;
1109           3040\t309F
1110           30A0\t30FF
1111           END
1112           }
1113
1114       Imagine that the here-doc end marker is at the beginning of the line.
1115       Now you can use "\p{InKana}" and "\P{InKana}".
1116
1117       You could also have used the existing block property names:
1118
1119           sub InKana {
1120               return <<'END';
1121           +utf8::InHiragana
1122           +utf8::InKatakana
1123           END
1124           }
1125
1126       Suppose you wanted to match only the allocated characters, not the raw
1127       block ranges: in other words, you want to remove the unassigned
1128       characters:
1129
1130           sub InKana {
1131               return <<'END';
1132           +utf8::InHiragana
1133           +utf8::InKatakana
1134           -utf8::IsCn
1135           END
1136           }
1137
1138       The negation is useful for defining (surprise!) negated classes.
1139
1140           sub InNotKana {
1141               return <<'END';
1142           !utf8::InHiragana
1143           -utf8::InKatakana
1144           +utf8::IsCn
1145           END
1146           }
1147
1148       This will match all non-Unicode code points, since every one of them is
1149       not in Kana.  You can use intersection to exclude these, if desired, as
1150       this modified example shows:
1151
1152           sub InNotKana {
1153               return <<'END';
1154           !utf8::InHiragana
1155           -utf8::InKatakana
1156           +utf8::IsCn
1157           &utf8::Any
1158           END
1159           }
1160
1161       &utf8::Any must be the last line in the definition.
1162
1163       Intersection is used generally for getting the common characters
1164       matched by two (or more) classes.  It's important to remember not to
1165       use "&" for the first set; that would be intersecting with nothing,
1166       resulting in an empty set.  (Similarly using "-" for the first set does
1167       nothing).
1168
1169       Unlike non-user-defined "\p{}" property matches, no warning is ever
1170       generated if these properties are matched against a non-Unicode code
1171       point (see "Beyond Unicode code points" below).
1172
1173   User-Defined Case Mappings (for serious hackers only)
1174       This feature has been removed as of Perl 5.16.  The CPAN module
1175       "Unicode::Casing" provides better functionality without the drawbacks
1176       that this feature had.  If you are using a Perl earlier than 5.16, this
1177       feature was most fully documented in the 5.14 version of this pod:
1178       <http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>
1179
1180   Character Encodings for Input and Output
1181       See Encode.
1182
1183   Unicode Regular Expression Support Level
1184       The following list of Unicode supported features for regular
1185       expressions describes all features currently directly supported by core
1186       Perl.  The references to "Level N" and the section numbers refer to
1187       UTS#18 "Unicode Regular Expressions"
1188       <https://www.unicode.org/reports/tr18>, version 18, October 2016.
1189
1190       Level 1 - Basic Unicode Support
1191
1192        RL1.1   Hex Notation                     - Done          [1]
1193        RL1.2   Properties                       - Done          [2]
1194        RL1.2a  Compatibility Properties         - Done          [3]
1195        RL1.3   Subtraction and Intersection     - Experimental  [4]
1196        RL1.4   Simple Word Boundaries           - Done          [5]
1197        RL1.5   Simple Loose Matches             - Done          [6]
1198        RL1.6   Line Boundaries                  - Partial       [7]
1199        RL1.7   Supplementary Code Points        - Done          [8]
1200
1201       [1] "\N{U+...}" and "\x{...}"
1202       [2] "\p{...}" "\P{...}".  This requirement is for a minimal list of
1203       properties.  Perl supports these.  See R2.7 for other properties.
1204       [3] Perl has "\d" "\D" "\s" "\S" "\w" "\W" "\X" "[:prop:]" "[:^prop:]",
1205       plus all the properties specified by
1206       <https://www.unicode.org/reports/tr18/#Compatibility_Properties>.
1207       These are described above in "Other Properties"
1208       [4] The experimental feature "(?[...])" starting in v5.18 accomplishes
1209           this.
1210
1211           See "(?[ ])" in perlre.  If you don't want to use an experimental
1212           feature, you can use one of the following:
1213
1214           ·   Regular expression lookahead
1215
1216               You can mimic class subtraction using lookahead.  For example,
1217               what UTS#18 might write as
1218
1219                   [{Block=Greek}-[{UNASSIGNED}]]
1220
1221               in Perl can be written as:
1222
1223                   (?!\p{Unassigned})\p{Block=Greek}
1224                   (?=\p{Assigned})\p{Block=Greek}
1225
1226               But in this particular example, you probably really want
1227
1228                   \p{Greek}
1229
1230               which will match assigned characters known to be part of the
1231               Greek script.
1232
1233           ·   CPAN module "Unicode::Regex::Set"
1234
1235               It does implement the full UTS#18 grouping, intersection,
1236               union, and removal (subtraction) syntax.
1237
1238           ·   "User-Defined Character Properties"
1239
1240               "+" for union, "-" for removal (set-difference), "&" for
1241               intersection
1242
1243       [5] "\b" "\B" meet most, but not all, the details of this requirement,
1244       but "\b{wb}" and "\B{wb}" do, as well as the stricter R2.3.
1245       [6] Note that Perl does Full case-folding in matching, not Simple:
1246
1247           For example "U+1F88" is equivalent to "U+1F00 U+03B9", instead of
1248           just "U+1F80".  This difference matters mainly for certain Greek
1249           capital letters with certain modifiers: the Full case-folding
1250           decomposes the letter, while the Simple case-folding would map it
1251           to a single character.
1252
1253       [7] The reason this is considered to be only partially implemented is
1254           that Perl has "qr/\b{lb}/" and "Unicode::LineBreak" that are
1255           conformant with UAX#14 "Unicode Line Breaking Algorithm"
1256           <https://www.unicode.org/reports/tr14>.  The regular expression
1257           construct provides default behavior, while the heavier-weight
1258           module provides customizable line breaking.
1259
1260           But Perl treats "\n" as the start- and end-line delimiter, whereas
1261           Unicode specifies more characters that should be so-interpreted.
1262
1263           These are:
1264
1265            VT   U+000B  (\v in C)
1266            FF   U+000C  (\f)
1267            CR   U+000D  (\r)
1268            NEL  U+0085
1269            LS   U+2028
1270            PS   U+2029
1271
1272           "^" and "$" in regular expression patterns are supposed to match
1273           all these, but don't.  These characters also don't, but should,
1274           affect "<>" $., and script line numbers.
1275
1276           Also, lines should not be split within "CRLF" (i.e. there is no
1277           empty line between "\r" and "\n").  For "CRLF", try the ":crlf"
1278           layer (see PerlIO).
1279
1280       [8] UTF-8/UTF-EBDDIC used in Perl allows not only "U+10000" to
1281       "U+10FFFF" but also beyond "U+10FFFF"
1282
1283       Level 2 - Extended Unicode Support
1284
1285        RL2.1   Canonical Equivalents           - Retracted     [9]
1286                                                  by Unicode
1287        RL2.2   Extended Grapheme Clusters and  - Partial       [10]
1288                Character Classes with Strings
1289        RL2.3   Default Word Boundaries         - Done          [11]
1290        RL2.4   Default Case Conversion         - Done
1291        RL2.5   Name Properties                 - Done
1292        RL2.6   Wildcards in Property Values    - Partial       [12]
1293        RL2.7   Full Properties                 - Partial       [13]
1294        RL2.8   Optional Properties             - Partial       [14]
1295
1296       [9] Unicode has rewritten this portion of UTS#18 to say that getting
1297       canonical equivalence (see UAX#15 "Unicode Normalization Forms"
1298       <https://www.unicode.org/reports/tr15>) is basically to be done at the
1299       programmer level.  Use NFD to write both your regular expressions and
1300       text to match them against (you can use Unicode::Normalize).
1301       [10] Perl has "\X" and "\b{gcb}".  Unicode has retracted their
1302       "Grapheme Cluster Mode", and recently added string properties, which
1303       Perl does not yet support.
1304       [11] see UAX#29 "Unicode Text Segmentation"
1305       <https://www.unicode.org/reports/tr29>,
1306       [12] see "Wildcards in Property Values" above.
1307       [13] Perl supports all the properties in the Unicode Character Database
1308       (UCD).  It does not yet support the listed properties that come from
1309       other Unicode sources.
1310       [14] The only optional property that Perl supports is Named Sequence.
1311       None of these properties are in the UCD.
1312
1313       Level 3 - Tailored Support
1314
1315       This has been retracted by Unicode.
1316
1317   Unicode Encodings
1318       Unicode characters are assigned to code points, which are abstract
1319       numbers.  To use these numbers, various encodings are needed.
1320
1321       ·   UTF-8
1322
1323           UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
1324           encoding.  In most of Perl's documentation, including elsewhere in
1325           this document, the term "UTF-8" means also "UTF-EBCDIC".  But in
1326           this section, "UTF-8" refers only to the encoding used on ASCII
1327           platforms.  It is a superset of 7-bit US-ASCII, so anything encoded
1328           in ASCII has the identical representation when encoded in UTF-8.
1329
1330           The following table is from Unicode 3.2.
1331
1332            Code Points            1st Byte  2nd Byte  3rd Byte 4th Byte
1333
1334              U+0000..U+007F       00..7F
1335              U+0080..U+07FF     * C2..DF    80..BF
1336              U+0800..U+0FFF       E0      * A0..BF    80..BF
1337              U+1000..U+CFFF       E1..EC    80..BF    80..BF
1338              U+D000..U+D7FF       ED        80..9F    80..BF
1339              U+D800..U+DFFF       +++++ utf16 surrogates, not legal utf8 +++++
1340              U+E000..U+FFFF       EE..EF    80..BF    80..BF
1341             U+10000..U+3FFFF      F0      * 90..BF    80..BF    80..BF
1342             U+40000..U+FFFFF      F1..F3    80..BF    80..BF    80..BF
1343            U+100000..U+10FFFF     F4        80..8F    80..BF    80..BF
1344
1345           Note the gaps marked by "*" before several of the byte entries
1346           above.  These are caused by legal UTF-8 avoiding non-shortest
1347           encodings: it is technically possible to UTF-8-encode a single code
1348           point in different ways, but that is explicitly forbidden, and the
1349           shortest possible encoding should always be used (and that is what
1350           Perl does).
1351
1352           Another way to look at it is via bits:
1353
1354                           Code Points  1st Byte  2nd Byte  3rd Byte  4th Byte
1355
1356                              0aaaaaaa  0aaaaaaa
1357                      00000bbbbbaaaaaa  110bbbbb  10aaaaaa
1358                      ccccbbbbbbaaaaaa  1110cccc  10bbbbbb  10aaaaaa
1359            00000dddccccccbbbbbbaaaaaa  11110ddd  10cccccc  10bbbbbb  10aaaaaa
1360
1361           As you can see, the continuation bytes all begin with "10", and the
1362           leading bits of the start byte tell how many bytes there are in the
1363           encoded character.
1364
1365           The original UTF-8 specification allowed up to 6 bytes, to allow
1366           encoding of numbers up to "0x7FFF_FFFF".  Perl continues to allow
1367           those, and has extended that up to 13 bytes to encode code points
1368           up to what can fit in a 64-bit word.  However, Perl will warn if
1369           you output any of these as being non-portable; and under strict
1370           UTF-8 input protocols, they are forbidden.  In addition, it is now
1371           illegal to use a code point larger than what a signed integer
1372           variable on your system can hold.  On 32-bit ASCII systems, this
1373           means "0x7FFF_FFFF" is the legal maximum (much higher on 64-bit
1374           systems).
1375
1376       ·   UTF-EBCDIC
1377
1378           Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
1379           This means that all the basic characters (which includes all those
1380           that have ASCII equivalents (like "A", "0", "%", etc.)  are the
1381           same in both EBCDIC and UTF-EBCDIC.)
1382
1383           UTF-EBCDIC is used on EBCDIC platforms.  It generally requires more
1384           bytes to represent a given code point than UTF-8 does; the largest
1385           Unicode code points take 5 bytes to represent (instead of 4 in
1386           UTF-8), and, extended for 64-bit words, it uses 14 bytes instead of
1387           13 bytes in UTF-8.
1388
1389       ·   UTF-16, UTF-16BE, UTF-16LE, Surrogates, and "BOM"'s (Byte Order
1390           Marks)
1391
1392           The followings items are mostly for reference and general Unicode
1393           knowledge, Perl doesn't use these constructs internally.
1394
1395           Like UTF-8, UTF-16 is a variable-width encoding, but where UTF-8
1396           uses 8-bit code units, UTF-16 uses 16-bit code units.  All code
1397           points occupy either 2 or 4 bytes in UTF-16: code points
1398           "U+0000..U+FFFF" are stored in a single 16-bit unit, and code
1399           points "U+10000..U+10FFFF" in two 16-bit units.  The latter case is
1400           using surrogates, the first 16-bit unit being the high surrogate,
1401           and the second being the low surrogate.
1402
1403           Surrogates are code points set aside to encode the
1404           "U+10000..U+10FFFF" range of Unicode code points in pairs of 16-bit
1405           units.  The high surrogates are the range "U+D800..U+DBFF" and the
1406           low surrogates are the range "U+DC00..U+DFFF".  The surrogate
1407           encoding is
1408
1409               $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1410               $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
1411
1412           and the decoding is
1413
1414               $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
1415
1416           Because of the 16-bitness, UTF-16 is byte-order dependent.  UTF-16
1417           itself can be used for in-memory computations, but if storage or
1418           transfer is required either UTF-16BE (big-endian) or UTF-16LE
1419           (little-endian) encodings must be chosen.
1420
1421           This introduces another problem: what if you just know that your
1422           data is UTF-16, but you don't know which endianness?  Byte Order
1423           Marks, or "BOM"'s, are a solution to this.  A special character has
1424           been reserved in Unicode to function as a byte order marker: the
1425           character with the code point "U+FEFF" is the "BOM".
1426
1427           The trick is that if you read a "BOM", you will know the byte
1428           order, since if it was written on a big-endian platform, you will
1429           read the bytes "0xFE 0xFF", but if it was written on a little-
1430           endian platform, you will read the bytes "0xFF 0xFE".  (And if the
1431           originating platform was writing in ASCII platform UTF-8, you will
1432           read the bytes "0xEF 0xBB 0xBF".)
1433
1434           The way this trick works is that the character with the code point
1435           "U+FFFE" is not supposed to be in input streams, so the sequence of
1436           bytes "0xFF 0xFE" is unambiguously ""BOM", represented in little-
1437           endian format" and cannot be "U+FFFE", represented in big-endian
1438           format".
1439
1440           Surrogates have no meaning in Unicode outside their use in pairs to
1441           represent other code points.  However, Perl allows them to be
1442           represented individually internally, for example by saying
1443           "chr(0xD801)", so that all code points, not just those valid for
1444           open interchange, are representable.  Unicode does define semantics
1445           for them, such as their "General_Category" is "Cs".  But because
1446           their use is somewhat dangerous, Perl will warn (using the warning
1447           category "surrogate", which is a sub-category of "utf8") if an
1448           attempt is made to do things like take the lower case of one, or
1449           match case-insensitively, or to output them.  (But don't try this
1450           on Perls before 5.14.)
1451
1452       ·   UTF-32, UTF-32BE, UTF-32LE
1453
1454           The UTF-32 family is pretty much like the UTF-16 family, except
1455           that the units are 32-bit, and therefore the surrogate scheme is
1456           not needed.  UTF-32 is a fixed-width encoding.  The "BOM"
1457           signatures are "0x00 0x00 0xFE 0xFF" for BE and "0xFF 0xFE 0x00
1458           0x00" for LE.
1459
1460       ·   UCS-2, UCS-4
1461
1462           Legacy, fixed-width encodings defined by the ISO 10646 standard.
1463           UCS-2 is a 16-bit encoding.  Unlike UTF-16, UCS-2 is not extensible
1464           beyond "U+FFFF", because it does not use surrogates.  UCS-4 is a
1465           32-bit encoding, functionally identical to UTF-32 (the difference
1466           being that UCS-4 forbids neither surrogates nor code points larger
1467           than "0x10_FFFF").
1468
1469       ·   UTF-7
1470
1471           A seven-bit safe (non-eight-bit) encoding, which is useful if the
1472           transport or storage is not eight-bit safe.  Defined by RFC 2152.
1473
1474   Noncharacter code points
1475       66 code points are set aside in Unicode as "noncharacter code points".
1476       These all have the "Unassigned" ("Cn") "General_Category", and no
1477       character will ever be assigned to any of them.  They are the 32 code
1478       points between "U+FDD0" and "U+FDEF" inclusive, and the 34 code points:
1479
1480        U+FFFE   U+FFFF
1481        U+1FFFE  U+1FFFF
1482        U+2FFFE  U+2FFFF
1483        ...
1484        U+EFFFE  U+EFFFF
1485        U+FFFFE  U+FFFFF
1486        U+10FFFE U+10FFFF
1487
1488       Until Unicode 7.0, the noncharacters were "forbidden for use in open
1489       interchange of Unicode text data", so that code that processed those
1490       streams could use these code points as sentinels that could be mixed in
1491       with character data, and would always be distinguishable from that
1492       data.  (Emphasis above and in the next paragraph are added in this
1493       document.)
1494
1495       Unicode 7.0 changed the wording so that they are "not recommended for
1496       use in open interchange of Unicode text data".  The 7.0 Standard goes
1497       on to say:
1498
1499           "If a noncharacter is received in open interchange, an application
1500           is not required to interpret it in any way.  It is good practice,
1501           however, to recognize it as a noncharacter and to take appropriate
1502           action, such as replacing it with "U+FFFD" replacement character,
1503           to indicate the problem in the text.  It is not recommended to
1504           simply delete noncharacter code points from such text, because of
1505           the potential security issues caused by deleting uninterpreted
1506           characters.  (See conformance clause C7 in Section 3.2, Conformance
1507           Requirements, and Unicode Technical Report #36, "Unicode Security
1508           Considerations"
1509           <https://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
1510
1511       This change was made because it was found that various commercial tools
1512       like editors, or for things like source code control, had been written
1513       so that they would not handle program files that used these code
1514       points, effectively precluding their use almost entirely!  And that was
1515       never the intent.  They've always been meant to be usable within an
1516       application, or cooperating set of applications, at will.
1517
1518       If you're writing code, such as an editor, that is supposed to be able
1519       to handle any Unicode text data, then you shouldn't be using these code
1520       points yourself, and instead allow them in the input.  If you need
1521       sentinels, they should instead be something that isn't legal Unicode.
1522       For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
1523       they never appear in well-formed UTF-8.  (There are equivalents for
1524       UTF-EBCDIC).  You can also store your Unicode code points in integer
1525       variables and use negative values as sentinels.
1526
1527       If you're not writing such a tool, then whether you accept
1528       noncharacters as input is up to you (though the Standard recommends
1529       that you not).  If you do strict input stream checking with Perl, these
1530       code points continue to be forbidden.  This is to maintain backward
1531       compatibility (otherwise potential security holes could open up, as an
1532       unsuspecting application that was written assuming the noncharacters
1533       would be filtered out before getting to it, could now, without warning,
1534       start getting them).  To do strict checking, you can use the layer
1535       ":encoding('UTF-8')".
1536
1537       Perl continues to warn (using the warning category "nonchar", which is
1538       a sub-category of "utf8") if an attempt is made to output
1539       noncharacters.
1540
1541   Beyond Unicode code points
1542       The maximum Unicode code point is "U+10FFFF", and Unicode only defines
1543       operations on code points up through that.  But Perl works on code
1544       points up to the maximum permissible signed number available on the
1545       platform.  However, Perl will not accept these from input streams
1546       unless lax rules are being used, and will warn (using the warning
1547       category "non_unicode", which is a sub-category of "utf8") if any are
1548       output.
1549
1550       Since Unicode rules are not defined on these code points, if a Unicode-
1551       defined operation is done on them, Perl uses what we believe are
1552       sensible rules, while generally warning, using the "non_unicode"
1553       category.  For example, "uc("\x{11_0000}")" will generate such a
1554       warning, returning the input parameter as its result, since Perl
1555       defines the uppercase of every non-Unicode code point to be the code
1556       point itself.  (All the case changing operations, not just uppercasing,
1557       work this way.)
1558
1559       The situation with matching Unicode properties in regular expressions,
1560       the "\p{}" and "\P{}" constructs, against these code points is not as
1561       clear cut, and how these are handled has changed as we've gained
1562       experience.
1563
1564       One possibility is to treat any match against these code points as
1565       undefined.  But since Perl doesn't have the concept of a match being
1566       undefined, it converts this to failing or "FALSE".  This is almost, but
1567       not quite, what Perl did from v5.14 (when use of these code points
1568       became generally reliable) through v5.18.  The difference is that Perl
1569       treated all "\p{}" matches as failing, but all "\P{}" matches as
1570       succeeding.
1571
1572       One problem with this is that it leads to unexpected, and confusing
1573       results in some cases:
1574
1575        chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Failed on <= v5.18
1576        chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Failed! on <= v5.18
1577
1578       That is, it treated both matches as undefined, and converted that to
1579       false (raising a warning on each).  The first case is the expected
1580       result, but the second is likely counterintuitive: "How could both be
1581       false when they are complements?"  Another problem was that the
1582       implementation optimized many Unicode property matches down to already
1583       existing simpler, faster operations, which don't raise the warning.  We
1584       chose to not forgo those optimizations, which help the vast majority of
1585       matches, just to generate a warning for the unlikely event that an
1586       above-Unicode code point is being matched against.
1587
1588       As a result of these problems, starting in v5.20, what Perl does is to
1589       treat non-Unicode code points as just typical unassigned Unicode
1590       characters, and matches accordingly.  (Note: Unicode has atypical
1591       unassigned code points.  For example, it has noncharacter code points,
1592       and ones that, when they do get assigned, are destined to be written
1593       Right-to-left, as Arabic and Hebrew are.  Perl assumes that no non-
1594       Unicode code point has any atypical properties.)
1595
1596       Perl, in most cases, will raise a warning when matching an above-
1597       Unicode code point against a Unicode property when the result is "TRUE"
1598       for "\p{}", and "FALSE" for "\P{}".  For example:
1599
1600        chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Fails, no warning
1601        chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Succeeds, with warning
1602
1603       In both these examples, the character being matched is non-Unicode, so
1604       Unicode doesn't define how it should match.  It clearly isn't an ASCII
1605       hex digit, so the first example clearly should fail, and so it does,
1606       with no warning.  But it is arguable that the second example should
1607       have an undefined, hence "FALSE", result.  So a warning is raised for
1608       it.
1609
1610       Thus the warning is raised for many fewer cases than in earlier Perls,
1611       and only when what the result is could be arguable.  It turns out that
1612       none of the optimizations made by Perl (or are ever likely to be made)
1613       cause the warning to be skipped, so it solves both problems of Perl's
1614       earlier approach.  The most commonly used property that is affected by
1615       this change is "\p{Unassigned}" which is a short form for
1616       "\p{General_Category=Unassigned}".  Starting in v5.20, all non-Unicode
1617       code points are considered "Unassigned".  In earlier releases the
1618       matches failed because the result was considered undefined.
1619
1620       The only place where the warning is not raised when it might ought to
1621       have been is if optimizations cause the whole pattern match to not even
1622       be attempted.  For example, Perl may figure out that for a string to
1623       match a certain regular expression pattern, the string has to contain
1624       the substring "foobar".  Before attempting the match, Perl may look for
1625       that substring, and if not found, immediately fail the match without
1626       actually trying it; so no warning gets generated even if the string
1627       contains an above-Unicode code point.
1628
1629       This behavior is more "Do what I mean" than in earlier Perls for most
1630       applications.  But it catches fewer issues for code that needs to be
1631       strictly Unicode compliant.  Therefore there is an additional mode of
1632       operation available to accommodate such code.  This mode is enabled if
1633       a regular expression pattern is compiled within the lexical scope where
1634       the "non_unicode" warning class has been made fatal, say by:
1635
1636        use warnings FATAL => "non_unicode"
1637
1638       (see warnings).  In this mode of operation, Perl will raise the warning
1639       for all matches against a non-Unicode code point (not just the arguable
1640       ones), and it skips the optimizations that might cause the warning to
1641       not be output.  (It currently still won't warn if the match isn't even
1642       attempted, like in the "foobar" example above.)
1643
1644       In summary, Perl now normally treats non-Unicode code points as typical
1645       Unicode unassigned code points for regular expression matches, raising
1646       a warning only when it is arguable what the result should be.  However,
1647       if this warning has been made fatal, it isn't skipped.
1648
1649       There is one exception to all this.  "\p{All}" looks like a Unicode
1650       property, but it is a Perl extension that is defined to be true for all
1651       possible code points, Unicode or not, so no warning is ever generated
1652       when matching this against a non-Unicode code point.  (Prior to v5.20,
1653       it was an exact synonym for "\p{Any}", matching code points 0 through
1654       0x10FFFF.)
1655
1656   Security Implications of Unicode
1657       First, read Unicode Security Considerations
1658       <https://www.unicode.org/reports/tr36>.
1659
1660       Also, note the following:
1661
1662       ·   Malformed UTF-8
1663
1664           UTF-8 is very structured, so many combinations of bytes are
1665           invalid.  In the past, Perl tried to soldier on and make some sense
1666           of invalid combinations, but this can lead to security holes, so
1667           now, if the Perl core needs to process an invalid combination, it
1668           will either raise a fatal error, or will replace those bytes by the
1669           sequence that forms the Unicode REPLACEMENT CHARACTER, for which
1670           purpose Unicode created it.
1671
1672           Every code point can be represented by more than one possible
1673           syntactically valid UTF-8 sequence.  Early on, both Unicode and
1674           Perl considered any of these to be valid, but now, all sequences
1675           longer than the shortest possible one are considered to be
1676           malformed.
1677
1678           Unicode considers many code points to be illegal, or to be avoided.
1679           Perl generally accepts them, once they have passed through any
1680           input filters that may try to exclude them.  These have been
1681           discussed above (see "Surrogates" under UTF-16 in "Unicode
1682           Encodings", "Noncharacter code points", and "Beyond Unicode code
1683           points").
1684
1685       ·   Regular expression pattern matching may surprise you if you're not
1686           accustomed to Unicode.  Starting in Perl 5.14, several pattern
1687           modifiers are available to control this, called the character set
1688           modifiers.  Details are given in "Character set modifiers" in
1689           perlre.
1690
1691       As discussed elsewhere, Perl has one foot (two hooves?) planted in each
1692       of two worlds: the old world of ASCII and single-byte locales, and the
1693       new world of Unicode, upgrading when necessary.  If your legacy code
1694       does not explicitly use Unicode, no automatic switch-over to Unicode
1695       should happen.
1696
1697   Unicode in Perl on EBCDIC
1698       Unicode is supported on EBCDIC platforms.  See perlebcdic.
1699
1700       Unless ASCII vs. EBCDIC issues are specifically being discussed,
1701       references to UTF-8 encoding in this document and elsewhere should be
1702       read as meaning UTF-EBCDIC on EBCDIC platforms.  See "Unicode and UTF"
1703       in perlebcdic.
1704
1705       Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
1706       hidden from you; "use utf8" (and NOT something like "use utfebcdic")
1707       declares the script is in the platform's "native" 8-bit encoding of
1708       Unicode.  (Similarly for the ":utf8" layer.)
1709
1710   Locales
1711       See "Unicode and UTF-8" in perllocale
1712
1713   When Unicode Does Not Happen
1714       There are still many places where Unicode (in some encoding or another)
1715       could be given as arguments or received as results, or both in Perl,
1716       but it is not, in spite of Perl having extensive ways to input and
1717       output in Unicode, and a few other "entry points" like the @ARGV array
1718       (which can sometimes be interpreted as UTF-8).
1719
1720       The following are such interfaces.  Also, see "The "Unicode Bug"".  For
1721       all of these interfaces Perl currently (as of v5.16.0) simply assumes
1722       byte strings both as arguments and results, or UTF-8 strings if the
1723       (deprecated) "encoding" pragma has been used.
1724
1725       One reason that Perl does not attempt to resolve the role of Unicode in
1726       these situations is that the answers are highly dependent on the
1727       operating system and the file system(s).  For example, whether
1728       filenames can be in Unicode and in exactly what kind of encoding, is
1729       not exactly a portable concept.  Similarly for "qx" and "system": how
1730       well will the "command-line interface" (and which of them?) handle
1731       Unicode?
1732
1733       ·   "chdir", "chmod", "chown", "chroot", "exec", "link", "lstat",
1734           "mkdir", "rename", "rmdir", "stat", "symlink", "truncate",
1735           "unlink", "utime", "-X"
1736
1737       ·   %ENV
1738
1739       ·   "glob" (aka the "<*>")
1740
1741       ·   "open", "opendir", "sysopen"
1742
1743       ·   "qx" (aka the backtick operator), "system"
1744
1745       ·   "readdir", "readlink"
1746
1747   The "Unicode Bug"
1748       The term, "Unicode bug" has been applied to an inconsistency with the
1749       code points in the "Latin-1 Supplement" block, that is, between 128 and
1750       255.  Without a locale specified, unlike all other characters or code
1751       points, these characters can have very different semantics depending on
1752       the rules in effect.  (Characters whose code points are above 255 force
1753       Unicode rules; whereas the rules for ASCII characters are the same
1754       under both ASCII and Unicode rules.)
1755
1756       Under Unicode rules, these upper-Latin1 characters are interpreted as
1757       Unicode code points, which means they have the same semantics as
1758       Latin-1 (ISO-8859-1) and C1 controls.
1759
1760       As explained in "ASCII Rules versus Unicode Rules", under ASCII rules,
1761       they are considered to be unassigned characters.
1762
1763       This can lead to unexpected results.  For example, a string's semantics
1764       can suddenly change if a code point above 255 is appended to it, which
1765       changes the rules from ASCII to Unicode.  As an example, consider the
1766       following program and its output:
1767
1768        $ perl -le'
1769            no feature "unicode_strings";
1770            $s1 = "\xC2";
1771            $s2 = "\x{2660}";
1772            for ($s1, $s2, $s1.$s2) {
1773                print /\w/ || 0;
1774            }
1775        '
1776        0
1777        0
1778        1
1779
1780       If there's no "\w" in "s1" nor in "s2", why does their concatenation
1781       have one?
1782
1783       This anomaly stems from Perl's attempt to not disturb older programs
1784       that didn't use Unicode, along with Perl's desire to add Unicode
1785       support seamlessly.  But the result turned out to not be seamless.  (By
1786       the way, you can choose to be warned when things like this happen.  See
1787       "encoding::warnings".)
1788
1789       "use feature 'unicode_strings'" was added, starting in Perl v5.12, to
1790       address this problem.  It affects these things:
1791
1792       ·   Changing the case of a scalar, that is, using "uc()", "ucfirst()",
1793           "lc()", and "lcfirst()", or "\L", "\U", "\u" and "\l" in double-
1794           quotish contexts, such as regular expression substitutions.
1795
1796           Under "unicode_strings" starting in Perl 5.12.0, Unicode rules are
1797           generally used.  See "lc" in perlfunc for details on how this works
1798           in combination with various other pragmas.
1799
1800       ·   Using caseless ("/i") regular expression matching.
1801
1802           Starting in Perl 5.14.0, regular expressions compiled within the
1803           scope of "unicode_strings" use Unicode rules even when executed or
1804           compiled into larger regular expressions outside the scope.
1805
1806       ·   Matching any of several properties in regular expressions.
1807
1808           These properties are "\b" (without braces), "\B" (without braces),
1809           "\s", "\S", "\w", "\W", and all the Posix character classes except
1810           "[[:ascii:]]".
1811
1812           Starting in Perl 5.14.0, regular expressions compiled within the
1813           scope of "unicode_strings" use Unicode rules even when executed or
1814           compiled into larger regular expressions outside the scope.
1815
1816       ·   In "quotemeta" or its inline equivalent "\Q".
1817
1818           Starting in Perl 5.16.0, consistent quoting rules are used within
1819           the scope of "unicode_strings", as described in "quotemeta" in
1820           perlfunc.  Prior to that, or outside its scope, no code points
1821           above 127 are quoted in UTF-8 encoded strings, but in byte encoded
1822           strings, code points between 128-255 are always quoted.
1823
1824       ·   In the ".." or range operator.
1825
1826           Starting in Perl 5.26.0, the range operator on strings treats their
1827           lengths consistently within the scope of "unicode_strings". Prior
1828           to that, or outside its scope, it could produce strings whose
1829           length in characters exceeded that of the right-hand side, where
1830           the right-hand side took up more bytes than the correct range
1831           endpoint.
1832
1833       ·   In "split"'s special-case whitespace splitting.
1834
1835           Starting in Perl 5.28.0, the "split" function with a pattern
1836           specified as a string containing a single space handles whitespace
1837           characters consistently within the scope of "unicode_strings".
1838           Prior to that, or outside its scope, characters that are whitespace
1839           according to Unicode rules but not according to ASCII rules were
1840           treated as field contents rather than field separators when they
1841           appear in byte-encoded strings.
1842
1843       You can see from the above that the effect of "unicode_strings"
1844       increased over several Perl releases.  (And Perl's support for Unicode
1845       continues to improve; it's best to use the latest available release in
1846       order to get the most complete and accurate results possible.)  Note
1847       that "unicode_strings" is automatically chosen if you "use 5.012" or
1848       higher.
1849
1850       For Perls earlier than those described above, or when a string is
1851       passed to a function outside the scope of "unicode_strings", see the
1852       next section.
1853
1854   Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
1855       Sometimes (see "When Unicode Does Not Happen" or "The "Unicode Bug"")
1856       there are situations where you simply need to force a byte string into
1857       UTF-8, or vice versa.  The standard module Encode can be used for this,
1858       or the low-level calls "utf8::upgrade($bytestring)" and
1859       "utf8::downgrade($utf8string[, FAIL_OK])".
1860
1861       Note that "utf8::downgrade()" can fail if the string contains
1862       characters that don't fit into a byte.
1863
1864       Calling either function on a string that already is in the desired
1865       state is a no-op.
1866
1867       "ASCII Rules versus Unicode Rules" gives all the ways that a string is
1868       made to use Unicode rules.
1869
1870   Using Unicode in XS
1871       See "Unicode Support" in perlguts for an introduction to Unicode at the
1872       XS level, and "Unicode Support" in perlapi for the API details.
1873
1874   Hacking Perl to work on earlier Unicode versions (for very serious hackers
1875       only)
1876       Perl by default comes with the latest supported Unicode version built-
1877       in, but the goal is to allow you to change to use any earlier one.  In
1878       Perls v5.20 and v5.22, however, the earliest usable version is Unicode
1879       5.1.  Perl v5.18 and v5.24 are able to handle all earlier versions.
1880
1881       Download the files in the desired version of Unicode from the Unicode
1882       web site <https://www.unicode.org>).  These should replace the existing
1883       files in lib/unicore in the Perl source tree.  Follow the instructions
1884       in README.perl in that directory to change some of their names, and
1885       then build perl (see INSTALL).
1886
1887   Porting code from perl-5.6.X
1888       Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6
1889       the programmer was required to use the "utf8" pragma to declare that a
1890       given scope expected to deal with Unicode data and had to make sure
1891       that only Unicode data were reaching that scope. If you have code that
1892       is working with 5.6, you will need some of the following adjustments to
1893       your code. The examples are written such that the code will continue to
1894       work under 5.6, so you should be safe to try them out.
1895
1896       ·  A filehandle that should read or write UTF-8
1897
1898            if ($] > 5.008) {
1899              binmode $fh, ":encoding(UTF-8)";
1900            }
1901
1902       ·  A scalar that is going to be passed to some extension
1903
1904          Be it "Compress::Zlib", "Apache::Request" or any extension that has
1905          no mention of Unicode in the manpage, you need to make sure that the
1906          UTF8 flag is stripped off. Note that at the time of this writing
1907          (January 2012) the mentioned modules are not UTF-8-aware. Please
1908          check the documentation to verify if this is still true.
1909
1910            if ($] > 5.008) {
1911              require Encode;
1912              $val = Encode::encode("UTF-8", $val); # make octets
1913            }
1914
1915       ·  A scalar we got back from an extension
1916
1917          If you believe the scalar comes back as UTF-8, you will most likely
1918          want the UTF8 flag restored:
1919
1920            if ($] > 5.008) {
1921              require Encode;
1922              $val = Encode::decode("UTF-8", $val);
1923            }
1924
1925       ·  Same thing, if you are really sure it is UTF-8
1926
1927            if ($] > 5.008) {
1928              require Encode;
1929              Encode::_utf8_on($val);
1930            }
1931
1932       ·  A wrapper for DBI "fetchrow_array" and "fetchrow_hashref"
1933
1934          When the database contains only UTF-8, a wrapper function or method
1935          is a convenient way to replace all your "fetchrow_array" and
1936          "fetchrow_hashref" calls. A wrapper function will also make it
1937          easier to adapt to future enhancements in your database driver. Note
1938          that at the time of this writing (January 2012), the DBI has no
1939          standardized way to deal with UTF-8 data. Please check the DBI
1940          documentation to verify if that is still true.
1941
1942            sub fetchrow {
1943              # $what is one of fetchrow_{array,hashref}
1944              my($self, $sth, $what) = @_;
1945              if ($] < 5.008) {
1946                return $sth->$what;
1947              } else {
1948                require Encode;
1949                if (wantarray) {
1950                  my @arr = $sth->$what;
1951                  for (@arr) {
1952                    defined && /[^\000-\177]/ && Encode::_utf8_on($_);
1953                  }
1954                  return @arr;
1955                } else {
1956                  my $ret = $sth->$what;
1957                  if (ref $ret) {
1958                    for my $k (keys %$ret) {
1959                      defined
1960                      && /[^\000-\177]/
1961                      && Encode::_utf8_on($_) for $ret->{$k};
1962                    }
1963                    return $ret;
1964                  } else {
1965                    defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
1966                    return $ret;
1967                  }
1968                }
1969              }
1970            }
1971
1972       ·  A large scalar that you know can only contain ASCII
1973
1974          Scalars that contain only ASCII and are marked as UTF-8 are
1975          sometimes a drag to your program. If you recognize such a situation,
1976          just remove the UTF8 flag:
1977
1978            utf8::downgrade($val) if $] > 5.008;
1979

BUGS

1981       See also "The "Unicode Bug"" above.
1982
1983   Interaction with Extensions
1984       When Perl exchanges data with an extension, the extension should be
1985       able to understand the UTF8 flag and act accordingly. If the extension
1986       doesn't recognize that flag, it's likely that the extension will return
1987       incorrectly-flagged data.
1988
1989       So if you're working with Unicode data, consult the documentation of
1990       every module you're using if there are any issues with Unicode data
1991       exchange. If the documentation does not talk about Unicode at all,
1992       suspect the worst and probably look at the source to learn how the
1993       module is implemented. Modules written completely in Perl shouldn't
1994       cause problems. Modules that directly or indirectly access code written
1995       in other programming languages are at risk.
1996
1997       For affected functions, the simple strategy to avoid data corruption is
1998       to always make the encoding of the exchanged data explicit. Choose an
1999       encoding that you know the extension can handle. Convert arguments
2000       passed to the extensions to that encoding and convert results back from
2001       that encoding. Write wrapper functions that do the conversions for you,
2002       so you can later change the functions when the extension catches up.
2003
2004       To provide an example, let's say the popular "Foo::Bar::escape_html"
2005       function doesn't deal with Unicode data yet. The wrapper function would
2006       convert the argument to raw UTF-8 and convert the result back to Perl's
2007       internal representation like so:
2008
2009           sub my_escape_html ($) {
2010               my($what) = shift;
2011               return unless defined $what;
2012               Encode::decode("UTF-8", Foo::Bar::escape_html(
2013                                            Encode::encode("UTF-8", $what)));
2014           }
2015
2016       Sometimes, when the extension does not convert data but just stores and
2017       retrieves it, you will be able to use the otherwise dangerous
2018       "Encode::_utf8_on()" function. Let's say the popular "Foo::Bar"
2019       extension, written in C, provides a "param" method that lets you store
2020       and retrieve data according to these prototypes:
2021
2022           $self->param($name, $value);            # set a scalar
2023           $value = $self->param($name);           # retrieve a scalar
2024
2025       If it does not yet provide support for any encoding, one could write a
2026       derived class with such a "param" method:
2027
2028           sub param {
2029             my($self,$name,$value) = @_;
2030             utf8::upgrade($name);     # make sure it is UTF-8 encoded
2031             if (defined $value) {
2032               utf8::upgrade($value);  # make sure it is UTF-8 encoded
2033               return $self->SUPER::param($name,$value);
2034             } else {
2035               my $ret = $self->SUPER::param($name);
2036               Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
2037               return $ret;
2038             }
2039           }
2040
2041       Some extensions provide filters on data entry/exit points, such as
2042       "DB_File::filter_store_key" and family. Look out for such filters in
2043       the documentation of your extensions; they can make the transition to
2044       Unicode data much easier.
2045
2046   Speed
2047       Some functions are slower when working on UTF-8 encoded strings than on
2048       byte encoded strings.  All functions that need to hop over characters
2049       such as "length()", "substr()" or "index()", or matching regular
2050       expressions can work much faster when the underlying data are byte-
2051       encoded.
2052
2053       In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a
2054       caching scheme was introduced which improved the situation.  In
2055       general, operations with UTF-8 encoded strings are still slower. As an
2056       example, the Unicode properties (character classes) like "\p{Nd}" are
2057       known to be quite a bit slower (5-20 times) than their simpler
2058       counterparts like "[0-9]" (then again, there are hundreds of Unicode
2059       characters matching "Nd" compared with the 10 ASCII characters matching
2060       "[0-9]").
2061

SEE ALSO

2063       perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes,
2064       perlretut, "${^UNICODE}" in perlvar,
2065       <https://www.unicode.org/reports/tr44>).
2066
2067
2068
2069perl v5.32.1                      2021-03-31                    PERLUNICODE(1)
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