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Add Unicode property wildcards
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1=head1 NAME
2
3perlunicode - Unicode support in Perl
4
5=head1 DESCRIPTION
6
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7If you haven't already, before reading this document, you should become
8familiar with both L<perlunitut> and L<perluniintro>.
9
10Unicode aims to B<UNI>-fy the en-B<CODE>-ings of all the world's
11character sets into a single Standard. For quite a few of the various
12coding standards that existed when Unicode was first created, converting
13from each to Unicode essentially meant adding a constant to each code
14point in the original standard, and converting back meant just
15subtracting that same constant. For ASCII and ISO-8859-1, the constant
16is 0. For ISO-8859-5, (Cyrillic) the constant is 864; for Hebrew
17(ISO-8859-8), it's 1488; Thai (ISO-8859-11), 3424; and so forth. This
18made it easy to do the conversions, and facilitated the adoption of
19Unicode.
20
21And it worked; nowadays, those legacy standards are rarely used. Most
22everyone uses Unicode.
23
24Unicode is a comprehensive standard. It specifies many things outside
25the scope of Perl, such as how to display sequences of characters. For
26a full discussion of all aspects of Unicode, see
27L<http://www.unicode.org>.
28
0a1f2d14 29=head2 Important Caveats
21bad921 30
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31Even though some of this section may not be understandable to you on
32first reading, we think it's important enough to highlight some of the
33gotchas before delving further, so here goes:
34
376d9008 35Unicode support is an extensive requirement. While Perl does not
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36implement the Unicode standard or the accompanying technical reports
37from cover to cover, Perl does support many Unicode features.
21bad921 38
f57d8456 39Also, the use of Unicode may present security issues that aren't
526f2ca9 40obvious, see L</Security Implications of Unicode> below.
9d1c51c1 41
13a2d996 42=over 4
21bad921 43
a9130ea9 44=item Safest if you C<use feature 'unicode_strings'>
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45
46In order to preserve backward compatibility, Perl does not turn
47on full internal Unicode support unless the pragma
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48L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
49is specified. (This is automatically
50selected if you S<C<use 5.012>> or higher.) Failure to do this can
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51trigger unexpected surprises. See L</The "Unicode Bug"> below.
52
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53This pragma doesn't affect I/O. Nor does it change the internal
54representation of strings, only their interpretation. There are still
55several places where Unicode isn't fully supported, such as in
56filenames.
42581d5d 57
fae2c0fb 58=item Input and Output Layers
21bad921 59
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60Use the C<:encoding(...)> layer to read from and write to
61filehandles using the specified encoding. (See L<open>.)
c349b1b9 62
01c3fbbc 63=item You must convert your non-ASCII, non-UTF-8 Perl scripts to be
a6a7eedc 64UTF-8.
21bad921 65
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66The L<encoding> module has been deprecated since perl 5.18 and the
67perl internals it requires have been removed with perl 5.26.
21bad921 68
a6a7eedc 69=item C<use utf8> still needed to enable L<UTF-8|/Unicode Encodings> in scripts
21bad921 70
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71If your Perl script is itself encoded in L<UTF-8|/Unicode Encodings>,
72the S<C<use utf8>> pragma must be explicitly included to enable
73recognition of that (in string or regular expression literals, or in
74identifier names). B<This is the only time when an explicit S<C<use
75utf8>> is needed.> (See L<utf8>).
7aa207d6 76
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77If a Perl script begins with the bytes that form the UTF-8 encoding of
78the Unicode BYTE ORDER MARK (C<BOM>, see L</Unicode Encodings>), those
79bytes are completely ignored.
80
81=item L<UTF-16|/Unicode Encodings> scripts autodetected
7aa207d6 82
fea12a3e 83If a Perl script begins with the Unicode C<BOM> (UTF-16LE,
27c74dfd 84UTF16-BE), or if the script looks like non-C<BOM>-marked
a6a7eedc 85UTF-16 of either endianness, Perl will correctly read in the script as
27c74dfd 86the appropriate Unicode encoding.
990e18f7 87
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88=back
89
376d9008 90=head2 Byte and Character Semantics
393fec97 91
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92Before Unicode, most encodings used 8 bits (a single byte) to encode
93each character. Thus a character was a byte, and a byte was a
94character, and there could be only 256 or fewer possible characters.
95"Byte Semantics" in the title of this section refers to
96this behavior. There was no need to distinguish between "Byte" and
97"Character".
98
99Then along comes Unicode which has room for over a million characters
100(and Perl allows for even more). This means that a character may
101require more than a single byte to represent it, and so the two terms
102are no longer equivalent. What matter are the characters as whole
103entities, and not usually the bytes that comprise them. That's what the
104term "Character Semantics" in the title of this section refers to.
105
106Perl had to change internally to decouple "bytes" from "characters".
107It is important that you too change your ideas, if you haven't already,
108so that "byte" and "character" no longer mean the same thing in your
109mind.
110
111The basic building block of Perl strings has always been a "character".
112The changes basically come down to that the implementation no longer
113thinks that a character is always just a single byte.
114
115There are various things to note:
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116
117=over 4
118
119=item *
120
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121String handling functions, for the most part, continue to operate in
122terms of characters. C<length()>, for example, returns the number of
123characters in a string, just as before. But that number no longer is
124necessarily the same as the number of bytes in the string (there may be
125more bytes than characters). The other such functions include
126C<chop()>, C<chomp()>, C<substr()>, C<pos()>, C<index()>, C<rindex()>,
127C<sort()>, C<sprintf()>, and C<write()>.
128
129The exceptions are:
130
131=over 4
132
133=item *
134
135the bit-oriented C<vec>
136
137E<nbsp>
138
139=item *
140
141the byte-oriented C<pack>/C<unpack> C<"C"> format
142
143However, the C<W> specifier does operate on whole characters, as does the
144C<U> specifier.
145
146=item *
147
148some operators that interact with the platform's operating system
149
150Operators dealing with filenames are examples.
151
152=item *
153
154when the functions are called from within the scope of the
155S<C<L<use bytes|bytes>>> pragma
156
157Likely, you should use this only for debugging anyway.
158
159=back
160
161=item *
162
376d9008 163Strings--including hash keys--and regular expression patterns may
b65e6125 164contain characters that have ordinal values larger than 255.
393fec97 165
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166If you use a Unicode editor to edit your program, Unicode characters may
167occur directly within the literal strings in UTF-8 encoding, or UTF-16.
27c74dfd 168(The former requires a C<use utf8>, the latter may require a C<BOM>.)
3e4dbfed 169
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170L<perluniintro/Creating Unicode> gives other ways to place non-ASCII
171characters in your strings.
6f335b04 172
a6a7eedc 173=item *
fbb93542 174
a6a7eedc 175The C<chr()> and C<ord()> functions work on whole characters.
376d9008 176
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177=item *
178
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179Regular expressions match whole characters. For example, C<"."> matches
180a whole character instead of only a single byte.
393fec97 181
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182=item *
183
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184The C<tr///> operator translates whole characters. (Note that the
185C<tr///CU> functionality has been removed. For similar functionality to
186that, see C<pack('U0', ...)> and C<pack('C0', ...)>).
393fec97 187
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188=item *
189
a6a7eedc 190C<scalar reverse()> reverses by character rather than by byte.
393fec97 191
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192=item *
193
a6a7eedc 194The bit string operators, C<& | ^ ~> and (starting in v5.22)
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195C<&. |. ^. ~.> can operate on bit strings encoded in UTF-8, but this
196can give unexpected results if any of the strings contain code points
197above 0xFF. Starting in v5.28, it is a fatal error to have such an
198operand. Otherwise, the operation is performed on a non-UTF-8 copy of
199the operand. If you're not sure about the encoding of a string,
200downgrade it before using any of these operators; you can use
a6a7eedc 201L<C<utf8::utf8_downgrade()>|utf8/Utility functions>.
822502e5 202
a6a7eedc 203=back
822502e5 204
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205The bottom line is that Perl has always practiced "Character Semantics",
206but with the advent of Unicode, that is now different than "Byte
207Semantics".
208
209=head2 ASCII Rules versus Unicode Rules
210
211Before Unicode, when a character was a byte was a character,
212Perl knew only about the 128 characters defined by ASCII, code points 0
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213through 127 (except for under L<S<C<use locale>>|perllocale>). That
214left the code
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215points 128 to 255 as unassigned, and available for whatever use a
216program might want. The only semantics they have is their ordinal
217numbers, and that they are members of none of the non-negative character
218classes. None are considered to match C<\w> for example, but all match
219C<\W>.
822502e5 220
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221Unicode, of course, assigns each of those code points a particular
222meaning (along with ones above 255). To preserve backward
223compatibility, Perl only uses the Unicode meanings when there is some
224indication that Unicode is what is intended; otherwise the non-ASCII
225code points remain treated as if they are unassigned.
226
227Here are the ways that Perl knows that a string should be treated as
228Unicode:
229
230=over
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231
232=item *
233
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234Within the scope of S<C<use utf8>>
235
236If the whole program is Unicode (signified by using 8-bit B<U>nicode
e423fa83 237B<T>ransformation B<F>ormat), then all literal strings within it must be
a6a7eedc 238Unicode.
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239
240=item *
241
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242Within the scope of
243L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
244
245This pragma was created so you can explicitly tell Perl that operations
246executed within its scope are to use Unicode rules. More operations are
247affected with newer perls. See L</The "Unicode Bug">.
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248
249=item *
250
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251Within the scope of S<C<use 5.012>> or higher
252
253This implicitly turns on S<C<use feature 'unicode_strings'>>.
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254
255=item *
256
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257Within the scope of
258L<S<C<use locale 'not_characters'>>|perllocale/Unicode and UTF-8>,
259or L<S<C<use locale>>|perllocale> and the current
260locale is a UTF-8 locale.
822502e5 261
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262The former is defined to imply Unicode handling; and the latter
263indicates a Unicode locale, hence a Unicode interpretation of all
264strings within it.
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265
266=item *
267
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268When the string contains a Unicode-only code point
269
270Perl has never accepted code points above 255 without them being
271Unicode, so their use implies Unicode for the whole string.
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272
273=item *
274
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275When the string contains a Unicode named code point C<\N{...}>
276
277The C<\N{...}> construct explicitly refers to a Unicode code point,
278even if it is one that is also in ASCII. Therefore the string
279containing it must be Unicode.
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280
281=item *
282
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283When the string has come from an external source marked as
284Unicode
285
286The L<C<-C>|perlrun/-C [numberE<sol>list]> command line option can
287specify that certain inputs to the program are Unicode, and the values
288of this can be read by your Perl code, see L<perlvar/"${^UNICODE}">.
289
290=item * When the string has been upgraded to UTF-8
291
292The function L<C<utf8::utf8_upgrade()>|utf8/Utility functions>
293can be explicitly used to permanently (unless a subsequent
294C<utf8::utf8_downgrade()> is called) cause a string to be treated as
295Unicode.
296
297=item * There are additional methods for regular expression patterns
298
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299A pattern that is compiled with the C<< /u >> or C<< /a >> modifiers is
300treated as Unicode (though there are some restrictions with C<< /a >>).
301Under the C<< /d >> and C<< /l >> modifiers, there are several other
302indications for Unicode; see L<perlre/Character set modifiers>.
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303
304=back
305
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306Note that all of the above are overridden within the scope of
307C<L<use bytes|bytes>>; but you should be using this pragma only for
308debugging.
309
310Note also that some interactions with the platform's operating system
311never use Unicode rules.
312
313When Unicode rules are in effect:
314
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315=over 4
316
317=item *
318
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319Case translation operators use the Unicode case translation tables.
320
321Note that C<uc()>, or C<\U> in interpolated strings, translates to
322uppercase, while C<ucfirst>, or C<\u> in interpolated strings,
323translates to titlecase in languages that make the distinction (which is
324equivalent to uppercase in languages without the distinction).
325
326There is a CPAN module, C<L<Unicode::Casing>>, which allows you to
327define your own mappings to be used in C<lc()>, C<lcfirst()>, C<uc()>,
328C<ucfirst()>, and C<fc> (or their double-quoted string inlined versions
329such as C<\U>). (Prior to Perl 5.16, this functionality was partially
330provided in the Perl core, but suffered from a number of insurmountable
331drawbacks, so the CPAN module was written instead.)
332
333=item *
334
335Character classes in regular expressions match based on the character
336properties specified in the Unicode properties database.
337
338C<\w> can be used to match a Japanese ideograph, for instance; and
339C<[[:digit:]]> a Bengali number.
340
341=item *
342
343Named Unicode properties, scripts, and block ranges may be used (like
344bracketed character classes) by using the C<\p{}> "matches property"
345construct and the C<\P{}> negation, "doesn't match property".
346
347See L</"Unicode Character Properties"> for more details.
348
349You can define your own character properties and use them
350in the regular expression with the C<\p{}> or C<\P{}> construct.
351See L</"User-Defined Character Properties"> for more details.
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352
353=back
354
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355=head2 Extended Grapheme Clusters (Logical characters)
356
357Consider a character, say C<H>. It could appear with various marks around it,
358such as an acute accent, or a circumflex, or various hooks, circles, arrows,
359I<etc.>, above, below, to one side or the other, I<etc>. There are many
360possibilities among the world's languages. The number of combinations is
361astronomical, and if there were a character for each combination, it would
362soon exhaust Unicode's more than a million possible characters. So Unicode
363took a different approach: there is a character for the base C<H>, and a
364character for each of the possible marks, and these can be variously combined
365to get a final logical character. So a logical character--what appears to be a
366single character--can be a sequence of more than one individual characters.
367The Unicode standard calls these "extended grapheme clusters" (which
368is an improved version of the no-longer much used "grapheme cluster");
369Perl furnishes the C<\X> regular expression construct to match such
370sequences in their entirety.
371
372But Unicode's intent is to unify the existing character set standards and
373practices, and several pre-existing standards have single characters that
374mean the same thing as some of these combinations, like ISO-8859-1,
375which has quite a few of them. For example, C<"LATIN CAPITAL LETTER E
376WITH ACUTE"> was already in this standard when Unicode came along.
377Unicode therefore added it to its repertoire as that single character.
378But this character is considered by Unicode to be equivalent to the
379sequence consisting of the character C<"LATIN CAPITAL LETTER E">
380followed by the character C<"COMBINING ACUTE ACCENT">.
381
382C<"LATIN CAPITAL LETTER E WITH ACUTE"> is called a "pre-composed"
383character, and its equivalence with the "E" and the "COMBINING ACCENT"
384sequence is called canonical equivalence. All pre-composed characters
385are said to have a decomposition (into the equivalent sequence), and the
386decomposition type is also called canonical. A string may be comprised
387as much as possible of precomposed characters, or it may be comprised of
388entirely decomposed characters. Unicode calls these respectively,
389"Normalization Form Composed" (NFC) and "Normalization Form Decomposed".
390The C<L<Unicode::Normalize>> module contains functions that convert
391between the two. A string may also have both composed characters and
392decomposed characters; this module can be used to make it all one or the
393other.
394
395You may be presented with strings in any of these equivalent forms.
396There is currently nothing in Perl 5 that ignores the differences. So
dabde021 397you'll have to specially handle it. The usual advice is to convert your
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398inputs to C<NFD> before processing further.
399
400For more detailed information, see L<http://unicode.org/reports/tr15/>.
401
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402=head2 Unicode Character Properties
403
ee88f7b6 404(The only time that Perl considers a sequence of individual code
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405points as a single logical character is in the C<\X> construct, already
406mentioned above. Therefore "character" in this discussion means a single
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407Unicode code point.)
408
409Very nearly all Unicode character properties are accessible through
410regular expressions by using the C<\p{}> "matches property" construct
411and the C<\P{}> "doesn't match property" for its negation.
51f494cc 412
9d1c51c1 413For instance, C<\p{Uppercase}> matches any single character with the Unicode
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414C<"Uppercase"> property, while C<\p{L}> matches any character with a
415C<General_Category> of C<"L"> (letter) property (see
416L</General_Category> below). Brackets are not
9d1c51c1 417required for single letter property names, so C<\p{L}> is equivalent to C<\pL>.
51f494cc 418
9d1c51c1 419More formally, C<\p{Uppercase}> matches any single character whose Unicode
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420C<Uppercase> property value is C<True>, and C<\P{Uppercase}> matches any character
421whose C<Uppercase> property value is C<False>, and they could have been written as
9d1c51c1 422C<\p{Uppercase=True}> and C<\p{Uppercase=False}>, respectively.
51f494cc 423
b19eb496 424This formality is needed when properties are not binary; that is, if they can
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425take on more values than just C<True> and C<False>. For example, the
426C<Bidi_Class> property (see L</"Bidirectional Character Types"> below),
427can take on several different
428values, such as C<Left>, C<Right>, C<Whitespace>, and others. To match these, one needs
429to specify both the property name (C<Bidi_Class>), AND the value being
5bff2035 430matched against
b65e6125 431(C<Left>, C<Right>, I<etc.>). This is done, as in the examples above, by having the
9f815e24 432two components separated by an equal sign (or interchangeably, a colon), like
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433C<\p{Bidi_Class: Left}>.
434
435All Unicode-defined character properties may be written in these compound forms
a9130ea9 436of C<\p{I<property>=I<value>}> or C<\p{I<property>:I<value>}>, but Perl provides some
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437additional properties that are written only in the single form, as well as
438single-form short-cuts for all binary properties and certain others described
439below, in which you may omit the property name and the equals or colon
440separator.
441
442Most Unicode character properties have at least two synonyms (or aliases if you
b19eb496 443prefer): a short one that is easier to type and a longer one that is more
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444descriptive and hence easier to understand. Thus the C<"L"> and
445C<"Letter"> properties above are equivalent and can be used
446interchangeably. Likewise, C<"Upper"> is a synonym for C<"Uppercase">,
447and we could have written C<\p{Uppercase}> equivalently as C<\p{Upper}>.
448Also, there are typically various synonyms for the values the property
449can be. For binary properties, C<"True"> has 3 synonyms: C<"T">,
450C<"Yes">, and C<"Y">; and C<"False"> has correspondingly C<"F">,
451C<"No">, and C<"N">. But be careful. A short form of a value for one
452property may not mean the same thing as the same short form for another.
453Thus, for the C<L</General_Category>> property, C<"L"> means
454C<"Letter">, but for the L<C<Bidi_Class>|/Bidirectional Character Types>
455property, C<"L"> means C<"Left">. A complete list of properties and
456synonyms is in L<perluniprops>.
51f494cc 457
b19eb496 458Upper/lower case differences in property names and values are irrelevant;
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459thus C<\p{Upper}> means the same thing as C<\p{upper}> or even C<\p{UpPeR}>.
460Similarly, you can add or subtract underscores anywhere in the middle of a
461word, so that these are also equivalent to C<\p{U_p_p_e_r}>. And white space
462is irrelevant adjacent to non-word characters, such as the braces and the equals
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463or colon separators, so C<\p{ Upper }> and C<\p{ Upper_case : Y }> are
464equivalent to these as well. In fact, white space and even
465hyphens can usually be added or deleted anywhere. So even C<\p{ Up-per case = Yes}> is
51f494cc 466equivalent. All this is called "loose-matching" by Unicode. The few places
b19eb496 467where stricter matching is used is in the middle of numbers, and in the Perl
51f494cc 468extension properties that begin or end with an underscore. Stricter matching
b19eb496 469cares about white space (except adjacent to non-word characters),
51f494cc 470hyphens, and non-interior underscores.
4193bef7 471
376d9008 472You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret
a9130ea9 473(C<^>) between the first brace and the property name: C<\p{^Tamil}> is
eb0cc9e3 474equal to C<\P{Tamil}>.
4193bef7 475
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476Almost all properties are immune to case-insensitive matching. That is,
477adding a C</i> regular expression modifier does not change what they
478match. There are two sets that are affected.
479The first set is
480C<Uppercase_Letter>,
481C<Lowercase_Letter>,
482and C<Titlecase_Letter>,
483all of which match C<Cased_Letter> under C</i> matching.
484And the second set is
485C<Uppercase>,
486C<Lowercase>,
487and C<Titlecase>,
488all of which match C<Cased> under C</i> matching.
489This set also includes its subsets C<PosixUpper> and C<PosixLower> both
a9130ea9 490of which under C</i> match C<PosixAlpha>.
56ca34ca 491(The difference between these sets is that some things, such as Roman
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492numerals, come in both upper and lower case so they are C<Cased>, but
493aren't considered letters, so they aren't C<Cased_Letter>'s.)
56ca34ca 494
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495See L</Beyond Unicode code points> for special considerations when
496matching Unicode properties against non-Unicode code points.
94b42e47 497
51f494cc 498=head3 B<General_Category>
14bb0a9a 499
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500Every Unicode character is assigned a general category, which is the "most
501usual categorization of a character" (from
502L<http://www.unicode.org/reports/tr44>).
822502e5 503
9f815e24 504The compound way of writing these is like C<\p{General_Category=Number}>
b65e6125 505(short: C<\p{gc:n}>). But Perl furnishes shortcuts in which everything up
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506through the equal or colon separator is omitted. So you can instead just write
507C<\pN>.
822502e5 508
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509Here are the short and long forms of the values the C<General Category> property
510can have:
393fec97 511
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512 Short Long
513
514 L Letter
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515 LC, L& Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
516 Lu Uppercase_Letter
517 Ll Lowercase_Letter
518 Lt Titlecase_Letter
519 Lm Modifier_Letter
520 Lo Other_Letter
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521
522 M Mark
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523 Mn Nonspacing_Mark
524 Mc Spacing_Mark
525 Me Enclosing_Mark
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526
527 N Number
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528 Nd Decimal_Number (also Digit)
529 Nl Letter_Number
530 No Other_Number
531
532 P Punctuation (also Punct)
533 Pc Connector_Punctuation
534 Pd Dash_Punctuation
535 Ps Open_Punctuation
536 Pe Close_Punctuation
537 Pi Initial_Punctuation
d73e5302 538 (may behave like Ps or Pe depending on usage)
51f494cc 539 Pf Final_Punctuation
d73e5302 540 (may behave like Ps or Pe depending on usage)
51f494cc 541 Po Other_Punctuation
d73e5302
JH
542
543 S Symbol
51f494cc
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544 Sm Math_Symbol
545 Sc Currency_Symbol
546 Sk Modifier_Symbol
547 So Other_Symbol
d73e5302
JH
548
549 Z Separator
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550 Zs Space_Separator
551 Zl Line_Separator
552 Zp Paragraph_Separator
d73e5302
JH
553
554 C Other
d88362ca 555 Cc Control (also Cntrl)
e150c829 556 Cf Format
6d4f9cf2 557 Cs Surrogate
51f494cc 558 Co Private_Use
e150c829 559 Cn Unassigned
1ac13f9a 560
376d9008 561Single-letter properties match all characters in any of the
3e4dbfed 562two-letter sub-properties starting with the same letter.
b19eb496 563C<LC> and C<L&> are special: both are aliases for the set consisting of everything matched by C<Ll>, C<Lu>, and C<Lt>.
32293815 564
51f494cc 565=head3 B<Bidirectional Character Types>
822502e5 566
b19eb496 567Because scripts differ in their directionality (Hebrew and Arabic are
a9130ea9 568written right to left, for example) Unicode supplies a C<Bidi_Class> property.
1850f57f 569Some of the values this property can have are:
32293815 570
88af3b93 571 Value Meaning
92e830a9 572
12ac2576
JP
573 L Left-to-Right
574 LRE Left-to-Right Embedding
575 LRO Left-to-Right Override
576 R Right-to-Left
51f494cc 577 AL Arabic Letter
12ac2576
JP
578 RLE Right-to-Left Embedding
579 RLO Right-to-Left Override
580 PDF Pop Directional Format
581 EN European Number
51f494cc
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582 ES European Separator
583 ET European Terminator
12ac2576 584 AN Arabic Number
51f494cc 585 CS Common Separator
12ac2576
JP
586 NSM Non-Spacing Mark
587 BN Boundary Neutral
588 B Paragraph Separator
589 S Segment Separator
590 WS Whitespace
591 ON Other Neutrals
592
51f494cc
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593This property is always written in the compound form.
594For example, C<\p{Bidi_Class:R}> matches characters that are normally
1850f57f 595written right to left. Unlike the
a9130ea9 596C<L</General_Category>> property, this
1850f57f
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597property can have more values added in a future Unicode release. Those
598listed above comprised the complete set for many Unicode releases, but
599others were added in Unicode 6.3; you can always find what the
20ada7da 600current ones are in L<perluniprops>. And
1850f57f 601L<http://www.unicode.org/reports/tr9/> describes how to use them.
eb0cc9e3 602
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603=head3 B<Scripts>
604
b19eb496 605The world's languages are written in many different scripts. This sentence
e1b711da 606(unless you're reading it in translation) is written in Latin, while Russian is
c69ca1d4 607written in Cyrillic, and Greek is written in, well, Greek; Japanese mainly in
e1b711da 608Hiragana or Katakana. There are many more.
51f494cc 609
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610The Unicode C<Script> and C<Script_Extensions> properties give what
611script a given character is in. The C<Script_Extensions> property is an
612improved version of C<Script>, as demonstrated below. Either property
613can be specified with the compound form like
82aed44a
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614C<\p{Script=Hebrew}> (short: C<\p{sc=hebr}>), or
615C<\p{Script_Extensions=Javanese}> (short: C<\p{scx=java}>).
616In addition, Perl furnishes shortcuts for all
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617C<Script_Extensions> property names. You can omit everything up through
618the equals (or colon), and simply write C<\p{Latin}> or C<\P{Cyrillic}>.
619(This is not true for C<Script>, which is required to be
620written in the compound form. Prior to Perl v5.26, the single form
621returned the plain old C<Script> version, but was changed because
622C<Script_Extensions> gives better results.)
82aed44a
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623
624The difference between these two properties involves characters that are
625used in multiple scripts. For example the digits '0' through '9' are
626used in many parts of the world. These are placed in a script named
627C<Common>. Other characters are used in just a few scripts. For
a9130ea9 628example, the C<"KATAKANA-HIRAGANA DOUBLE HYPHEN"> is used in both Japanese
82aed44a
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629scripts, Katakana and Hiragana, but nowhere else. The C<Script>
630property places all characters that are used in multiple scripts in the
631C<Common> script, while the C<Script_Extensions> property places those
632that are used in only a few scripts into each of those scripts; while
633still using C<Common> for those used in many scripts. Thus both these
634match:
635
636 "0" =~ /\p{sc=Common}/ # Matches
637 "0" =~ /\p{scx=Common}/ # Matches
638
639and only the first of these match:
640
641 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common} # Matches
642 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match
643
644And only the last two of these match:
645
646 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana} # No match
647 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana} # No match
648 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
649 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches
650
651C<Script_Extensions> is thus an improved C<Script>, in which there are
652fewer characters in the C<Common> script, and correspondingly more in
653other scripts. It is new in Unicode version 6.0, and its data are likely
654to change significantly in later releases, as things get sorted out.
b65e6125 655New code should probably be using C<Script_Extensions> and not plain
48791bf1
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656C<Script>. If you compile perl with a Unicode release that doesn't have
657C<Script_Extensions>, the single form Perl extensions will instead refer
658to the plain C<Script> property. If you compile with a version of
659Unicode that doesn't have the C<Script> property, these extensions will
660not be defined at all.
82aed44a
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661
662(Actually, besides C<Common>, the C<Inherited> script, contains
663characters that are used in multiple scripts. These are modifier
b65e6125 664characters which inherit the script value
82aed44a
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665of the controlling character. Some of these are used in many scripts,
666and so go into C<Inherited> in both C<Script> and C<Script_Extensions>.
667Others are used in just a few scripts, so are in C<Inherited> in
668C<Script>, but not in C<Script_Extensions>.)
669
670It is worth stressing that there are several different sets of digits in
671Unicode that are equivalent to 0-9 and are matchable by C<\d> in a
672regular expression. If they are used in a single language only, they
48791bf1 673are in that language's C<Script> and C<Script_Extensions>. If they are
82aed44a
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674used in more than one script, they will be in C<sc=Common>, but only
675if they are used in many scripts should they be in C<scx=Common>.
51f494cc 676
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677The explanation above has omitted some detail; refer to UAX#24 "Unicode
678Script Property": L<http://www.unicode.org/reports/tr24>.
679
51f494cc
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680A complete list of scripts and their shortcuts is in L<perluniprops>.
681
a9130ea9 682=head3 B<Use of the C<"Is"> Prefix>
822502e5 683
7b0ac457 684For backward compatibility (with ancient Perl 5.6), all properties writable
b65e6125 685without using the compound form mentioned
51f494cc
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686so far may have C<Is> or C<Is_> prepended to their name, so C<\P{Is_Lu}>, for
687example, is equal to C<\P{Lu}>, and C<\p{IsScript:Arabic}> is equal to
688C<\p{Arabic}>.
eb0cc9e3 689
51f494cc 690=head3 B<Blocks>
2796c109 691
1bfb14c4
JH
692In addition to B<scripts>, Unicode also defines B<blocks> of
693characters. The difference between scripts and blocks is that the
694concept of scripts is closer to natural languages, while the concept
51f494cc 695of blocks is more of an artificial grouping based on groups of Unicode
a9130ea9 696characters with consecutive ordinal values. For example, the C<"Basic Latin">
b65e6125 697block is all the characters whose ordinals are between 0 and 127, inclusive; in
a9130ea9
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698other words, the ASCII characters. The C<"Latin"> script contains some letters
699from this as well as several other blocks, like C<"Latin-1 Supplement">,
b65e6125 700C<"Latin Extended-A">, I<etc.>, but it does not contain all the characters from
7be67b37 701those blocks. It does not, for example, contain the digits 0-9, because
82aed44a
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702those digits are shared across many scripts, and hence are in the
703C<Common> script.
51f494cc
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704
705For more about scripts versus blocks, see UAX#24 "Unicode Script Property":
706L<http://www.unicode.org/reports/tr24>
707
48791bf1 708The C<Script_Extensions> or C<Script> properties are likely to be the
82aed44a 709ones you want to use when processing
a9130ea9 710natural language; the C<Block> property may occasionally be useful in working
b19eb496 711with the nuts and bolts of Unicode.
51f494cc
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712
713Block names are matched in the compound form, like C<\p{Block: Arrows}> or
b19eb496 714C<\p{Blk=Hebrew}>. Unlike most other properties, only a few block names have a
6b5cf123
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715Unicode-defined short name.
716
717Perl also defines single form synonyms for the block property in cases
718where these do not conflict with something else. But don't use any of
719these, because they are unstable. Since these are Perl extensions, they
720are subordinate to official Unicode property names; Unicode doesn't know
721nor care about Perl's extensions. It may happen that a name that
722currently means the Perl extension will later be changed without warning
723to mean a different Unicode property in a future version of the perl
724interpreter that uses a later Unicode release, and your code would no
725longer work. The extensions are mentioned here for completeness: Take
726the block name and prefix it with one of: C<In> (for example
727C<\p{Blk=Arrows}> can currently be written as C<\p{In_Arrows}>); or
728sometimes C<Is> (like C<\p{Is_Arrows}>); or sometimes no prefix at all
48791bf1 729(C<\p{Arrows}>). As of this writing (Unicode 9.0) there are no
6b5cf123
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730conflicts with using the C<In_> prefix, but there are plenty with the
731other two forms. For example, C<\p{Is_Hebrew}> and C<\p{Hebrew}> mean
48791bf1
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732C<\p{Script_Extensions=Hebrew}> which is NOT the same thing as
733C<\p{Blk=Hebrew}>. Our
6b5cf123
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734advice used to be to use the C<In_> prefix as a single form way of
735specifying a block. But Unicode 8.0 added properties whose names begin
736with C<In>, and it's now clear that it's only luck that's so far
737prevented a conflict. Using C<In> is only marginally less typing than
738C<Blk:>, and the latter's meaning is clearer anyway, and guaranteed to
739never conflict. So don't take chances. Use C<\p{Blk=foo}> for new
740code. And be sure that block is what you really really want to do. In
741most cases scripts are what you want instead.
742
743A complete list of blocks is in L<perluniprops>.
51f494cc 744
9f815e24
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745=head3 B<Other Properties>
746
747There are many more properties than the very basic ones described here.
748A complete list is in L<perluniprops>.
749
750Unicode defines all its properties in the compound form, so all single-form
b19eb496
TC
751properties are Perl extensions. Most of these are just synonyms for the
752Unicode ones, but some are genuine extensions, including several that are in
9f815e24
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753the compound form. And quite a few of these are actually recommended by Unicode
754(in L<http://www.unicode.org/reports/tr18>).
755
5bff2035
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756This section gives some details on all extensions that aren't just
757synonyms for compound-form Unicode properties
758(for those properties, you'll have to refer to the
9f815e24
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759L<Unicode Standard|http://www.unicode.org/reports/tr44>.
760
761=over
762
763=item B<C<\p{All}>>
764
2d88a86a
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765This matches every possible code point. It is equivalent to C<qr/./s>.
766Unlike all the other non-user-defined C<\p{}> property matches, no
767warning is ever generated if this is property is matched against a
768non-Unicode code point (see L</Beyond Unicode code points> below).
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769
770=item B<C<\p{Alnum}>>
771
772This matches any C<\p{Alphabetic}> or C<\p{Decimal_Number}> character.
773
774=item B<C<\p{Any}>>
775
2d88a86a
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776This matches any of the 1_114_112 Unicode code points. It is a synonym
777for C<\p{Unicode}>.
9f815e24 778
42581d5d
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779=item B<C<\p{ASCII}>>
780
781This matches any of the 128 characters in the US-ASCII character set,
782which is a subset of Unicode.
783
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784=item B<C<\p{Assigned}>>
785
a9130ea9
KW
786This matches any assigned code point; that is, any code point whose L<general
787category|/General_Category> is not C<Unassigned> (or equivalently, not C<Cn>).
9f815e24
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788
789=item B<C<\p{Blank}>>
790
791This is the same as C<\h> and C<\p{HorizSpace}>: A character that changes the
792spacing horizontally.
793
794=item B<C<\p{Decomposition_Type: Non_Canonical}>> (Short: C<\p{Dt=NonCanon}>)
795
796Matches a character that has a non-canonical decomposition.
797
a6a7eedc
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798The L</Extended Grapheme Clusters (Logical characters)> section above
799talked about canonical decompositions. However, many more characters
800have a different type of decomposition, a "compatible" or
801"non-canonical" decomposition. The sequences that form these
802decompositions are not considered canonically equivalent to the
803pre-composed character. An example is the C<"SUPERSCRIPT ONE">. It is
804somewhat like a regular digit 1, but not exactly; its decomposition into
805the digit 1 is called a "compatible" decomposition, specifically a
9f815e24 806"super" decomposition. There are several such compatibility
b65e6125
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807decompositions (see L<http://www.unicode.org/reports/tr44>), including
808one called "compat", which means some miscellaneous type of
809decomposition that doesn't fit into the other decomposition categories
810that Unicode has chosen.
9f815e24
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811
812Note that most Unicode characters don't have a decomposition, so their
a9130ea9 813decomposition type is C<"None">.
9f815e24 814
b19eb496
TC
815For your convenience, Perl has added the C<Non_Canonical> decomposition
816type to mean any of the several compatibility decompositions.
9f815e24
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817
818=item B<C<\p{Graph}>>
819
820Matches any character that is graphic. Theoretically, this means a character
821that on a printer would cause ink to be used.
822
823=item B<C<\p{HorizSpace}>>
824
b19eb496 825This is the same as C<\h> and C<\p{Blank}>: a character that changes the
9f815e24
KW
826spacing horizontally.
827
42581d5d 828=item B<C<\p{In=*}>>
9f815e24
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829
830This is a synonym for C<\p{Present_In=*}>
831
832=item B<C<\p{PerlSpace}>>
833
d28d8023 834This is the same as C<\s>, restricted to ASCII, namely C<S<[ \f\n\r\t]>>
779cf272 835and starting in Perl v5.18, a vertical tab.
9f815e24
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836
837Mnemonic: Perl's (original) space
838
839=item B<C<\p{PerlWord}>>
840
841This is the same as C<\w>, restricted to ASCII, namely C<[A-Za-z0-9_]>
842
843Mnemonic: Perl's (original) word.
844
42581d5d 845=item B<C<\p{Posix...}>>
9f815e24 846
b65e6125
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847There are several of these, which are equivalents, using the C<\p{}>
848notation, for Posix classes and are described in
42581d5d 849L<perlrecharclass/POSIX Character Classes>.
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850
851=item B<C<\p{Present_In: *}>> (Short: C<\p{In=*}>)
852
853This property is used when you need to know in what Unicode version(s) a
854character is.
855
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856The "*" above stands for some Unicode version number, such as
857C<1.1> or C<12.0>; or the "*" can also be C<Unassigned>. This property will
9f815e24
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858match the code points whose final disposition has been settled as of the
859Unicode release given by the version number; C<\p{Present_In: Unassigned}>
860will match those code points whose meaning has yet to be assigned.
861
a9130ea9 862For example, C<U+0041> C<"LATIN CAPITAL LETTER A"> was present in the very first
9f815e24
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863Unicode release available, which is C<1.1>, so this property is true for all
864valid "*" versions. On the other hand, C<U+1EFF> was not assigned until version
a9130ea9 8655.1 when it became C<"LATIN SMALL LETTER Y WITH LOOP">, so the only "*" that
9f815e24
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866would match it are 5.1, 5.2, and later.
867
868Unicode furnishes the C<Age> property from which this is derived. The problem
869with Age is that a strict interpretation of it (which Perl takes) has it
870matching the precise release a code point's meaning is introduced in. Thus
871C<U+0041> would match only 1.1; and C<U+1EFF> only 5.1. This is not usually what
872you want.
873
874Some non-Perl implementations of the Age property may change its meaning to be
a9130ea9 875the same as the Perl C<Present_In> property; just be aware of that.
9f815e24
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876
877Another confusion with both these properties is that the definition is not
b19eb496
TC
878that the code point has been I<assigned>, but that the meaning of the code point
879has been I<determined>. This is because 66 code points will always be
a9130ea9 880unassigned, and so the C<Age> for them is the Unicode version in which the decision
b19eb496 881to make them so was made. For example, C<U+FDD0> is to be permanently
9f815e24 882unassigned to a character, and the decision to do that was made in version 3.1,
b19eb496 883so C<\p{Age=3.1}> matches this character, as also does C<\p{Present_In: 3.1}> and up.
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884
885=item B<C<\p{Print}>>
886
ae5b72c8 887This matches any character that is graphical or blank, except controls.
9f815e24
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888
889=item B<C<\p{SpacePerl}>>
890
891This is the same as C<\s>, including beyond ASCII.
892
4d4acfba 893Mnemonic: Space, as modified by Perl. (It doesn't include the vertical tab
779cf272 894until v5.18, which both the Posix standard and Unicode consider white space.)
9f815e24 895
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896=item B<C<\p{Title}>> and B<C<\p{Titlecase}>>
897
898Under case-sensitive matching, these both match the same code points as
899C<\p{General Category=Titlecase_Letter}> (C<\p{gc=lt}>). The difference
900is that under C</i> caseless matching, these match the same as
901C<\p{Cased}>, whereas C<\p{gc=lt}> matches C<\p{Cased_Letter>).
902
2d88a86a
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903=item B<C<\p{Unicode}>>
904
905This matches any of the 1_114_112 Unicode code points.
906C<\p{Any}>.
907
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908=item B<C<\p{VertSpace}>>
909
910This is the same as C<\v>: A character that changes the spacing vertically.
911
912=item B<C<\p{Word}>>
913
b19eb496 914This is the same as C<\w>, including over 100_000 characters beyond ASCII.
9f815e24 915
42581d5d
KW
916=item B<C<\p{XPosix...}>>
917
b19eb496 918There are several of these, which are the standard Posix classes
42581d5d
KW
919extended to the full Unicode range. They are described in
920L<perlrecharclass/POSIX Character Classes>.
921
9f815e24
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922=back
923
1532347b
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924=head2 Wildcards in Property Values
925
926Starting in Perl 5.30, it is possible to do do something like this:
927
928 qr!\p{numeric_value=/\A[0-5]\z/}!
929
930or, by abbreviating and adding C</x>,
931
932 qr! \p{nv= /(?x) \A [0-5] \z / }!
933
934This matches all code points whose numeric value is one of 0, 1, 2, 3,
9354, or 5. This particular example could instead have been written as
936
937 qr! \A [ \p{nv=0}\p{nv=1}\p{nv=2}\p{nv=3}\p{nv=4}\p{nv=5} ] \z !xx
938
939in earlier perls, so in this case this feature just makes things easier
940and shorter to write. If we hadn't included the C<\A> and C<\z>, these
941would have matched things like C<1E<sol>2> because that contains a 1 (as
942well as a 2). As written, it matches things like subscripts that have
943these numeric values. If we only wanted the decimal digits with those
944numeric values, we could say,
945
946 qr! (?[ \d & \p{nv=/[0-5]/ ]) }!x
947
948The C<\d> gets rid of needing to anchor the pattern, since it forces the
949result to only match C<[0-9]>, and the C<[0-5]> further restricts it.
950
951The text in the above examples enclosed between the C<"E<sol>">
952characters can be just about any regular expression. It is independent
953of the main pattern, so doesn't share any capturing groups, I<etc>. The
954delimiters for it must be ASCII punctuation, but it may NOT be
955delimited by C<"{">, nor C<"}"> nor contain a literal C<"}">, as that
956delimits the end of the enclosing C<\p{}>. Like any pattern, certain
957other delimiters are terminated by their mirror images. These are
958C<"(">, C<"[>", and C<"E<lt>">. If the delimiter is any of C<"-">,
959C<"_">, C<"+">, or C<"\">, or is the same delimiter as is used for the
960enclosing pattern, it must be be preceded by a backslash escape, both
961fore and aft.
962
963Beware of using C<"$"> to indicate to match the end of the string. It
964can too easily be interpreted as being a punctuation variable, like
965C<$/>.
966
967No modifiers may follow the final delimiter. Instead, use
968L<perlre/(?adlupimnsx-imnsx)> and/or
969L<perlre/(?adluimnsx-imnsx:pattern)> to specify modifiers.
970
971This feature is not available when the left-hand side is prefixed by
972C<Is_>, nor for any form that is marked as "Discouraged" in
973L<perluniprops/Discouraged>.
974
975Perl wraps your pattern with C<(?iaa: ... )>. This is because nothing
976outside ASCII can match the Unicode property values available in this
977release, and they should match caselessly. If your pattern has a syntax
978error, this wrapping will be shown in the error message, even though you
979didn't specify it yourself. This could be confusing if you don't know
980about this.
981
982This experimental feature has been added to begin to implement
983L<https://www.unicode.org/reports/tr18/#Wildcard_Properties>. Using it
984will raise a (default-on) warning in the
985C<experimental::uniprop_wildcards> category. We reserve the right to
986change its operation as we gain experience.
987
988Your subpattern can be just about anything, but for it to have some
989utility, it should match when called with either or both of
990a) the full name of the property value with underscores (and/or spaces
991in the Block property) and some things uppercase; or b) the property
992value in all lowercase with spaces and underscores squeezed out. For
993example,
994
995 qr!\p{Blk=/Old I.*/}!
996 qr!\p{Blk=/oldi.*/}!
997
998would match the same things.
999
1000A warning is issued if none of the legal values for a property are
1001matched by your pattern. It's likely that a future release will raise a
1002warning if your pattern ends up causing every possible code point to
1003match.
1004
1005Another example that shows that within C<\p{...}>, C</x> isn't needed to
1006have spaces:
1007
1008 qr!\p{scx= /Hebrew|Greek/ }!
1009
1010To be safe, we should have anchored the above example, to prevent
1011matches for something like C<Hebrew_Braile>, but there aren't
1012any script names like that.
1013
1014There are certain properties that it doesn't currently work with. These
1015are:
1016
1017 Bidi Mirroring Glyph
1018 Bidi Paired Bracket
1019 Case Folding
1020 Decomposition Mapping
1021 Equivalent Unified Ideograph
1022 Name
1023 Name Alias
1024 Lowercase Mapping
1025 NFKC Case Fold
1026 Titlecase Mapping
1027 Uppercase Mapping
1028
1029Nor is the C<@I<unicode_property>@> form implemented.
1030
1031Here's a complete example of matching IPV4 internet protocol addresses
1032in any (single) script
1033
1034 no warnings 'experimental::script_run';
1035 no warnings 'experimental::regex_sets';
1036 no warnings 'experimental::uniprop_wildcards';
1037
1038 # Can match a substring, so this intermediate regex needs to have
1039 # context or anchoring in its final use. Using nt=de yields decimal
1040 # digits. When specifying a subset of these, we must include \d to
1041 # prevent things like U+00B2 SUPERSCRIPT TWO from matching
1042 my $zero_through_255 =
1043 qr/ \b (*sr: # All from same sript
1044 (?[ \p{nv=0} & \d ])* # Optional leading zeros
1045 ( # Then one of:
1046 \d{1,2} # 0 - 99
1047 | (?[ \p{nv=1} & \d ]) \d{2} # 100 - 199
1048 | (?[ \p{nv=2} & \d ])
1049 ( (?[ \p{nv=:[0-4]:} & \d ]) \d # 200 - 249
1050 | (?[ \p{nv=5} & \d ])
1051 (?[ \p{nv=:[0-5]:} & \d ]) # 250 - 255
1052 )
1053 )
1054 )
1055 \b
1056 /x;
1057
1058 my $ipv4 = qr/ \A (*sr: $zero_through_255
1059 (?: [.] $zero_through_255 ) {3}
1060 )
1061 \z
1062 /x;
a9130ea9 1063
376d9008 1064=head2 User-Defined Character Properties
491fd90a 1065
51f494cc 1066You can define your own binary character properties by defining subroutines
a9130ea9 1067whose names begin with C<"In"> or C<"Is">. (The experimental feature
9d1a5160
KW
1068L<perlre/(?[ ])> provides an alternative which allows more complex
1069definitions.) The subroutines can be defined in any
51f494cc 1070package. The user-defined properties can be used in the regular expression
a9130ea9 1071C<\p{}> and C<\P{}> constructs; if you are using a user-defined property from a
51f494cc 1072package other than the one you are in, you must specify its package in the
a9130ea9 1073C<\p{}> or C<\P{}> construct.
bac0b425 1074
51f494cc 1075 # assuming property Is_Foreign defined in Lang::
bac0b425
JP
1076 package main; # property package name required
1077 if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
1078
1079 package Lang; # property package name not required
1080 if ($txt =~ /\p{IsForeign}+/) { ... }
1081
1082
1083Note that the effect is compile-time and immutable once defined.
b19eb496
TC
1084However, the subroutines are passed a single parameter, which is 0 if
1085case-sensitive matching is in effect and non-zero if caseless matching
56ca34ca
KW
1086is in effect. The subroutine may return different values depending on
1087the value of the flag, and one set of values will immutably be in effect
b19eb496 1088for all case-sensitive matches, and the other set for all case-insensitive
56ca34ca 1089matches.
491fd90a 1090
b19eb496 1091Note that if the regular expression is tainted, then Perl will die rather
a9130ea9 1092than calling the subroutine when the name of the subroutine is
0e9be77f
DM
1093determined by the tainted data.
1094
376d9008
JB
1095The subroutines must return a specially-formatted string, with one
1096or more newline-separated lines. Each line must be one of the following:
491fd90a
JH
1097
1098=over 4
1099
1100=item *
1101
df9e1087 1102A single hexadecimal number denoting a code point to include.
510254c9
A
1103
1104=item *
1105
99a6b1f0 1106Two hexadecimal numbers separated by horizontal whitespace (space or
73b95840
KW
1107tabular characters) denoting a range of code points to include. The
1108second number must not be smaller than the first.
491fd90a
JH
1109
1110=item *
1111
a9130ea9
KW
1112Something to include, prefixed by C<"+">: a built-in character
1113property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 1114name) user-defined character property,
bac0b425
JP
1115to represent all the characters in that property; two hexadecimal code
1116points for a range; or a single hexadecimal code point.
491fd90a
JH
1117
1118=item *
1119
a9130ea9
KW
1120Something to exclude, prefixed by C<"-">: an existing character
1121property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 1122name) user-defined character property,
bac0b425
JP
1123to represent all the characters in that property; two hexadecimal code
1124points for a range; or a single hexadecimal code point.
491fd90a
JH
1125
1126=item *
1127
a9130ea9
KW
1128Something to negate, prefixed C<"!">: an existing character
1129property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 1130name) user-defined character property,
bac0b425
JP
1131to represent all the characters in that property; two hexadecimal code
1132points for a range; or a single hexadecimal code point.
1133
1134=item *
1135
a9130ea9
KW
1136Something to intersect with, prefixed by C<"&">: an existing character
1137property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 1138name) user-defined character property,
bac0b425
JP
1139for all the characters except the characters in the property; two
1140hexadecimal code points for a range; or a single hexadecimal code point.
491fd90a
JH
1141
1142=back
1143
1144For example, to define a property that covers both the Japanese
1145syllabaries (hiragana and katakana), you can define
1146
1147 sub InKana {
d88362ca 1148 return <<END;
d5822f25
A
1149 3040\t309F
1150 30A0\t30FF
491fd90a
JH
1151 END
1152 }
1153
d5822f25
A
1154Imagine that the here-doc end marker is at the beginning of the line.
1155Now you can use C<\p{InKana}> and C<\P{InKana}>.
491fd90a
JH
1156
1157You could also have used the existing block property names:
1158
1159 sub InKana {
d88362ca 1160 return <<'END';
491fd90a
JH
1161 +utf8::InHiragana
1162 +utf8::InKatakana
1163 END
1164 }
1165
1166Suppose you wanted to match only the allocated characters,
d5822f25 1167not the raw block ranges: in other words, you want to remove
b65e6125 1168the unassigned characters:
491fd90a
JH
1169
1170 sub InKana {
d88362ca 1171 return <<'END';
491fd90a
JH
1172 +utf8::InHiragana
1173 +utf8::InKatakana
1174 -utf8::IsCn
1175 END
1176 }
1177
1178The negation is useful for defining (surprise!) negated classes.
1179
1180 sub InNotKana {
d88362ca 1181 return <<'END';
491fd90a
JH
1182 !utf8::InHiragana
1183 -utf8::InKatakana
1184 +utf8::IsCn
1185 END
1186 }
1187
461020ad
KW
1188This will match all non-Unicode code points, since every one of them is
1189not in Kana. You can use intersection to exclude these, if desired, as
1190this modified example shows:
bac0b425 1191
461020ad 1192 sub InNotKana {
bac0b425 1193 return <<'END';
461020ad
KW
1194 !utf8::InHiragana
1195 -utf8::InKatakana
1196 +utf8::IsCn
1197 &utf8::Any
bac0b425
JP
1198 END
1199 }
1200
461020ad
KW
1201C<&utf8::Any> must be the last line in the definition.
1202
1203Intersection is used generally for getting the common characters matched
a9130ea9 1204by two (or more) classes. It's important to remember not to use C<"&"> for
461020ad 1205the first set; that would be intersecting with nothing, resulting in an
5acbde07 1206empty set. (Similarly using C<"-"> for the first set does nothing).
461020ad 1207
2d88a86a
KW
1208Unlike non-user-defined C<\p{}> property matches, no warning is ever
1209generated if these properties are matched against a non-Unicode code
1210point (see L</Beyond Unicode code points> below).
bac0b425 1211
68585b5e 1212=head2 User-Defined Case Mappings (for serious hackers only)
822502e5 1213
5d1892be 1214B<This feature has been removed as of Perl 5.16.>
a9130ea9 1215The CPAN module C<L<Unicode::Casing>> provides better functionality without
5d1892be
KW
1216the drawbacks that this feature had. If you are using a Perl earlier
1217than 5.16, this feature was most fully documented in the 5.14 version of
1218this pod:
1219L<http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>
3a2263fe 1220
376d9008 1221=head2 Character Encodings for Input and Output
8cbd9a7a 1222
7221edc9 1223See L<Encode>.
8cbd9a7a 1224
c29a771d 1225=head2 Unicode Regular Expression Support Level
776f8809 1226
b19eb496 1227The following list of Unicode supported features for regular expressions describes
fea12a3e
KW
1228all features currently directly supported by core Perl. The references
1229to "Level I<N>" and the section numbers refer to
1230L<UTS#18 "Unicode Regular Expressions"|http://www.unicode.org/reports/tr18>,
526f2ca9 1231version 18, October 2016.
fea12a3e
KW
1232
1233=head3 Level 1 - Basic Unicode Support
1234
1235 RL1.1 Hex Notation - Done [1]
1236 RL1.2 Properties - Done [2]
1237 RL1.2a Compatibility Properties - Done [3]
1238 RL1.3 Subtraction and Intersection - Experimental [4]
1239 RL1.4 Simple Word Boundaries - Done [5]
1240 RL1.5 Simple Loose Matches - Done [6]
1241 RL1.6 Line Boundaries - Partial [7]
1242 RL1.7 Supplementary Code Points - Done [8]
755789c0 1243
6f33e417
KW
1244=over 4
1245
a6a7eedc 1246=item [1] C<\N{U+...}> and C<\x{...}>
6f33e417 1247
fea12a3e
KW
1248=item [2]
1249C<\p{...}> C<\P{...}>. This requirement is for a minimal list of
1250properties. Perl supports these and all other Unicode character
1251properties, as R2.7 asks (see L</"Unicode Character Properties"> above).
6f33e417 1252
fea12a3e
KW
1253=item [3]
1254Perl has C<\d> C<\D> C<\s> C<\S> C<\w> C<\W> C<\X> C<[:I<prop>:]>
1255C<[:^I<prop>:]>, plus all the properties specified by
1256L<http://www.unicode.org/reports/tr18/#Compatibility_Properties>. These
1257are described above in L</Other Properties>
6f33e417 1258
fea12a3e 1259=item [4]
6f33e417 1260
fea12a3e 1261The experimental feature C<"(?[...])"> starting in v5.18 accomplishes
a6a7eedc 1262this.
6f33e417 1263
a6a7eedc
KW
1264See L<perlre/(?[ ])>. If you don't want to use an experimental
1265feature, you can use one of the following:
6f33e417
KW
1266
1267=over 4
1268
a6a7eedc 1269=item *
f67a5002 1270Regular expression lookahead
6f33e417
KW
1271
1272You can mimic class subtraction using lookahead.
8158862b 1273For example, what UTS#18 might write as
29bdacb8 1274
209c9685 1275 [{Block=Greek}-[{UNASSIGNED}]]
dbe420b4
JH
1276
1277in Perl can be written as:
1278
209c9685
KW
1279 (?!\p{Unassigned})\p{Block=Greek}
1280 (?=\p{Assigned})\p{Block=Greek}
dbe420b4
JH
1281
1282But in this particular example, you probably really want
1283
209c9685 1284 \p{Greek}
dbe420b4
JH
1285
1286which will match assigned characters known to be part of the Greek script.
29bdacb8 1287
a6a7eedc
KW
1288=item *
1289
1290CPAN module C<L<Unicode::Regex::Set>>
8158862b 1291
6f33e417
KW
1292It does implement the full UTS#18 grouping, intersection, union, and
1293removal (subtraction) syntax.
8158862b 1294
a6a7eedc
KW
1295=item *
1296
1297L</"User-Defined Character Properties">
6f33e417 1298
a9130ea9 1299C<"+"> for union, C<"-"> for removal (set-difference), C<"&"> for intersection
6f33e417
KW
1300
1301=back
1302
fea12a3e
KW
1303=item [5]
1304C<\b> C<\B> meet most, but not all, the details of this requirement, but
1305C<\b{wb}> and C<\B{wb}> do, as well as the stricter R2.3.
1306
1307=item [6]
6f33e417 1308
a6a7eedc 1309Note that Perl does Full case-folding in matching, not Simple:
6f33e417 1310
a6a7eedc
KW
1311For example C<U+1F88> is equivalent to C<U+1F00 U+03B9>, instead of just
1312C<U+1F80>. This difference matters mainly for certain Greek capital
a9130ea9
KW
1313letters with certain modifiers: the Full case-folding decomposes the
1314letter, while the Simple case-folding would map it to a single
1315character.
6f33e417 1316
fea12a3e
KW
1317=item [7]
1318
1319The reason this is considered to be only partially implemented is that
1320Perl has L<C<qrE<sol>\b{lb}E<sol>>|perlrebackslash/\b{lb}> and
1321C<L<Unicode::LineBreak>> that are conformant with
1322L<UAX#14 "Unicode Line Breaking Algorithm"|http://www.unicode.org/reports/tr14>.
1323The regular expression construct provides default behavior, while the
1324heavier-weight module provides customizable line breaking.
1325
1326But Perl treats C<\n> as the start- and end-line
1327delimiter, whereas Unicode specifies more characters that should be
1328so-interpreted.
6f33e417 1329
a6a7eedc 1330These are:
6f33e417 1331
a6a7eedc
KW
1332 VT U+000B (\v in C)
1333 FF U+000C (\f)
1334 CR U+000D (\r)
1335 NEL U+0085
1336 LS U+2028
1337 PS U+2029
6f33e417 1338
a6a7eedc
KW
1339C<^> and C<$> in regular expression patterns are supposed to match all
1340these, but don't.
1341These characters also don't, but should, affect C<< <> >> C<$.>, and
1342script line numbers.
6f33e417 1343
a6a7eedc
KW
1344Also, lines should not be split within C<CRLF> (i.e. there is no
1345empty line between C<\r> and C<\n>). For C<CRLF>, try the C<:crlf>
1346layer (see L<PerlIO>).
1347
fea12a3e 1348=item [8]
a9130ea9
KW
1349UTF-8/UTF-EBDDIC used in Perl allows not only C<U+10000> to
1350C<U+10FFFF> but also beyond C<U+10FFFF>
6f33e417
KW
1351
1352=back
5ca1ac52 1353
fea12a3e 1354=head3 Level 2 - Extended Unicode Support
776f8809 1355
fea12a3e
KW
1356 RL2.1 Canonical Equivalents - Retracted [9]
1357 by Unicode
1358 RL2.2 Extended Grapheme Clusters - Partial [10]
1359 RL2.3 Default Word Boundaries - Done [11]
1360 RL2.4 Default Case Conversion - Done
1361 RL2.5 Name Properties - Done
1532347b 1362 RL2.6 Wildcards in Property Values - Partial [12]
fea12a3e 1363 RL2.7 Full Properties - Done
776f8809 1364
fea12a3e 1365=over 4
8158862b 1366
fea12a3e
KW
1367=item [9]
1368Unicode has rewritten this portion of UTS#18 to say that getting
1369canonical equivalence (see UAX#15
1370L<"Unicode Normalization Forms"|http://www.unicode.org/reports/tr15>)
1371is basically to be done at the programmer level. Use NFD to write
1372both your regular expressions and text to match them against (you
1373can use L<Unicode::Normalize>).
776f8809 1374
fea12a3e
KW
1375=item [10]
1376Perl has C<\X> and C<\b{gcb}> but we don't have a "Grapheme Cluster Mode".
1377
1378=item [11] see
1379L<UAX#29 "Unicode Text Segmentation"|http://www.unicode.org/reports/tr29>,
1380
1532347b
KW
1381=item [12] see
1382L</Wildcards in Property Values> above.
1383
fea12a3e
KW
1384=back
1385
1386=head3 Level 3 - Tailored Support
1387
1388 RL3.1 Tailored Punctuation - Missing
526f2ca9 1389 RL3.2 Tailored Grapheme Clusters - Missing [13]
fea12a3e
KW
1390 RL3.3 Tailored Word Boundaries - Missing
1391 RL3.4 Tailored Loose Matches - Retracted by Unicode
1392 RL3.5 Tailored Ranges - Retracted by Unicode
526f2ca9 1393 RL3.6 Context Matching - Partial [14]
fea12a3e 1394 RL3.7 Incremental Matches - Missing
526f2ca9 1395 RL3.8 Unicode Set Sharing - Retracted by Unicode
fea12a3e
KW
1396 RL3.9 Possible Match Sets - Missing
1397 RL3.10 Folded Matching - Retracted by Unicode
526f2ca9 1398 RL3.11 Submatchers - Partial [15]
fea12a3e
KW
1399
1400=over 4
1401
526f2ca9 1402=item [13]
fea12a3e
KW
1403Perl has L<Unicode::Collate>, but it isn't integrated with regular
1404expressions. See
1405L<UTS#10 "Unicode Collation Algorithms"|http://www.unicode.org/reports/tr10>.
776f8809 1406
526f2ca9
KW
1407=item [14]
1408Perl has C<(?<=x)> and C<(?=x)>, but this requirement says that it
1409should be possible to specify that matches may occur only in a substring
1410with the lookaheads and lookbehinds able to see beyond that matchable
1411portion.
1412
1413=item [15]
1414Perl has user-defined properties (L</"User-Defined Character
1415Properties">) to look at single code points in ways beyond Unicode, and
1416it might be possible, though probably not very clean, to use code blocks
1532347b 1417and things like C<(?(DEFINE)...)> (see L<perlre>) to do more specialized
526f2ca9 1418matching.
776f8809
JH
1419
1420=back
1421
c349b1b9
JH
1422=head2 Unicode Encodings
1423
376d9008
JB
1424Unicode characters are assigned to I<code points>, which are abstract
1425numbers. To use these numbers, various encodings are needed.
c349b1b9
JH
1426
1427=over 4
1428
c29a771d 1429=item *
5cb3728c
RB
1430
1431UTF-8
c349b1b9 1432
6d4f9cf2 1433UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
a6a7eedc
KW
1434encoding. In most of Perl's documentation, including elsewhere in this
1435document, the term "UTF-8" means also "UTF-EBCDIC". But in this section,
1436"UTF-8" refers only to the encoding used on ASCII platforms. It is a
1437superset of 7-bit US-ASCII, so anything encoded in ASCII has the
1438identical representation when encoded in UTF-8.
c349b1b9 1439
8c007b5a 1440The following table is from Unicode 3.2.
05632f9a 1441
755789c0 1442 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
05632f9a 1443
d88362ca 1444 U+0000..U+007F 00..7F
e1b711da 1445 U+0080..U+07FF * C2..DF 80..BF
d88362ca 1446 U+0800..U+0FFF E0 * A0..BF 80..BF
ec90690f
ST
1447 U+1000..U+CFFF E1..EC 80..BF 80..BF
1448 U+D000..U+D7FF ED 80..9F 80..BF
755789c0 1449 U+D800..U+DFFF +++++ utf16 surrogates, not legal utf8 +++++
ec90690f 1450 U+E000..U+FFFF EE..EF 80..BF 80..BF
d88362ca
KW
1451 U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF
1452 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
1453 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
e1b711da 1454
b19eb496 1455Note the gaps marked by "*" before several of the byte entries above. These are
e1b711da
KW
1456caused by legal UTF-8 avoiding non-shortest encodings: it is technically
1457possible to UTF-8-encode a single code point in different ways, but that is
1458explicitly forbidden, and the shortest possible encoding should always be used
1459(and that is what Perl does).
37361303 1460
376d9008 1461Another way to look at it is via bits:
05632f9a 1462
755789c0 1463 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
05632f9a 1464
755789c0
KW
1465 0aaaaaaa 0aaaaaaa
1466 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
1467 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
1468 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
05632f9a 1469
a9130ea9 1470As you can see, the continuation bytes all begin with C<"10">, and the
e1b711da 1471leading bits of the start byte tell how many bytes there are in the
05632f9a
JH
1472encoded character.
1473
6d4f9cf2 1474The original UTF-8 specification allowed up to 6 bytes, to allow
a9130ea9 1475encoding of numbers up to C<0x7FFF_FFFF>. Perl continues to allow those,
6d4f9cf2
KW
1476and has extended that up to 13 bytes to encode code points up to what
1477can fit in a 64-bit word. However, Perl will warn if you output any of
b19eb496 1478these as being non-portable; and under strict UTF-8 input protocols,
526f2ca9 1479they are forbidden. In addition, it is now illegal to use a code point
760c7c2f
KW
1480larger than what a signed integer variable on your system can hold. On
148132-bit ASCII systems, this means C<0x7FFF_FFFF> is the legal maximum
526f2ca9 1482(much higher on 64-bit systems).
6d4f9cf2 1483
c29a771d 1484=item *
5cb3728c
RB
1485
1486UTF-EBCDIC
dbe420b4 1487
b65e6125 1488Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
a6a7eedc
KW
1489This means that all the basic characters (which includes all
1490those that have ASCII equivalents (like C<"A">, C<"0">, C<"%">, I<etc.>)
1491are the same in both EBCDIC and UTF-EBCDIC.)
1492
c0236afe
KW
1493UTF-EBCDIC is used on EBCDIC platforms. It generally requires more
1494bytes to represent a given code point than UTF-8 does; the largest
1495Unicode code points take 5 bytes to represent (instead of 4 in UTF-8),
1496and, extended for 64-bit words, it uses 14 bytes instead of 13 bytes in
1497UTF-8.
dbe420b4 1498
c29a771d 1499=item *
5cb3728c 1500
b65e6125 1501UTF-16, UTF-16BE, UTF-16LE, Surrogates, and C<BOM>'s (Byte Order Marks)
c349b1b9 1502
1bfb14c4
JH
1503The followings items are mostly for reference and general Unicode
1504knowledge, Perl doesn't use these constructs internally.
dbe420b4 1505
b19eb496
TC
1506Like UTF-8, UTF-16 is a variable-width encoding, but where
1507UTF-8 uses 8-bit code units, UTF-16 uses 16-bit code units.
1508All code points occupy either 2 or 4 bytes in UTF-16: code points
1509C<U+0000..U+FFFF> are stored in a single 16-bit unit, and code
1bfb14c4 1510points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is
c349b1b9
JH
1511using I<surrogates>, the first 16-bit unit being the I<high
1512surrogate>, and the second being the I<low surrogate>.
1513
376d9008 1514Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
c349b1b9 1515range of Unicode code points in pairs of 16-bit units. The I<high
9f815e24 1516surrogates> are the range C<U+D800..U+DBFF> and the I<low surrogates>
376d9008 1517are the range C<U+DC00..U+DFFF>. The surrogate encoding is
c349b1b9 1518
d88362ca
KW
1519 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1520 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
c349b1b9
JH
1521
1522and the decoding is
1523
d88362ca 1524 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
c349b1b9 1525
376d9008 1526Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
c349b1b9 1527itself can be used for in-memory computations, but if storage or
376d9008
JB
1528transfer is required either UTF-16BE (big-endian) or UTF-16LE
1529(little-endian) encodings must be chosen.
c349b1b9
JH
1530
1531This introduces another problem: what if you just know that your data
376d9008 1532is UTF-16, but you don't know which endianness? Byte Order Marks, or
b65e6125 1533C<BOM>'s, are a solution to this. A special character has been reserved
86bbd6d1 1534in Unicode to function as a byte order marker: the character with the
a9130ea9 1535code point C<U+FEFF> is the C<BOM>.
042da322 1536
a9130ea9 1537The trick is that if you read a C<BOM>, you will know the byte order,
376d9008
JB
1538since if it was written on a big-endian platform, you will read the
1539bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
1540you will read the bytes C<0xFF 0xFE>. (And if the originating platform
b65e6125
KW
1541was writing in ASCII platform UTF-8, you will read the bytes
1542C<0xEF 0xBB 0xBF>.)
042da322 1543
86bbd6d1 1544The way this trick works is that the character with the code point
6d4f9cf2 1545C<U+FFFE> is not supposed to be in input streams, so the
a9130ea9 1546sequence of bytes C<0xFF 0xFE> is unambiguously "C<BOM>, represented in
1bfb14c4 1547little-endian format" and cannot be C<U+FFFE>, represented in big-endian
6d4f9cf2
KW
1548format".
1549
1550Surrogates have no meaning in Unicode outside their use in pairs to
1551represent other code points. However, Perl allows them to be
1552represented individually internally, for example by saying
f651977e
TC
1553C<chr(0xD801)>, so that all code points, not just those valid for open
1554interchange, are
6d4f9cf2 1555representable. Unicode does define semantics for them, such as their
a9130ea9
KW
1556C<L</General_Category>> is C<"Cs">. But because their use is somewhat dangerous,
1557Perl will warn (using the warning category C<"surrogate">, which is a
1558sub-category of C<"utf8">) if an attempt is made
6d4f9cf2
KW
1559to do things like take the lower case of one, or match
1560case-insensitively, or to output them. (But don't try this on Perls
1561before 5.14.)
c349b1b9 1562
c29a771d 1563=item *
5cb3728c 1564
1e54db1a 1565UTF-32, UTF-32BE, UTF-32LE
c349b1b9 1566
b65e6125 1567The UTF-32 family is pretty much like the UTF-16 family, except that
042da322 1568the units are 32-bit, and therefore the surrogate scheme is not
a9130ea9 1569needed. UTF-32 is a fixed-width encoding. The C<BOM> signatures are
b19eb496 1570C<0x00 0x00 0xFE 0xFF> for BE and C<0xFF 0xFE 0x00 0x00> for LE.
c349b1b9 1571
c29a771d 1572=item *
5cb3728c
RB
1573
1574UCS-2, UCS-4
c349b1b9 1575
b19eb496 1576Legacy, fixed-width encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
376d9008 1577encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
339cfa0e 1578because it does not use surrogates. UCS-4 is a 32-bit encoding,
b19eb496 1579functionally identical to UTF-32 (the difference being that
a9130ea9 1580UCS-4 forbids neither surrogates nor code points larger than C<0x10_FFFF>).
c349b1b9 1581
c29a771d 1582=item *
5cb3728c
RB
1583
1584UTF-7
c349b1b9 1585
376d9008
JB
1586A seven-bit safe (non-eight-bit) encoding, which is useful if the
1587transport or storage is not eight-bit safe. Defined by RFC 2152.
c349b1b9 1588
95a1a48b
JH
1589=back
1590
57e88091 1591=head2 Noncharacter code points
6d4f9cf2 1592
57e88091 159366 code points are set aside in Unicode as "noncharacter code points".
a9130ea9 1594These all have the C<Unassigned> (C<Cn>) C<L</General_Category>>, and
57e88091
KW
1595no character will ever be assigned to any of them. They are the 32 code
1596points between C<U+FDD0> and C<U+FDEF> inclusive, and the 34 code
1597points:
1598
1599 U+FFFE U+FFFF
1600 U+1FFFE U+1FFFF
1601 U+2FFFE U+2FFFF
1602 ...
1603 U+EFFFE U+EFFFF
1604 U+FFFFE U+FFFFF
1605 U+10FFFE U+10FFFF
1606
1607Until Unicode 7.0, the noncharacters were "B<forbidden> for use in open
1608interchange of Unicode text data", so that code that processed those
1609streams could use these code points as sentinels that could be mixed in
1610with character data, and would always be distinguishable from that data.
1611(Emphasis above and in the next paragraph are added in this document.)
1612
1613Unicode 7.0 changed the wording so that they are "B<not recommended> for
1614use in open interchange of Unicode text data". The 7.0 Standard goes on
1615to say:
1616
1617=over 4
1618
1619"If a noncharacter is received in open interchange, an application is
1620not required to interpret it in any way. It is good practice, however,
1621to recognize it as a noncharacter and to take appropriate action, such
1622as replacing it with C<U+FFFD> replacement character, to indicate the
1623problem in the text. It is not recommended to simply delete
1624noncharacter code points from such text, because of the potential
1625security issues caused by deleting uninterpreted characters. (See
1626conformance clause C7 in Section 3.2, Conformance Requirements, and
1627L<Unicode Technical Report #36, "Unicode Security
1628Considerations"|http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
1629
1630=back
1631
1632This change was made because it was found that various commercial tools
1633like editors, or for things like source code control, had been written
1634so that they would not handle program files that used these code points,
1635effectively precluding their use almost entirely! And that was never
1636the intent. They've always been meant to be usable within an
1637application, or cooperating set of applications, at will.
1638
1639If you're writing code, such as an editor, that is supposed to be able
1640to handle any Unicode text data, then you shouldn't be using these code
1641points yourself, and instead allow them in the input. If you need
1642sentinels, they should instead be something that isn't legal Unicode.
1643For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
1644they never appear in well-formed UTF-8. (There are equivalents for
1645UTF-EBCDIC). You can also store your Unicode code points in integer
1646variables and use negative values as sentinels.
1647
1648If you're not writing such a tool, then whether you accept noncharacters
1649as input is up to you (though the Standard recommends that you not). If
1650you do strict input stream checking with Perl, these code points
1651continue to be forbidden. This is to maintain backward compatibility
1652(otherwise potential security holes could open up, as an unsuspecting
1653application that was written assuming the noncharacters would be
1654filtered out before getting to it, could now, without warning, start
1655getting them). To do strict checking, you can use the layer
1656C<:encoding('UTF-8')>.
1657
1658Perl continues to warn (using the warning category C<"nonchar">, which
1659is a sub-category of C<"utf8">) if an attempt is made to output
1660noncharacters.
42581d5d
KW
1661
1662=head2 Beyond Unicode code points
1663
a9130ea9
KW
1664The maximum Unicode code point is C<U+10FFFF>, and Unicode only defines
1665operations on code points up through that. But Perl works on code
526f2ca9 1666points up to the maximum permissible signed number available on the
42581d5d
KW
1667platform. However, Perl will not accept these from input streams unless
1668lax rules are being used, and will warn (using the warning category
2d88a86a
KW
1669C<"non_unicode">, which is a sub-category of C<"utf8">) if any are output.
1670
1671Since Unicode rules are not defined on these code points, if a
1672Unicode-defined operation is done on them, Perl uses what we believe are
1673sensible rules, while generally warning, using the C<"non_unicode">
1674category. For example, C<uc("\x{11_0000}")> will generate such a
1675warning, returning the input parameter as its result, since Perl defines
1676the uppercase of every non-Unicode code point to be the code point
b65e6125
KW
1677itself. (All the case changing operations, not just uppercasing, work
1678this way.)
2d88a86a
KW
1679
1680The situation with matching Unicode properties in regular expressions,
1681the C<\p{}> and C<\P{}> constructs, against these code points is not as
1682clear cut, and how these are handled has changed as we've gained
1683experience.
1684
1685One possibility is to treat any match against these code points as
1686undefined. But since Perl doesn't have the concept of a match being
1687undefined, it converts this to failing or C<FALSE>. This is almost, but
1688not quite, what Perl did from v5.14 (when use of these code points
1689became generally reliable) through v5.18. The difference is that Perl
1690treated all C<\p{}> matches as failing, but all C<\P{}> matches as
1691succeeding.
1692
f66ccb6c 1693One problem with this is that it leads to unexpected, and confusing
2d88a86a
KW
1694results in some cases:
1695
1696 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Failed on <= v5.18
1697 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Failed! on <= v5.18
1698
1699That is, it treated both matches as undefined, and converted that to
1700false (raising a warning on each). The first case is the expected
1701result, but the second is likely counterintuitive: "How could both be
1702false when they are complements?" Another problem was that the
1703implementation optimized many Unicode property matches down to already
1704existing simpler, faster operations, which don't raise the warning. We
1705chose to not forgo those optimizations, which help the vast majority of
1706matches, just to generate a warning for the unlikely event that an
1707above-Unicode code point is being matched against.
1708
1709As a result of these problems, starting in v5.20, what Perl does is
1710to treat non-Unicode code points as just typical unassigned Unicode
1711characters, and matches accordingly. (Note: Unicode has atypical
57e88091 1712unassigned code points. For example, it has noncharacter code points,
2d88a86a
KW
1713and ones that, when they do get assigned, are destined to be written
1714Right-to-left, as Arabic and Hebrew are. Perl assumes that no
1715non-Unicode code point has any atypical properties.)
1716
1717Perl, in most cases, will raise a warning when matching an above-Unicode
1718code point against a Unicode property when the result is C<TRUE> for
1719C<\p{}>, and C<FALSE> for C<\P{}>. For example:
1720
1721 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails, no warning
1722 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Succeeds, with warning
1723
1724In both these examples, the character being matched is non-Unicode, so
1725Unicode doesn't define how it should match. It clearly isn't an ASCII
1726hex digit, so the first example clearly should fail, and so it does,
1727with no warning. But it is arguable that the second example should have
1728an undefined, hence C<FALSE>, result. So a warning is raised for it.
1729
1730Thus the warning is raised for many fewer cases than in earlier Perls,
1731and only when what the result is could be arguable. It turns out that
1732none of the optimizations made by Perl (or are ever likely to be made)
1733cause the warning to be skipped, so it solves both problems of Perl's
1734earlier approach. The most commonly used property that is affected by
1735this change is C<\p{Unassigned}> which is a short form for
1736C<\p{General_Category=Unassigned}>. Starting in v5.20, all non-Unicode
1737code points are considered C<Unassigned>. In earlier releases the
1738matches failed because the result was considered undefined.
1739
1740The only place where the warning is not raised when it might ought to
1741have been is if optimizations cause the whole pattern match to not even
1742be attempted. For example, Perl may figure out that for a string to
1743match a certain regular expression pattern, the string has to contain
1744the substring C<"foobar">. Before attempting the match, Perl may look
1745for that substring, and if not found, immediately fail the match without
1746actually trying it; so no warning gets generated even if the string
1747contains an above-Unicode code point.
1748
1749This behavior is more "Do what I mean" than in earlier Perls for most
1750applications. But it catches fewer issues for code that needs to be
1751strictly Unicode compliant. Therefore there is an additional mode of
1752operation available to accommodate such code. This mode is enabled if a
1753regular expression pattern is compiled within the lexical scope where
1754the C<"non_unicode"> warning class has been made fatal, say by:
1755
1756 use warnings FATAL => "non_unicode"
1757
44ecbbd8 1758(see L<warnings>). In this mode of operation, Perl will raise the
2d88a86a
KW
1759warning for all matches against a non-Unicode code point (not just the
1760arguable ones), and it skips the optimizations that might cause the
1761warning to not be output. (It currently still won't warn if the match
1762isn't even attempted, like in the C<"foobar"> example above.)
1763
1764In summary, Perl now normally treats non-Unicode code points as typical
1765Unicode unassigned code points for regular expression matches, raising a
1766warning only when it is arguable what the result should be. However, if
1767this warning has been made fatal, it isn't skipped.
1768
1769There is one exception to all this. C<\p{All}> looks like a Unicode
1770property, but it is a Perl extension that is defined to be true for all
1771possible code points, Unicode or not, so no warning is ever generated
1772when matching this against a non-Unicode code point. (Prior to v5.20,
1773it was an exact synonym for C<\p{Any}>, matching code points C<0>
1774through C<0x10FFFF>.)
6d4f9cf2 1775
0d7c09bb
JH
1776=head2 Security Implications of Unicode
1777
b65e6125
KW
1778First, read
1779L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
1780
e1b711da
KW
1781Also, note the following:
1782
0d7c09bb
JH
1783=over 4
1784
1785=item *
1786
1787Malformed UTF-8
bf0fa0b2 1788
f57d8456
KW
1789UTF-8 is very structured, so many combinations of bytes are invalid. In
1790the past, Perl tried to soldier on and make some sense of invalid
1791combinations, but this can lead to security holes, so now, if the Perl
1792core needs to process an invalid combination, it will either raise a
1793fatal error, or will replace those bytes by the sequence that forms the
1794Unicode REPLACEMENT CHARACTER, for which purpose Unicode created it.
1795
1796Every code point can be represented by more than one possible
1797syntactically valid UTF-8 sequence. Early on, both Unicode and Perl
1798considered any of these to be valid, but now, all sequences longer
1799than the shortest possible one are considered to be malformed.
1800
1801Unicode considers many code points to be illegal, or to be avoided.
1802Perl generally accepts them, once they have passed through any input
1803filters that may try to exclude them. These have been discussed above
1804(see "Surrogates" under UTF-16 in L</Unicode Encodings>,
1805L</Noncharacter code points>, and L</Beyond Unicode code points>).
bf0fa0b2 1806
0d7c09bb
JH
1807=item *
1808
68693f9e 1809Regular expression pattern matching may surprise you if you're not
b19eb496
TC
1810accustomed to Unicode. Starting in Perl 5.14, several pattern
1811modifiers are available to control this, called the character set
42581d5d
KW
1812modifiers. Details are given in L<perlre/Character set modifiers>.
1813
1814=back
0d7c09bb 1815
376d9008 1816As discussed elsewhere, Perl has one foot (two hooves?) planted in
a6a7eedc
KW
1817each of two worlds: the old world of ASCII and single-byte locales, and
1818the new world of Unicode, upgrading when necessary.
376d9008 1819If your legacy code does not explicitly use Unicode, no automatic
a6a7eedc 1820switch-over to Unicode should happen.
0d7c09bb 1821
c349b1b9
JH
1822=head2 Unicode in Perl on EBCDIC
1823
a6a7eedc
KW
1824Unicode is supported on EBCDIC platforms. See L<perlebcdic>.
1825
1826Unless ASCII vs. EBCDIC issues are specifically being discussed,
1827references to UTF-8 encoding in this document and elsewhere should be
1828read as meaning UTF-EBCDIC on EBCDIC platforms.
1829See L<perlebcdic/Unicode and UTF>.
1830
1831Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
1832hidden from you; S<C<use utf8>> (and NOT something like
dabde021 1833S<C<use utfebcdic>>) declares the script is in the platform's
a6a7eedc
KW
1834"native" 8-bit encoding of Unicode. (Similarly for the C<":utf8">
1835layer.)
c349b1b9 1836
b310b053
JH
1837=head2 Locales
1838
42581d5d 1839See L<perllocale/Unicode and UTF-8>
b310b053 1840
1aad1664
JH
1841=head2 When Unicode Does Not Happen
1842
b65e6125
KW
1843There are still many places where Unicode (in some encoding or
1844another) could be given as arguments or received as results, or both in
1845Perl, but it is not, in spite of Perl having extensive ways to input and
1846output in Unicode, and a few other "entry points" like the C<@ARGV>
1847array (which can sometimes be interpreted as UTF-8).
1aad1664 1848
e1b711da
KW
1849The following are such interfaces. Also, see L</The "Unicode Bug">.
1850For all of these interfaces Perl
b9cedb1b 1851currently (as of v5.16.0) simply assumes byte strings both as arguments
b65e6125 1852and results, or UTF-8 strings if the (deprecated) C<encoding> pragma has been used.
1aad1664 1853
b19eb496
TC
1854One reason that Perl does not attempt to resolve the role of Unicode in
1855these situations is that the answers are highly dependent on the operating
1aad1664 1856system and the file system(s). For example, whether filenames can be
b19eb496
TC
1857in Unicode and in exactly what kind of encoding, is not exactly a
1858portable concept. Similarly for C<qx> and C<system>: how well will the
1859"command-line interface" (and which of them?) handle Unicode?
1aad1664
JH
1860
1861=over 4
1862
557a2462
RB
1863=item *
1864
a9130ea9
KW
1865C<chdir>, C<chmod>, C<chown>, C<chroot>, C<exec>, C<link>, C<lstat>, C<mkdir>,
1866C<rename>, C<rmdir>, C<stat>, C<symlink>, C<truncate>, C<unlink>, C<utime>, C<-X>
557a2462
RB
1867
1868=item *
1869
a9130ea9 1870C<%ENV>
557a2462
RB
1871
1872=item *
1873
a9130ea9 1874C<glob> (aka the C<E<lt>*E<gt>>)
557a2462
RB
1875
1876=item *
1aad1664 1877
a9130ea9 1878C<open>, C<opendir>, C<sysopen>
1aad1664 1879
557a2462 1880=item *
1aad1664 1881
a9130ea9 1882C<qx> (aka the backtick operator), C<system>
1aad1664 1883
557a2462 1884=item *
1aad1664 1885
a9130ea9 1886C<readdir>, C<readlink>
1aad1664
JH
1887
1888=back
1889
e1b711da
KW
1890=head2 The "Unicode Bug"
1891
a6a7eedc
KW
1892The term, "Unicode bug" has been applied to an inconsistency with the
1893code points in the C<Latin-1 Supplement> block, that is, between
1894128 and 255. Without a locale specified, unlike all other characters or
1895code points, these characters can have very different semantics
1896depending on the rules in effect. (Characters whose code points are
1897above 255 force Unicode rules; whereas the rules for ASCII characters
1898are the same under both ASCII and Unicode rules.)
1899
1900Under Unicode rules, these upper-Latin1 characters are interpreted as
1901Unicode code points, which means they have the same semantics as Latin-1
1902(ISO-8859-1) and C1 controls.
1903
1904As explained in L</ASCII Rules versus Unicode Rules>, under ASCII rules,
1905they are considered to be unassigned characters.
1906
1907This can lead to unexpected results. For example, a string's
1908semantics can suddenly change if a code point above 255 is appended to
1909it, which changes the rules from ASCII to Unicode. As an
1910example, consider the following program and its output:
1911
1912 $ perl -le'
f434f357 1913 no feature "unicode_strings";
a6a7eedc
KW
1914 $s1 = "\xC2";
1915 $s2 = "\x{2660}";
1916 for ($s1, $s2, $s1.$s2) {
1917 print /\w/ || 0;
1918 }
1919 '
1920 0
1921 0
1922 1
1923
1924If there's no C<\w> in C<s1> nor in C<s2>, why does their concatenation
1925have one?
1926
1927This anomaly stems from Perl's attempt to not disturb older programs that
1928didn't use Unicode, along with Perl's desire to add Unicode support
1929seamlessly. But the result turned out to not be seamless. (By the way,
1930you can choose to be warned when things like this happen. See
1931C<L<encoding::warnings>>.)
1932
1933L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
1934was added, starting in Perl v5.12, to address this problem. It affects
1935these things:
e1b711da
KW
1936
1937=over 4
1938
1939=item *
1940
1941Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
2e2b2571
KW
1942and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in double-quotish
1943contexts, such as regular expression substitutions.
a6a7eedc
KW
1944
1945Under C<unicode_strings> starting in Perl 5.12.0, Unicode rules are
2e2b2571
KW
1946generally used. See L<perlfunc/lc> for details on how this works
1947in combination with various other pragmas.
e1b711da
KW
1948
1949=item *
1950
2e2b2571 1951Using caseless (C</i>) regular expression matching.
a6a7eedc 1952
2e2b2571 1953Starting in Perl 5.14.0, regular expressions compiled within
a6a7eedc 1954the scope of C<unicode_strings> use Unicode rules
2e2b2571
KW
1955even when executed or compiled into larger
1956regular expressions outside the scope.
e1b711da
KW
1957
1958=item *
1959
a6a7eedc
KW
1960Matching any of several properties in regular expressions.
1961
1962These properties are C<\b> (without braces), C<\B> (without braces),
1963C<\s>, C<\S>, C<\w>, C<\W>, and all the Posix character classes
630d17dc 1964I<except> C<[[:ascii:]]>.
a6a7eedc 1965
2e2b2571 1966Starting in Perl 5.14.0, regular expressions compiled within
a6a7eedc 1967the scope of C<unicode_strings> use Unicode rules
2e2b2571
KW
1968even when executed or compiled into larger
1969regular expressions outside the scope.
e1b711da
KW
1970
1971=item *
1972
a6a7eedc
KW
1973In C<quotemeta> or its inline equivalent C<\Q>.
1974
2e2b2571
KW
1975Starting in Perl 5.16.0, consistent quoting rules are used within the
1976scope of C<unicode_strings>, as described in L<perlfunc/quotemeta>.
a6a7eedc
KW
1977Prior to that, or outside its scope, no code points above 127 are quoted
1978in UTF-8 encoded strings, but in byte encoded strings, code points
1979between 128-255 are always quoted.
eb88ed9e 1980
d6c970c7
AC
1981=item *
1982
1983In the C<..> or L<range|perlop/Range Operators> operator.
1984
1985Starting in Perl 5.26.0, the range operator on strings treats their lengths
1986consistently within the scope of C<unicode_strings>. Prior to that, or
1987outside its scope, it could produce strings whose length in characters
1988exceeded that of the right-hand side, where the right-hand side took up more
1989bytes than the correct range endpoint.
1990
20ae58f7
AC
1991=item *
1992
1993In L<< C<split>'s special-case whitespace splitting|perlfunc/split >>.
1994
1995Starting in Perl 5.28.0, the C<split> function with a pattern specified as
1996a string containing a single space handles whitespace characters consistently
1997within the scope of of C<unicode_strings>. Prior to that, or outside its scope,
1998characters that are whitespace according to Unicode rules but not according to
1999ASCII rules were treated as field contents rather than field separators when
2000they appear in byte-encoded strings.
2001
e1b711da
KW
2002=back
2003
a6a7eedc
KW
2004You can see from the above that the effect of C<unicode_strings>
2005increased over several Perl releases. (And Perl's support for Unicode
2006continues to improve; it's best to use the latest available release in
2007order to get the most complete and accurate results possible.) Note that
2008C<unicode_strings> is automatically chosen if you S<C<use 5.012>> or
2009higher.
e1b711da 2010
2e2b2571 2011For Perls earlier than those described above, or when a string is passed
a6a7eedc 2012to a function outside the scope of C<unicode_strings>, see the next section.
e1b711da 2013
1aad1664
JH
2014=head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
2015
e1b711da
KW
2016Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
2017there are situations where you simply need to force a byte
a6a7eedc
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2018string into UTF-8, or vice versa. The standard module L<Encode> can be
2019used for this, or the low-level calls
a9130ea9 2020L<C<utf8::upgrade($bytestring)>|utf8/Utility functions> and
a6a7eedc 2021L<C<utf8::downgrade($utf8string[, FAIL_OK])>|utf8/Utility functions>.
1aad1664 2022
a9130ea9 2023Note that C<utf8::downgrade()> can fail if the string contains characters
2bbc8d55 2024that don't fit into a byte.
1aad1664 2025
e1b711da
KW
2026Calling either function on a string that already is in the desired state is a
2027no-op.
2028
a6a7eedc
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2029L</ASCII Rules versus Unicode Rules> gives all the ways that a string is
2030made to use Unicode rules.
95a1a48b 2031
37b3b608 2032=head2 Using Unicode in XS
c349b1b9 2033
37b3b608
KW
2034See L<perlguts/"Unicode Support"> for an introduction to Unicode at
2035the XS level, and L<perlapi/Unicode Support> for the API details.
95a1a48b 2036
e1b711da
KW
2037=head2 Hacking Perl to work on earlier Unicode versions (for very serious hackers only)
2038
a6a7eedc
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2039Perl by default comes with the latest supported Unicode version built-in, but
2040the goal is to allow you to change to use any earlier one. In Perls
2041v5.20 and v5.22, however, the earliest usable version is Unicode 5.1.
c55dd03d 2042Perl v5.18 and v5.24 are able to handle all earlier versions.
e1b711da 2043
42581d5d 2044Download the files in the desired version of Unicode from the Unicode web
e1b711da 2045site L<http://www.unicode.org>). These should replace the existing files in
b19eb496 2046F<lib/unicore> in the Perl source tree. Follow the instructions in
116693e8 2047F<README.perl> in that directory to change some of their names, and then build
26e391dd 2048perl (see L<INSTALL>).
116693e8 2049
c8d992ba
A
2050=head2 Porting code from perl-5.6.X
2051
a6a7eedc
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2052Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6 the
2053programmer was required to use the C<utf8> pragma to declare that a
2054given scope expected to deal with Unicode data and had to make sure that
2055only Unicode data were reaching that scope. If you have code that is
c8d992ba 2056working with 5.6, you will need some of the following adjustments to
a6a7eedc
KW
2057your code. The examples are written such that the code will continue to
2058work under 5.6, so you should be safe to try them out.
c8d992ba 2059
755789c0 2060=over 3
c8d992ba
A
2061
2062=item *
2063
2064A filehandle that should read or write UTF-8
2065
b9cedb1b 2066 if ($] > 5.008) {
6d8e7450 2067 binmode $fh, ":encoding(UTF-8)";
c8d992ba
A
2068 }
2069
2070=item *
2071
2072A scalar that is going to be passed to some extension
2073
a9130ea9 2074Be it C<Compress::Zlib>, C<Apache::Request> or any extension that has no
c8d992ba 2075mention of Unicode in the manpage, you need to make sure that the
2575c402 2076UTF8 flag is stripped off. Note that at the time of this writing
b9cedb1b 2077(January 2012) the mentioned modules are not UTF-8-aware. Please
c8d992ba
A
2078check the documentation to verify if this is still true.
2079
b9cedb1b 2080 if ($] > 5.008) {
c8d992ba 2081 require Encode;
8e179dd8 2082 $val = Encode::encode("UTF-8", $val); # make octets
c8d992ba
A
2083 }
2084
2085=item *
2086
2087A scalar we got back from an extension
2088
2089If you believe the scalar comes back as UTF-8, you will most likely
2575c402 2090want the UTF8 flag restored:
c8d992ba 2091
b9cedb1b 2092 if ($] > 5.008) {
c8d992ba 2093 require Encode;
8e179dd8 2094 $val = Encode::decode("UTF-8", $val);
c8d992ba
A
2095 }
2096
2097=item *
2098
2099Same thing, if you are really sure it is UTF-8
2100
b9cedb1b 2101 if ($] > 5.008) {
c8d992ba
A
2102 require Encode;
2103 Encode::_utf8_on($val);
2104 }
2105
2106=item *
2107
a9130ea9 2108A wrapper for L<DBI> C<fetchrow_array> and C<fetchrow_hashref>
c8d992ba
A
2109
2110When the database contains only UTF-8, a wrapper function or method is
a9130ea9
KW
2111a convenient way to replace all your C<fetchrow_array> and
2112C<fetchrow_hashref> calls. A wrapper function will also make it easier to
c8d992ba 2113adapt to future enhancements in your database driver. Note that at the
b9cedb1b 2114time of this writing (January 2012), the DBI has no standardized way
a9130ea9 2115to deal with UTF-8 data. Please check the L<DBI documentation|DBI> to verify if
c8d992ba
A
2116that is still true.
2117
2118 sub fetchrow {
d88362ca
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2119 # $what is one of fetchrow_{array,hashref}
2120 my($self, $sth, $what) = @_;
b9cedb1b 2121 if ($] < 5.008) {
c8d992ba
A
2122 return $sth->$what;
2123 } else {
2124 require Encode;
2125 if (wantarray) {
2126 my @arr = $sth->$what;
2127 for (@arr) {
2128 defined && /[^\000-\177]/ && Encode::_utf8_on($_);
2129 }
2130 return @arr;
2131 } else {
2132 my $ret = $sth->$what;
2133 if (ref $ret) {
2134 for my $k (keys %$ret) {
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2135 defined
2136 && /[^\000-\177]/
2137 && Encode::_utf8_on($_) for $ret->{$k};
c8d992ba
A
2138 }
2139 return $ret;
2140 } else {
2141 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
2142 return $ret;
2143 }
2144 }
2145 }
2146 }
2147
2148
2149=item *
2150
2151A large scalar that you know can only contain ASCII
2152
2153Scalars that contain only ASCII and are marked as UTF-8 are sometimes
2154a drag to your program. If you recognize such a situation, just remove
2575c402 2155the UTF8 flag:
c8d992ba 2156
b9cedb1b 2157 utf8::downgrade($val) if $] > 5.008;
c8d992ba
A
2158
2159=back
2160
a6a7eedc
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2161=head1 BUGS
2162
2163See also L</The "Unicode Bug"> above.
2164
2165=head2 Interaction with Extensions
2166
2167When Perl exchanges data with an extension, the extension should be
2168able to understand the UTF8 flag and act accordingly. If the
2169extension doesn't recognize that flag, it's likely that the extension
2170will return incorrectly-flagged data.
2171
2172So if you're working with Unicode data, consult the documentation of
2173every module you're using if there are any issues with Unicode data
2174exchange. If the documentation does not talk about Unicode at all,
2175suspect the worst and probably look at the source to learn how the
2176module is implemented. Modules written completely in Perl shouldn't
2177cause problems. Modules that directly or indirectly access code written
2178in other programming languages are at risk.
2179
2180For affected functions, the simple strategy to avoid data corruption is
2181to always make the encoding of the exchanged data explicit. Choose an
2182encoding that you know the extension can handle. Convert arguments passed
2183to the extensions to that encoding and convert results back from that
2184encoding. Write wrapper functions that do the conversions for you, so
2185you can later change the functions when the extension catches up.
2186
2187To provide an example, let's say the popular C<Foo::Bar::escape_html>
2188function doesn't deal with Unicode data yet. The wrapper function
2189would convert the argument to raw UTF-8 and convert the result back to
2190Perl's internal representation like so:
2191
2192 sub my_escape_html ($) {
2193 my($what) = shift;
2194 return unless defined $what;
8e179dd8
P
2195 Encode::decode("UTF-8", Foo::Bar::escape_html(
2196 Encode::encode("UTF-8", $what)));
a6a7eedc
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2197 }
2198
2199Sometimes, when the extension does not convert data but just stores
2200and retrieves it, you will be able to use the otherwise
2201dangerous L<C<Encode::_utf8_on()>|Encode/_utf8_on> function. Let's say
2202the popular C<Foo::Bar> extension, written in C, provides a C<param>
2203method that lets you store and retrieve data according to these prototypes:
2204
2205 $self->param($name, $value); # set a scalar
2206 $value = $self->param($name); # retrieve a scalar
2207
2208If it does not yet provide support for any encoding, one could write a
2209derived class with such a C<param> method:
2210
2211 sub param {
2212 my($self,$name,$value) = @_;
2213 utf8::upgrade($name); # make sure it is UTF-8 encoded
2214 if (defined $value) {
2215 utf8::upgrade($value); # make sure it is UTF-8 encoded
2216 return $self->SUPER::param($name,$value);
2217 } else {
2218 my $ret = $self->SUPER::param($name);
2219 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
2220 return $ret;
2221 }
2222 }
2223
2224Some extensions provide filters on data entry/exit points, such as
2225C<DB_File::filter_store_key> and family. Look out for such filters in
2226the documentation of your extensions; they can make the transition to
2227Unicode data much easier.
2228
2229=head2 Speed
2230
2231Some functions are slower when working on UTF-8 encoded strings than
2232on byte encoded strings. All functions that need to hop over
2233characters such as C<length()>, C<substr()> or C<index()>, or matching
2234regular expressions can work B<much> faster when the underlying data are
2235byte-encoded.
2236
2237In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
2238a caching scheme was introduced which improved the situation. In general,
2239operations with UTF-8 encoded strings are still slower. As an example,
2240the Unicode properties (character classes) like C<\p{Nd}> are known to
2241be quite a bit slower (5-20 times) than their simpler counterparts
2242like C<[0-9]> (then again, there are hundreds of Unicode characters matching
2243C<Nd> compared with the 10 ASCII characters matching C<[0-9]>).
2244
393fec97
GS
2245=head1 SEE ALSO
2246
51f494cc 2247L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>,
b65e6125 2248L<perlretut>, L<perlvar/"${^UNICODE}">,
51f494cc 2249L<http://www.unicode.org/reports/tr44>).
393fec97
GS
2250
2251=cut