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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
51f494cc
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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
KW
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
KW
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
51f494cc
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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
48791bf1
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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
48791bf1
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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
KW
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
KW
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
KW
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
KW
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
KW
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
KW
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
48791bf1
KW
677The explanation above has omitted some detail; refer to UAX#24 "Unicode
678Script Property": L<http://www.unicode.org/reports/tr24>.
679
51f494cc
KW
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
KW
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
KW
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
KW
702those digits are shared across many scripts, and hence are in the
703C<Common> script.
51f494cc
KW
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
KW
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
KW
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
KW
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
KW
732C<\p{Script_Extensions=Hebrew}> which is NOT the same thing as
733C<\p{Blk=Hebrew}>. Our
6b5cf123
KW
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
KW
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
KW
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
KW
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
KW
759L<Unicode Standard|http://www.unicode.org/reports/tr44>.
760
761=over
762
763=item B<C<\p{All}>>
764
2d88a86a
KW
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).
9f815e24
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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
KW
776This matches any of the 1_114_112 Unicode code points. It is a synonym
777for C<\p{Unicode}>.
9f815e24 778
42581d5d
KW
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
9f815e24
KW
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
KW
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
KW
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
KW
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
KW
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
KW
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
KW
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
KW
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
KW
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>.
9f815e24
KW
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
526f2ca9
KW
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
KW
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
KW
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
KW
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
KW
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.
9f815e24
KW
884
885=item B<C<\p{Print}>>
886
ae5b72c8 887This matches any character that is graphical or blank, except controls.
9f815e24
KW
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
4364919a
KW
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
KW
903=item B<C<\p{Unicode}>>
904
905This matches any of the 1_114_112 Unicode code points.
906C<\p{Any}>.
907
9f815e24
KW
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
KW
922=back
923
a9130ea9 924
376d9008 925=head2 User-Defined Character Properties
491fd90a 926
51f494cc 927You can define your own binary character properties by defining subroutines
a9130ea9 928whose names begin with C<"In"> or C<"Is">. (The experimental feature
9d1a5160
KW
929L<perlre/(?[ ])> provides an alternative which allows more complex
930definitions.) The subroutines can be defined in any
51f494cc 931package. The user-defined properties can be used in the regular expression
a9130ea9 932C<\p{}> and C<\P{}> constructs; if you are using a user-defined property from a
51f494cc 933package other than the one you are in, you must specify its package in the
a9130ea9 934C<\p{}> or C<\P{}> construct.
bac0b425 935
51f494cc 936 # assuming property Is_Foreign defined in Lang::
bac0b425
JP
937 package main; # property package name required
938 if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
939
940 package Lang; # property package name not required
941 if ($txt =~ /\p{IsForeign}+/) { ... }
942
943
944Note that the effect is compile-time and immutable once defined.
b19eb496
TC
945However, the subroutines are passed a single parameter, which is 0 if
946case-sensitive matching is in effect and non-zero if caseless matching
56ca34ca
KW
947is in effect. The subroutine may return different values depending on
948the value of the flag, and one set of values will immutably be in effect
b19eb496 949for all case-sensitive matches, and the other set for all case-insensitive
56ca34ca 950matches.
491fd90a 951
b19eb496 952Note that if the regular expression is tainted, then Perl will die rather
a9130ea9 953than calling the subroutine when the name of the subroutine is
0e9be77f
DM
954determined by the tainted data.
955
376d9008
JB
956The subroutines must return a specially-formatted string, with one
957or more newline-separated lines. Each line must be one of the following:
491fd90a
JH
958
959=over 4
960
961=item *
962
df9e1087 963A single hexadecimal number denoting a code point to include.
510254c9
A
964
965=item *
966
99a6b1f0 967Two hexadecimal numbers separated by horizontal whitespace (space or
73b95840
KW
968tabular characters) denoting a range of code points to include. The
969second number must not be smaller than the first.
491fd90a
JH
970
971=item *
972
a9130ea9
KW
973Something to include, prefixed by C<"+">: a built-in character
974property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 975name) user-defined character property,
bac0b425
JP
976to represent all the characters in that property; two hexadecimal code
977points for a range; or a single hexadecimal code point.
491fd90a
JH
978
979=item *
980
a9130ea9
KW
981Something to exclude, prefixed by C<"-">: an existing character
982property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 983name) user-defined character property,
bac0b425
JP
984to represent all the characters in that property; two hexadecimal code
985points for a range; or a single hexadecimal code point.
491fd90a
JH
986
987=item *
988
a9130ea9
KW
989Something to negate, prefixed C<"!">: an existing character
990property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 991name) user-defined character property,
bac0b425
JP
992to represent all the characters in that property; two hexadecimal code
993points for a range; or a single hexadecimal code point.
994
995=item *
996
a9130ea9
KW
997Something to intersect with, prefixed by C<"&">: an existing character
998property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 999name) user-defined character property,
bac0b425
JP
1000for all the characters except the characters in the property; two
1001hexadecimal code points for a range; or a single hexadecimal code point.
491fd90a
JH
1002
1003=back
1004
1005For example, to define a property that covers both the Japanese
1006syllabaries (hiragana and katakana), you can define
1007
1008 sub InKana {
d88362ca 1009 return <<END;
d5822f25
A
1010 3040\t309F
1011 30A0\t30FF
491fd90a
JH
1012 END
1013 }
1014
d5822f25
A
1015Imagine that the here-doc end marker is at the beginning of the line.
1016Now you can use C<\p{InKana}> and C<\P{InKana}>.
491fd90a
JH
1017
1018You could also have used the existing block property names:
1019
1020 sub InKana {
d88362ca 1021 return <<'END';
491fd90a
JH
1022 +utf8::InHiragana
1023 +utf8::InKatakana
1024 END
1025 }
1026
1027Suppose you wanted to match only the allocated characters,
d5822f25 1028not the raw block ranges: in other words, you want to remove
b65e6125 1029the unassigned characters:
491fd90a
JH
1030
1031 sub InKana {
d88362ca 1032 return <<'END';
491fd90a
JH
1033 +utf8::InHiragana
1034 +utf8::InKatakana
1035 -utf8::IsCn
1036 END
1037 }
1038
1039The negation is useful for defining (surprise!) negated classes.
1040
1041 sub InNotKana {
d88362ca 1042 return <<'END';
491fd90a
JH
1043 !utf8::InHiragana
1044 -utf8::InKatakana
1045 +utf8::IsCn
1046 END
1047 }
1048
461020ad
KW
1049This will match all non-Unicode code points, since every one of them is
1050not in Kana. You can use intersection to exclude these, if desired, as
1051this modified example shows:
bac0b425 1052
461020ad 1053 sub InNotKana {
bac0b425 1054 return <<'END';
461020ad
KW
1055 !utf8::InHiragana
1056 -utf8::InKatakana
1057 +utf8::IsCn
1058 &utf8::Any
bac0b425
JP
1059 END
1060 }
1061
461020ad
KW
1062C<&utf8::Any> must be the last line in the definition.
1063
1064Intersection is used generally for getting the common characters matched
a9130ea9 1065by two (or more) classes. It's important to remember not to use C<"&"> for
461020ad 1066the first set; that would be intersecting with nothing, resulting in an
5acbde07 1067empty set. (Similarly using C<"-"> for the first set does nothing).
461020ad 1068
2d88a86a
KW
1069Unlike non-user-defined C<\p{}> property matches, no warning is ever
1070generated if these properties are matched against a non-Unicode code
1071point (see L</Beyond Unicode code points> below).
bac0b425 1072
68585b5e 1073=head2 User-Defined Case Mappings (for serious hackers only)
822502e5 1074
5d1892be 1075B<This feature has been removed as of Perl 5.16.>
a9130ea9 1076The CPAN module C<L<Unicode::Casing>> provides better functionality without
5d1892be
KW
1077the drawbacks that this feature had. If you are using a Perl earlier
1078than 5.16, this feature was most fully documented in the 5.14 version of
1079this pod:
1080L<http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>
3a2263fe 1081
376d9008 1082=head2 Character Encodings for Input and Output
8cbd9a7a 1083
7221edc9 1084See L<Encode>.
8cbd9a7a 1085
c29a771d 1086=head2 Unicode Regular Expression Support Level
776f8809 1087
b19eb496 1088The following list of Unicode supported features for regular expressions describes
fea12a3e
KW
1089all features currently directly supported by core Perl. The references
1090to "Level I<N>" and the section numbers refer to
1091L<UTS#18 "Unicode Regular Expressions"|http://www.unicode.org/reports/tr18>,
526f2ca9 1092version 18, October 2016.
fea12a3e
KW
1093
1094=head3 Level 1 - Basic Unicode Support
1095
1096 RL1.1 Hex Notation - Done [1]
1097 RL1.2 Properties - Done [2]
1098 RL1.2a Compatibility Properties - Done [3]
1099 RL1.3 Subtraction and Intersection - Experimental [4]
1100 RL1.4 Simple Word Boundaries - Done [5]
1101 RL1.5 Simple Loose Matches - Done [6]
1102 RL1.6 Line Boundaries - Partial [7]
1103 RL1.7 Supplementary Code Points - Done [8]
755789c0 1104
6f33e417
KW
1105=over 4
1106
a6a7eedc 1107=item [1] C<\N{U+...}> and C<\x{...}>
6f33e417 1108
fea12a3e
KW
1109=item [2]
1110C<\p{...}> C<\P{...}>. This requirement is for a minimal list of
1111properties. Perl supports these and all other Unicode character
1112properties, as R2.7 asks (see L</"Unicode Character Properties"> above).
6f33e417 1113
fea12a3e
KW
1114=item [3]
1115Perl has C<\d> C<\D> C<\s> C<\S> C<\w> C<\W> C<\X> C<[:I<prop>:]>
1116C<[:^I<prop>:]>, plus all the properties specified by
1117L<http://www.unicode.org/reports/tr18/#Compatibility_Properties>. These
1118are described above in L</Other Properties>
6f33e417 1119
fea12a3e 1120=item [4]
6f33e417 1121
fea12a3e 1122The experimental feature C<"(?[...])"> starting in v5.18 accomplishes
a6a7eedc 1123this.
6f33e417 1124
a6a7eedc
KW
1125See L<perlre/(?[ ])>. If you don't want to use an experimental
1126feature, you can use one of the following:
6f33e417
KW
1127
1128=over 4
1129
a6a7eedc 1130=item *
f67a5002 1131Regular expression lookahead
6f33e417
KW
1132
1133You can mimic class subtraction using lookahead.
8158862b 1134For example, what UTS#18 might write as
29bdacb8 1135
209c9685 1136 [{Block=Greek}-[{UNASSIGNED}]]
dbe420b4
JH
1137
1138in Perl can be written as:
1139
209c9685
KW
1140 (?!\p{Unassigned})\p{Block=Greek}
1141 (?=\p{Assigned})\p{Block=Greek}
dbe420b4
JH
1142
1143But in this particular example, you probably really want
1144
209c9685 1145 \p{Greek}
dbe420b4
JH
1146
1147which will match assigned characters known to be part of the Greek script.
29bdacb8 1148
a6a7eedc
KW
1149=item *
1150
1151CPAN module C<L<Unicode::Regex::Set>>
8158862b 1152
6f33e417
KW
1153It does implement the full UTS#18 grouping, intersection, union, and
1154removal (subtraction) syntax.
8158862b 1155
a6a7eedc
KW
1156=item *
1157
1158L</"User-Defined Character Properties">
6f33e417 1159
a9130ea9 1160C<"+"> for union, C<"-"> for removal (set-difference), C<"&"> for intersection
6f33e417
KW
1161
1162=back
1163
fea12a3e
KW
1164=item [5]
1165C<\b> C<\B> meet most, but not all, the details of this requirement, but
1166C<\b{wb}> and C<\B{wb}> do, as well as the stricter R2.3.
1167
1168=item [6]
6f33e417 1169
a6a7eedc 1170Note that Perl does Full case-folding in matching, not Simple:
6f33e417 1171
a6a7eedc
KW
1172For example C<U+1F88> is equivalent to C<U+1F00 U+03B9>, instead of just
1173C<U+1F80>. This difference matters mainly for certain Greek capital
a9130ea9
KW
1174letters with certain modifiers: the Full case-folding decomposes the
1175letter, while the Simple case-folding would map it to a single
1176character.
6f33e417 1177
fea12a3e
KW
1178=item [7]
1179
1180The reason this is considered to be only partially implemented is that
1181Perl has L<C<qrE<sol>\b{lb}E<sol>>|perlrebackslash/\b{lb}> and
1182C<L<Unicode::LineBreak>> that are conformant with
1183L<UAX#14 "Unicode Line Breaking Algorithm"|http://www.unicode.org/reports/tr14>.
1184The regular expression construct provides default behavior, while the
1185heavier-weight module provides customizable line breaking.
1186
1187But Perl treats C<\n> as the start- and end-line
1188delimiter, whereas Unicode specifies more characters that should be
1189so-interpreted.
6f33e417 1190
a6a7eedc 1191These are:
6f33e417 1192
a6a7eedc
KW
1193 VT U+000B (\v in C)
1194 FF U+000C (\f)
1195 CR U+000D (\r)
1196 NEL U+0085
1197 LS U+2028
1198 PS U+2029
6f33e417 1199
a6a7eedc
KW
1200C<^> and C<$> in regular expression patterns are supposed to match all
1201these, but don't.
1202These characters also don't, but should, affect C<< <> >> C<$.>, and
1203script line numbers.
6f33e417 1204
a6a7eedc
KW
1205Also, lines should not be split within C<CRLF> (i.e. there is no
1206empty line between C<\r> and C<\n>). For C<CRLF>, try the C<:crlf>
1207layer (see L<PerlIO>).
1208
fea12a3e 1209=item [8]
a9130ea9
KW
1210UTF-8/UTF-EBDDIC used in Perl allows not only C<U+10000> to
1211C<U+10FFFF> but also beyond C<U+10FFFF>
6f33e417
KW
1212
1213=back
5ca1ac52 1214
fea12a3e 1215=head3 Level 2 - Extended Unicode Support
776f8809 1216
fea12a3e
KW
1217 RL2.1 Canonical Equivalents - Retracted [9]
1218 by Unicode
1219 RL2.2 Extended Grapheme Clusters - Partial [10]
1220 RL2.3 Default Word Boundaries - Done [11]
1221 RL2.4 Default Case Conversion - Done
1222 RL2.5 Name Properties - Done
1223 RL2.6 Wildcard Properties - Missing
1224 RL2.7 Full Properties - Done
776f8809 1225
fea12a3e 1226=over 4
8158862b 1227
fea12a3e
KW
1228=item [9]
1229Unicode has rewritten this portion of UTS#18 to say that getting
1230canonical equivalence (see UAX#15
1231L<"Unicode Normalization Forms"|http://www.unicode.org/reports/tr15>)
1232is basically to be done at the programmer level. Use NFD to write
1233both your regular expressions and text to match them against (you
1234can use L<Unicode::Normalize>).
776f8809 1235
fea12a3e
KW
1236=item [10]
1237Perl has C<\X> and C<\b{gcb}> but we don't have a "Grapheme Cluster Mode".
1238
1239=item [11] see
1240L<UAX#29 "Unicode Text Segmentation"|http://www.unicode.org/reports/tr29>,
1241
1242=back
1243
1244=head3 Level 3 - Tailored Support
1245
1246 RL3.1 Tailored Punctuation - Missing
526f2ca9 1247 RL3.2 Tailored Grapheme Clusters - Missing [13]
fea12a3e
KW
1248 RL3.3 Tailored Word Boundaries - Missing
1249 RL3.4 Tailored Loose Matches - Retracted by Unicode
1250 RL3.5 Tailored Ranges - Retracted by Unicode
526f2ca9 1251 RL3.6 Context Matching - Partial [14]
fea12a3e 1252 RL3.7 Incremental Matches - Missing
526f2ca9 1253 RL3.8 Unicode Set Sharing - Retracted by Unicode
fea12a3e
KW
1254 RL3.9 Possible Match Sets - Missing
1255 RL3.10 Folded Matching - Retracted by Unicode
526f2ca9 1256 RL3.11 Submatchers - Partial [15]
fea12a3e
KW
1257
1258=over 4
1259
526f2ca9 1260=item [13]
fea12a3e
KW
1261Perl has L<Unicode::Collate>, but it isn't integrated with regular
1262expressions. See
1263L<UTS#10 "Unicode Collation Algorithms"|http://www.unicode.org/reports/tr10>.
776f8809 1264
526f2ca9
KW
1265=item [14]
1266Perl has C<(?<=x)> and C<(?=x)>, but this requirement says that it
1267should be possible to specify that matches may occur only in a substring
1268with the lookaheads and lookbehinds able to see beyond that matchable
1269portion.
1270
1271=item [15]
1272Perl has user-defined properties (L</"User-Defined Character
1273Properties">) to look at single code points in ways beyond Unicode, and
1274it might be possible, though probably not very clean, to use code blocks
1275and things like C<(?(DEFINE)...)> (see L<perlre> to do more specialized
1276matching.
776f8809
JH
1277
1278=back
1279
c349b1b9
JH
1280=head2 Unicode Encodings
1281
376d9008
JB
1282Unicode characters are assigned to I<code points>, which are abstract
1283numbers. To use these numbers, various encodings are needed.
c349b1b9
JH
1284
1285=over 4
1286
c29a771d 1287=item *
5cb3728c
RB
1288
1289UTF-8
c349b1b9 1290
6d4f9cf2 1291UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
a6a7eedc
KW
1292encoding. In most of Perl's documentation, including elsewhere in this
1293document, the term "UTF-8" means also "UTF-EBCDIC". But in this section,
1294"UTF-8" refers only to the encoding used on ASCII platforms. It is a
1295superset of 7-bit US-ASCII, so anything encoded in ASCII has the
1296identical representation when encoded in UTF-8.
c349b1b9 1297
8c007b5a 1298The following table is from Unicode 3.2.
05632f9a 1299
755789c0 1300 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
05632f9a 1301
d88362ca 1302 U+0000..U+007F 00..7F
e1b711da 1303 U+0080..U+07FF * C2..DF 80..BF
d88362ca 1304 U+0800..U+0FFF E0 * A0..BF 80..BF
ec90690f
ST
1305 U+1000..U+CFFF E1..EC 80..BF 80..BF
1306 U+D000..U+D7FF ED 80..9F 80..BF
755789c0 1307 U+D800..U+DFFF +++++ utf16 surrogates, not legal utf8 +++++
ec90690f 1308 U+E000..U+FFFF EE..EF 80..BF 80..BF
d88362ca
KW
1309 U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF
1310 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
1311 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
e1b711da 1312
b19eb496 1313Note the gaps marked by "*" before several of the byte entries above. These are
e1b711da
KW
1314caused by legal UTF-8 avoiding non-shortest encodings: it is technically
1315possible to UTF-8-encode a single code point in different ways, but that is
1316explicitly forbidden, and the shortest possible encoding should always be used
1317(and that is what Perl does).
37361303 1318
376d9008 1319Another way to look at it is via bits:
05632f9a 1320
755789c0 1321 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
05632f9a 1322
755789c0
KW
1323 0aaaaaaa 0aaaaaaa
1324 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
1325 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
1326 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
05632f9a 1327
a9130ea9 1328As you can see, the continuation bytes all begin with C<"10">, and the
e1b711da 1329leading bits of the start byte tell how many bytes there are in the
05632f9a
JH
1330encoded character.
1331
6d4f9cf2 1332The original UTF-8 specification allowed up to 6 bytes, to allow
a9130ea9 1333encoding of numbers up to C<0x7FFF_FFFF>. Perl continues to allow those,
6d4f9cf2
KW
1334and has extended that up to 13 bytes to encode code points up to what
1335can fit in a 64-bit word. However, Perl will warn if you output any of
b19eb496 1336these as being non-portable; and under strict UTF-8 input protocols,
526f2ca9 1337they are forbidden. In addition, it is now illegal to use a code point
760c7c2f
KW
1338larger than what a signed integer variable on your system can hold. On
133932-bit ASCII systems, this means C<0x7FFF_FFFF> is the legal maximum
526f2ca9 1340(much higher on 64-bit systems).
6d4f9cf2 1341
c29a771d 1342=item *
5cb3728c
RB
1343
1344UTF-EBCDIC
dbe420b4 1345
b65e6125 1346Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
a6a7eedc
KW
1347This means that all the basic characters (which includes all
1348those that have ASCII equivalents (like C<"A">, C<"0">, C<"%">, I<etc.>)
1349are the same in both EBCDIC and UTF-EBCDIC.)
1350
c0236afe
KW
1351UTF-EBCDIC is used on EBCDIC platforms. It generally requires more
1352bytes to represent a given code point than UTF-8 does; the largest
1353Unicode code points take 5 bytes to represent (instead of 4 in UTF-8),
1354and, extended for 64-bit words, it uses 14 bytes instead of 13 bytes in
1355UTF-8.
dbe420b4 1356
c29a771d 1357=item *
5cb3728c 1358
b65e6125 1359UTF-16, UTF-16BE, UTF-16LE, Surrogates, and C<BOM>'s (Byte Order Marks)
c349b1b9 1360
1bfb14c4
JH
1361The followings items are mostly for reference and general Unicode
1362knowledge, Perl doesn't use these constructs internally.
dbe420b4 1363
b19eb496
TC
1364Like UTF-8, UTF-16 is a variable-width encoding, but where
1365UTF-8 uses 8-bit code units, UTF-16 uses 16-bit code units.
1366All code points occupy either 2 or 4 bytes in UTF-16: code points
1367C<U+0000..U+FFFF> are stored in a single 16-bit unit, and code
1bfb14c4 1368points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is
c349b1b9
JH
1369using I<surrogates>, the first 16-bit unit being the I<high
1370surrogate>, and the second being the I<low surrogate>.
1371
376d9008 1372Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
c349b1b9 1373range of Unicode code points in pairs of 16-bit units. The I<high
9f815e24 1374surrogates> are the range C<U+D800..U+DBFF> and the I<low surrogates>
376d9008 1375are the range C<U+DC00..U+DFFF>. The surrogate encoding is
c349b1b9 1376
d88362ca
KW
1377 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1378 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
c349b1b9
JH
1379
1380and the decoding is
1381
d88362ca 1382 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
c349b1b9 1383
376d9008 1384Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
c349b1b9 1385itself can be used for in-memory computations, but if storage or
376d9008
JB
1386transfer is required either UTF-16BE (big-endian) or UTF-16LE
1387(little-endian) encodings must be chosen.
c349b1b9
JH
1388
1389This introduces another problem: what if you just know that your data
376d9008 1390is UTF-16, but you don't know which endianness? Byte Order Marks, or
b65e6125 1391C<BOM>'s, are a solution to this. A special character has been reserved
86bbd6d1 1392in Unicode to function as a byte order marker: the character with the
a9130ea9 1393code point C<U+FEFF> is the C<BOM>.
042da322 1394
a9130ea9 1395The trick is that if you read a C<BOM>, you will know the byte order,
376d9008
JB
1396since if it was written on a big-endian platform, you will read the
1397bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
1398you will read the bytes C<0xFF 0xFE>. (And if the originating platform
b65e6125
KW
1399was writing in ASCII platform UTF-8, you will read the bytes
1400C<0xEF 0xBB 0xBF>.)
042da322 1401
86bbd6d1 1402The way this trick works is that the character with the code point
6d4f9cf2 1403C<U+FFFE> is not supposed to be in input streams, so the
a9130ea9 1404sequence of bytes C<0xFF 0xFE> is unambiguously "C<BOM>, represented in
1bfb14c4 1405little-endian format" and cannot be C<U+FFFE>, represented in big-endian
6d4f9cf2
KW
1406format".
1407
1408Surrogates have no meaning in Unicode outside their use in pairs to
1409represent other code points. However, Perl allows them to be
1410represented individually internally, for example by saying
f651977e
TC
1411C<chr(0xD801)>, so that all code points, not just those valid for open
1412interchange, are
6d4f9cf2 1413representable. Unicode does define semantics for them, such as their
a9130ea9
KW
1414C<L</General_Category>> is C<"Cs">. But because their use is somewhat dangerous,
1415Perl will warn (using the warning category C<"surrogate">, which is a
1416sub-category of C<"utf8">) if an attempt is made
6d4f9cf2
KW
1417to do things like take the lower case of one, or match
1418case-insensitively, or to output them. (But don't try this on Perls
1419before 5.14.)
c349b1b9 1420
c29a771d 1421=item *
5cb3728c 1422
1e54db1a 1423UTF-32, UTF-32BE, UTF-32LE
c349b1b9 1424
b65e6125 1425The UTF-32 family is pretty much like the UTF-16 family, except that
042da322 1426the units are 32-bit, and therefore the surrogate scheme is not
a9130ea9 1427needed. UTF-32 is a fixed-width encoding. The C<BOM> signatures are
b19eb496 1428C<0x00 0x00 0xFE 0xFF> for BE and C<0xFF 0xFE 0x00 0x00> for LE.
c349b1b9 1429
c29a771d 1430=item *
5cb3728c
RB
1431
1432UCS-2, UCS-4
c349b1b9 1433
b19eb496 1434Legacy, fixed-width encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
376d9008 1435encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
339cfa0e 1436because it does not use surrogates. UCS-4 is a 32-bit encoding,
b19eb496 1437functionally identical to UTF-32 (the difference being that
a9130ea9 1438UCS-4 forbids neither surrogates nor code points larger than C<0x10_FFFF>).
c349b1b9 1439
c29a771d 1440=item *
5cb3728c
RB
1441
1442UTF-7
c349b1b9 1443
376d9008
JB
1444A seven-bit safe (non-eight-bit) encoding, which is useful if the
1445transport or storage is not eight-bit safe. Defined by RFC 2152.
c349b1b9 1446
95a1a48b
JH
1447=back
1448
57e88091 1449=head2 Noncharacter code points
6d4f9cf2 1450
57e88091 145166 code points are set aside in Unicode as "noncharacter code points".
a9130ea9 1452These all have the C<Unassigned> (C<Cn>) C<L</General_Category>>, and
57e88091
KW
1453no character will ever be assigned to any of them. They are the 32 code
1454points between C<U+FDD0> and C<U+FDEF> inclusive, and the 34 code
1455points:
1456
1457 U+FFFE U+FFFF
1458 U+1FFFE U+1FFFF
1459 U+2FFFE U+2FFFF
1460 ...
1461 U+EFFFE U+EFFFF
1462 U+FFFFE U+FFFFF
1463 U+10FFFE U+10FFFF
1464
1465Until Unicode 7.0, the noncharacters were "B<forbidden> for use in open
1466interchange of Unicode text data", so that code that processed those
1467streams could use these code points as sentinels that could be mixed in
1468with character data, and would always be distinguishable from that data.
1469(Emphasis above and in the next paragraph are added in this document.)
1470
1471Unicode 7.0 changed the wording so that they are "B<not recommended> for
1472use in open interchange of Unicode text data". The 7.0 Standard goes on
1473to say:
1474
1475=over 4
1476
1477"If a noncharacter is received in open interchange, an application is
1478not required to interpret it in any way. It is good practice, however,
1479to recognize it as a noncharacter and to take appropriate action, such
1480as replacing it with C<U+FFFD> replacement character, to indicate the
1481problem in the text. It is not recommended to simply delete
1482noncharacter code points from such text, because of the potential
1483security issues caused by deleting uninterpreted characters. (See
1484conformance clause C7 in Section 3.2, Conformance Requirements, and
1485L<Unicode Technical Report #36, "Unicode Security
1486Considerations"|http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
1487
1488=back
1489
1490This change was made because it was found that various commercial tools
1491like editors, or for things like source code control, had been written
1492so that they would not handle program files that used these code points,
1493effectively precluding their use almost entirely! And that was never
1494the intent. They've always been meant to be usable within an
1495application, or cooperating set of applications, at will.
1496
1497If you're writing code, such as an editor, that is supposed to be able
1498to handle any Unicode text data, then you shouldn't be using these code
1499points yourself, and instead allow them in the input. If you need
1500sentinels, they should instead be something that isn't legal Unicode.
1501For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
1502they never appear in well-formed UTF-8. (There are equivalents for
1503UTF-EBCDIC). You can also store your Unicode code points in integer
1504variables and use negative values as sentinels.
1505
1506If you're not writing such a tool, then whether you accept noncharacters
1507as input is up to you (though the Standard recommends that you not). If
1508you do strict input stream checking with Perl, these code points
1509continue to be forbidden. This is to maintain backward compatibility
1510(otherwise potential security holes could open up, as an unsuspecting
1511application that was written assuming the noncharacters would be
1512filtered out before getting to it, could now, without warning, start
1513getting them). To do strict checking, you can use the layer
1514C<:encoding('UTF-8')>.
1515
1516Perl continues to warn (using the warning category C<"nonchar">, which
1517is a sub-category of C<"utf8">) if an attempt is made to output
1518noncharacters.
42581d5d
KW
1519
1520=head2 Beyond Unicode code points
1521
a9130ea9
KW
1522The maximum Unicode code point is C<U+10FFFF>, and Unicode only defines
1523operations on code points up through that. But Perl works on code
526f2ca9 1524points up to the maximum permissible signed number available on the
42581d5d
KW
1525platform. However, Perl will not accept these from input streams unless
1526lax rules are being used, and will warn (using the warning category
2d88a86a
KW
1527C<"non_unicode">, which is a sub-category of C<"utf8">) if any are output.
1528
1529Since Unicode rules are not defined on these code points, if a
1530Unicode-defined operation is done on them, Perl uses what we believe are
1531sensible rules, while generally warning, using the C<"non_unicode">
1532category. For example, C<uc("\x{11_0000}")> will generate such a
1533warning, returning the input parameter as its result, since Perl defines
1534the uppercase of every non-Unicode code point to be the code point
b65e6125
KW
1535itself. (All the case changing operations, not just uppercasing, work
1536this way.)
2d88a86a
KW
1537
1538The situation with matching Unicode properties in regular expressions,
1539the C<\p{}> and C<\P{}> constructs, against these code points is not as
1540clear cut, and how these are handled has changed as we've gained
1541experience.
1542
1543One possibility is to treat any match against these code points as
1544undefined. But since Perl doesn't have the concept of a match being
1545undefined, it converts this to failing or C<FALSE>. This is almost, but
1546not quite, what Perl did from v5.14 (when use of these code points
1547became generally reliable) through v5.18. The difference is that Perl
1548treated all C<\p{}> matches as failing, but all C<\P{}> matches as
1549succeeding.
1550
f66ccb6c 1551One problem with this is that it leads to unexpected, and confusing
2d88a86a
KW
1552results in some cases:
1553
1554 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Failed on <= v5.18
1555 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Failed! on <= v5.18
1556
1557That is, it treated both matches as undefined, and converted that to
1558false (raising a warning on each). The first case is the expected
1559result, but the second is likely counterintuitive: "How could both be
1560false when they are complements?" Another problem was that the
1561implementation optimized many Unicode property matches down to already
1562existing simpler, faster operations, which don't raise the warning. We
1563chose to not forgo those optimizations, which help the vast majority of
1564matches, just to generate a warning for the unlikely event that an
1565above-Unicode code point is being matched against.
1566
1567As a result of these problems, starting in v5.20, what Perl does is
1568to treat non-Unicode code points as just typical unassigned Unicode
1569characters, and matches accordingly. (Note: Unicode has atypical
57e88091 1570unassigned code points. For example, it has noncharacter code points,
2d88a86a
KW
1571and ones that, when they do get assigned, are destined to be written
1572Right-to-left, as Arabic and Hebrew are. Perl assumes that no
1573non-Unicode code point has any atypical properties.)
1574
1575Perl, in most cases, will raise a warning when matching an above-Unicode
1576code point against a Unicode property when the result is C<TRUE> for
1577C<\p{}>, and C<FALSE> for C<\P{}>. For example:
1578
1579 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails, no warning
1580 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Succeeds, with warning
1581
1582In both these examples, the character being matched is non-Unicode, so
1583Unicode doesn't define how it should match. It clearly isn't an ASCII
1584hex digit, so the first example clearly should fail, and so it does,
1585with no warning. But it is arguable that the second example should have
1586an undefined, hence C<FALSE>, result. So a warning is raised for it.
1587
1588Thus the warning is raised for many fewer cases than in earlier Perls,
1589and only when what the result is could be arguable. It turns out that
1590none of the optimizations made by Perl (or are ever likely to be made)
1591cause the warning to be skipped, so it solves both problems of Perl's
1592earlier approach. The most commonly used property that is affected by
1593this change is C<\p{Unassigned}> which is a short form for
1594C<\p{General_Category=Unassigned}>. Starting in v5.20, all non-Unicode
1595code points are considered C<Unassigned>. In earlier releases the
1596matches failed because the result was considered undefined.
1597
1598The only place where the warning is not raised when it might ought to
1599have been is if optimizations cause the whole pattern match to not even
1600be attempted. For example, Perl may figure out that for a string to
1601match a certain regular expression pattern, the string has to contain
1602the substring C<"foobar">. Before attempting the match, Perl may look
1603for that substring, and if not found, immediately fail the match without
1604actually trying it; so no warning gets generated even if the string
1605contains an above-Unicode code point.
1606
1607This behavior is more "Do what I mean" than in earlier Perls for most
1608applications. But it catches fewer issues for code that needs to be
1609strictly Unicode compliant. Therefore there is an additional mode of
1610operation available to accommodate such code. This mode is enabled if a
1611regular expression pattern is compiled within the lexical scope where
1612the C<"non_unicode"> warning class has been made fatal, say by:
1613
1614 use warnings FATAL => "non_unicode"
1615
44ecbbd8 1616(see L<warnings>). In this mode of operation, Perl will raise the
2d88a86a
KW
1617warning for all matches against a non-Unicode code point (not just the
1618arguable ones), and it skips the optimizations that might cause the
1619warning to not be output. (It currently still won't warn if the match
1620isn't even attempted, like in the C<"foobar"> example above.)
1621
1622In summary, Perl now normally treats non-Unicode code points as typical
1623Unicode unassigned code points for regular expression matches, raising a
1624warning only when it is arguable what the result should be. However, if
1625this warning has been made fatal, it isn't skipped.
1626
1627There is one exception to all this. C<\p{All}> looks like a Unicode
1628property, but it is a Perl extension that is defined to be true for all
1629possible code points, Unicode or not, so no warning is ever generated
1630when matching this against a non-Unicode code point. (Prior to v5.20,
1631it was an exact synonym for C<\p{Any}>, matching code points C<0>
1632through C<0x10FFFF>.)
6d4f9cf2 1633
0d7c09bb
JH
1634=head2 Security Implications of Unicode
1635
b65e6125
KW
1636First, read
1637L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
1638
e1b711da
KW
1639Also, note the following:
1640
0d7c09bb
JH
1641=over 4
1642
1643=item *
1644
1645Malformed UTF-8
bf0fa0b2 1646
f57d8456
KW
1647UTF-8 is very structured, so many combinations of bytes are invalid. In
1648the past, Perl tried to soldier on and make some sense of invalid
1649combinations, but this can lead to security holes, so now, if the Perl
1650core needs to process an invalid combination, it will either raise a
1651fatal error, or will replace those bytes by the sequence that forms the
1652Unicode REPLACEMENT CHARACTER, for which purpose Unicode created it.
1653
1654Every code point can be represented by more than one possible
1655syntactically valid UTF-8 sequence. Early on, both Unicode and Perl
1656considered any of these to be valid, but now, all sequences longer
1657than the shortest possible one are considered to be malformed.
1658
1659Unicode considers many code points to be illegal, or to be avoided.
1660Perl generally accepts them, once they have passed through any input
1661filters that may try to exclude them. These have been discussed above
1662(see "Surrogates" under UTF-16 in L</Unicode Encodings>,
1663L</Noncharacter code points>, and L</Beyond Unicode code points>).
bf0fa0b2 1664
0d7c09bb
JH
1665=item *
1666
68693f9e 1667Regular expression pattern matching may surprise you if you're not
b19eb496
TC
1668accustomed to Unicode. Starting in Perl 5.14, several pattern
1669modifiers are available to control this, called the character set
42581d5d
KW
1670modifiers. Details are given in L<perlre/Character set modifiers>.
1671
1672=back
0d7c09bb 1673
376d9008 1674As discussed elsewhere, Perl has one foot (two hooves?) planted in
a6a7eedc
KW
1675each of two worlds: the old world of ASCII and single-byte locales, and
1676the new world of Unicode, upgrading when necessary.
376d9008 1677If your legacy code does not explicitly use Unicode, no automatic
a6a7eedc 1678switch-over to Unicode should happen.
0d7c09bb 1679
c349b1b9
JH
1680=head2 Unicode in Perl on EBCDIC
1681
a6a7eedc
KW
1682Unicode is supported on EBCDIC platforms. See L<perlebcdic>.
1683
1684Unless ASCII vs. EBCDIC issues are specifically being discussed,
1685references to UTF-8 encoding in this document and elsewhere should be
1686read as meaning UTF-EBCDIC on EBCDIC platforms.
1687See L<perlebcdic/Unicode and UTF>.
1688
1689Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
1690hidden from you; S<C<use utf8>> (and NOT something like
dabde021 1691S<C<use utfebcdic>>) declares the script is in the platform's
a6a7eedc
KW
1692"native" 8-bit encoding of Unicode. (Similarly for the C<":utf8">
1693layer.)
c349b1b9 1694
b310b053
JH
1695=head2 Locales
1696
42581d5d 1697See L<perllocale/Unicode and UTF-8>
b310b053 1698
1aad1664
JH
1699=head2 When Unicode Does Not Happen
1700
b65e6125
KW
1701There are still many places where Unicode (in some encoding or
1702another) could be given as arguments or received as results, or both in
1703Perl, but it is not, in spite of Perl having extensive ways to input and
1704output in Unicode, and a few other "entry points" like the C<@ARGV>
1705array (which can sometimes be interpreted as UTF-8).
1aad1664 1706
e1b711da
KW
1707The following are such interfaces. Also, see L</The "Unicode Bug">.
1708For all of these interfaces Perl
b9cedb1b 1709currently (as of v5.16.0) simply assumes byte strings both as arguments
b65e6125 1710and results, or UTF-8 strings if the (deprecated) C<encoding> pragma has been used.
1aad1664 1711
b19eb496
TC
1712One reason that Perl does not attempt to resolve the role of Unicode in
1713these situations is that the answers are highly dependent on the operating
1aad1664 1714system and the file system(s). For example, whether filenames can be
b19eb496
TC
1715in Unicode and in exactly what kind of encoding, is not exactly a
1716portable concept. Similarly for C<qx> and C<system>: how well will the
1717"command-line interface" (and which of them?) handle Unicode?
1aad1664
JH
1718
1719=over 4
1720
557a2462
RB
1721=item *
1722
a9130ea9
KW
1723C<chdir>, C<chmod>, C<chown>, C<chroot>, C<exec>, C<link>, C<lstat>, C<mkdir>,
1724C<rename>, C<rmdir>, C<stat>, C<symlink>, C<truncate>, C<unlink>, C<utime>, C<-X>
557a2462
RB
1725
1726=item *
1727
a9130ea9 1728C<%ENV>
557a2462
RB
1729
1730=item *
1731
a9130ea9 1732C<glob> (aka the C<E<lt>*E<gt>>)
557a2462
RB
1733
1734=item *
1aad1664 1735
a9130ea9 1736C<open>, C<opendir>, C<sysopen>
1aad1664 1737
557a2462 1738=item *
1aad1664 1739
a9130ea9 1740C<qx> (aka the backtick operator), C<system>
1aad1664 1741
557a2462 1742=item *
1aad1664 1743
a9130ea9 1744C<readdir>, C<readlink>
1aad1664
JH
1745
1746=back
1747
e1b711da
KW
1748=head2 The "Unicode Bug"
1749
a6a7eedc
KW
1750The term, "Unicode bug" has been applied to an inconsistency with the
1751code points in the C<Latin-1 Supplement> block, that is, between
1752128 and 255. Without a locale specified, unlike all other characters or
1753code points, these characters can have very different semantics
1754depending on the rules in effect. (Characters whose code points are
1755above 255 force Unicode rules; whereas the rules for ASCII characters
1756are the same under both ASCII and Unicode rules.)
1757
1758Under Unicode rules, these upper-Latin1 characters are interpreted as
1759Unicode code points, which means they have the same semantics as Latin-1
1760(ISO-8859-1) and C1 controls.
1761
1762As explained in L</ASCII Rules versus Unicode Rules>, under ASCII rules,
1763they are considered to be unassigned characters.
1764
1765This can lead to unexpected results. For example, a string's
1766semantics can suddenly change if a code point above 255 is appended to
1767it, which changes the rules from ASCII to Unicode. As an
1768example, consider the following program and its output:
1769
1770 $ perl -le'
f434f357 1771 no feature "unicode_strings";
a6a7eedc
KW
1772 $s1 = "\xC2";
1773 $s2 = "\x{2660}";
1774 for ($s1, $s2, $s1.$s2) {
1775 print /\w/ || 0;
1776 }
1777 '
1778 0
1779 0
1780 1
1781
1782If there's no C<\w> in C<s1> nor in C<s2>, why does their concatenation
1783have one?
1784
1785This anomaly stems from Perl's attempt to not disturb older programs that
1786didn't use Unicode, along with Perl's desire to add Unicode support
1787seamlessly. But the result turned out to not be seamless. (By the way,
1788you can choose to be warned when things like this happen. See
1789C<L<encoding::warnings>>.)
1790
1791L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
1792was added, starting in Perl v5.12, to address this problem. It affects
1793these things:
e1b711da
KW
1794
1795=over 4
1796
1797=item *
1798
1799Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
2e2b2571
KW
1800and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in double-quotish
1801contexts, such as regular expression substitutions.
a6a7eedc
KW
1802
1803Under C<unicode_strings> starting in Perl 5.12.0, Unicode rules are
2e2b2571
KW
1804generally used. See L<perlfunc/lc> for details on how this works
1805in combination with various other pragmas.
e1b711da
KW
1806
1807=item *
1808
2e2b2571 1809Using caseless (C</i>) regular expression matching.
a6a7eedc 1810
2e2b2571 1811Starting in Perl 5.14.0, regular expressions compiled within
a6a7eedc 1812the scope of C<unicode_strings> use Unicode rules
2e2b2571
KW
1813even when executed or compiled into larger
1814regular expressions outside the scope.
e1b711da
KW
1815
1816=item *
1817
a6a7eedc
KW
1818Matching any of several properties in regular expressions.
1819
1820These properties are C<\b> (without braces), C<\B> (without braces),
1821C<\s>, C<\S>, C<\w>, C<\W>, and all the Posix character classes
630d17dc 1822I<except> C<[[:ascii:]]>.
a6a7eedc 1823
2e2b2571 1824Starting in Perl 5.14.0, regular expressions compiled within
a6a7eedc 1825the scope of C<unicode_strings> use Unicode rules
2e2b2571
KW
1826even when executed or compiled into larger
1827regular expressions outside the scope.
e1b711da
KW
1828
1829=item *
1830
a6a7eedc
KW
1831In C<quotemeta> or its inline equivalent C<\Q>.
1832
2e2b2571
KW
1833Starting in Perl 5.16.0, consistent quoting rules are used within the
1834scope of C<unicode_strings>, as described in L<perlfunc/quotemeta>.
a6a7eedc
KW
1835Prior to that, or outside its scope, no code points above 127 are quoted
1836in UTF-8 encoded strings, but in byte encoded strings, code points
1837between 128-255 are always quoted.
eb88ed9e 1838
d6c970c7
AC
1839=item *
1840
1841In the C<..> or L<range|perlop/Range Operators> operator.
1842
1843Starting in Perl 5.26.0, the range operator on strings treats their lengths
1844consistently within the scope of C<unicode_strings>. Prior to that, or
1845outside its scope, it could produce strings whose length in characters
1846exceeded that of the right-hand side, where the right-hand side took up more
1847bytes than the correct range endpoint.
1848
20ae58f7
AC
1849=item *
1850
1851In L<< C<split>'s special-case whitespace splitting|perlfunc/split >>.
1852
1853Starting in Perl 5.28.0, the C<split> function with a pattern specified as
1854a string containing a single space handles whitespace characters consistently
1855within the scope of of C<unicode_strings>. Prior to that, or outside its scope,
1856characters that are whitespace according to Unicode rules but not according to
1857ASCII rules were treated as field contents rather than field separators when
1858they appear in byte-encoded strings.
1859
e1b711da
KW
1860=back
1861
a6a7eedc
KW
1862You can see from the above that the effect of C<unicode_strings>
1863increased over several Perl releases. (And Perl's support for Unicode
1864continues to improve; it's best to use the latest available release in
1865order to get the most complete and accurate results possible.) Note that
1866C<unicode_strings> is automatically chosen if you S<C<use 5.012>> or
1867higher.
e1b711da 1868
2e2b2571 1869For Perls earlier than those described above, or when a string is passed
a6a7eedc 1870to a function outside the scope of C<unicode_strings>, see the next section.
e1b711da 1871
1aad1664
JH
1872=head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
1873
e1b711da
KW
1874Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
1875there are situations where you simply need to force a byte
a6a7eedc
KW
1876string into UTF-8, or vice versa. The standard module L<Encode> can be
1877used for this, or the low-level calls
a9130ea9 1878L<C<utf8::upgrade($bytestring)>|utf8/Utility functions> and
a6a7eedc 1879L<C<utf8::downgrade($utf8string[, FAIL_OK])>|utf8/Utility functions>.
1aad1664 1880
a9130ea9 1881Note that C<utf8::downgrade()> can fail if the string contains characters
2bbc8d55 1882that don't fit into a byte.
1aad1664 1883
e1b711da
KW
1884Calling either function on a string that already is in the desired state is a
1885no-op.
1886
a6a7eedc
KW
1887L</ASCII Rules versus Unicode Rules> gives all the ways that a string is
1888made to use Unicode rules.
95a1a48b 1889
37b3b608 1890=head2 Using Unicode in XS
c349b1b9 1891
37b3b608
KW
1892See L<perlguts/"Unicode Support"> for an introduction to Unicode at
1893the XS level, and L<perlapi/Unicode Support> for the API details.
95a1a48b 1894
e1b711da
KW
1895=head2 Hacking Perl to work on earlier Unicode versions (for very serious hackers only)
1896
a6a7eedc
KW
1897Perl by default comes with the latest supported Unicode version built-in, but
1898the goal is to allow you to change to use any earlier one. In Perls
1899v5.20 and v5.22, however, the earliest usable version is Unicode 5.1.
c55dd03d 1900Perl v5.18 and v5.24 are able to handle all earlier versions.
e1b711da 1901
42581d5d 1902Download the files in the desired version of Unicode from the Unicode web
e1b711da 1903site L<http://www.unicode.org>). These should replace the existing files in
b19eb496 1904F<lib/unicore> in the Perl source tree. Follow the instructions in
116693e8 1905F<README.perl> in that directory to change some of their names, and then build
26e391dd 1906perl (see L<INSTALL>).
116693e8 1907
c8d992ba
A
1908=head2 Porting code from perl-5.6.X
1909
a6a7eedc
KW
1910Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6 the
1911programmer was required to use the C<utf8> pragma to declare that a
1912given scope expected to deal with Unicode data and had to make sure that
1913only Unicode data were reaching that scope. If you have code that is
c8d992ba 1914working with 5.6, you will need some of the following adjustments to
a6a7eedc
KW
1915your code. The examples are written such that the code will continue to
1916work under 5.6, so you should be safe to try them out.
c8d992ba 1917
755789c0 1918=over 3
c8d992ba
A
1919
1920=item *
1921
1922A filehandle that should read or write UTF-8
1923
b9cedb1b 1924 if ($] > 5.008) {
6d8e7450 1925 binmode $fh, ":encoding(UTF-8)";
c8d992ba
A
1926 }
1927
1928=item *
1929
1930A scalar that is going to be passed to some extension
1931
a9130ea9 1932Be it C<Compress::Zlib>, C<Apache::Request> or any extension that has no
c8d992ba 1933mention of Unicode in the manpage, you need to make sure that the
2575c402 1934UTF8 flag is stripped off. Note that at the time of this writing
b9cedb1b 1935(January 2012) the mentioned modules are not UTF-8-aware. Please
c8d992ba
A
1936check the documentation to verify if this is still true.
1937
b9cedb1b 1938 if ($] > 5.008) {
c8d992ba 1939 require Encode;
8e179dd8 1940 $val = Encode::encode("UTF-8", $val); # make octets
c8d992ba
A
1941 }
1942
1943=item *
1944
1945A scalar we got back from an extension
1946
1947If you believe the scalar comes back as UTF-8, you will most likely
2575c402 1948want the UTF8 flag restored:
c8d992ba 1949
b9cedb1b 1950 if ($] > 5.008) {
c8d992ba 1951 require Encode;
8e179dd8 1952 $val = Encode::decode("UTF-8", $val);
c8d992ba
A
1953 }
1954
1955=item *
1956
1957Same thing, if you are really sure it is UTF-8
1958
b9cedb1b 1959 if ($] > 5.008) {
c8d992ba
A
1960 require Encode;
1961 Encode::_utf8_on($val);
1962 }
1963
1964=item *
1965
a9130ea9 1966A wrapper for L<DBI> C<fetchrow_array> and C<fetchrow_hashref>
c8d992ba
A
1967
1968When the database contains only UTF-8, a wrapper function or method is
a9130ea9
KW
1969a convenient way to replace all your C<fetchrow_array> and
1970C<fetchrow_hashref> calls. A wrapper function will also make it easier to
c8d992ba 1971adapt to future enhancements in your database driver. Note that at the
b9cedb1b 1972time of this writing (January 2012), the DBI has no standardized way
a9130ea9 1973to deal with UTF-8 data. Please check the L<DBI documentation|DBI> to verify if
c8d992ba
A
1974that is still true.
1975
1976 sub fetchrow {
d88362ca
KW
1977 # $what is one of fetchrow_{array,hashref}
1978 my($self, $sth, $what) = @_;
b9cedb1b 1979 if ($] < 5.008) {
c8d992ba
A
1980 return $sth->$what;
1981 } else {
1982 require Encode;
1983 if (wantarray) {
1984 my @arr = $sth->$what;
1985 for (@arr) {
1986 defined && /[^\000-\177]/ && Encode::_utf8_on($_);
1987 }
1988 return @arr;
1989 } else {
1990 my $ret = $sth->$what;
1991 if (ref $ret) {
1992 for my $k (keys %$ret) {
d88362ca
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1993 defined
1994 && /[^\000-\177]/
1995 && Encode::_utf8_on($_) for $ret->{$k};
c8d992ba
A
1996 }
1997 return $ret;
1998 } else {
1999 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
2000 return $ret;
2001 }
2002 }
2003 }
2004 }
2005
2006
2007=item *
2008
2009A large scalar that you know can only contain ASCII
2010
2011Scalars that contain only ASCII and are marked as UTF-8 are sometimes
2012a drag to your program. If you recognize such a situation, just remove
2575c402 2013the UTF8 flag:
c8d992ba 2014
b9cedb1b 2015 utf8::downgrade($val) if $] > 5.008;
c8d992ba
A
2016
2017=back
2018
a6a7eedc
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2019=head1 BUGS
2020
2021See also L</The "Unicode Bug"> above.
2022
2023=head2 Interaction with Extensions
2024
2025When Perl exchanges data with an extension, the extension should be
2026able to understand the UTF8 flag and act accordingly. If the
2027extension doesn't recognize that flag, it's likely that the extension
2028will return incorrectly-flagged data.
2029
2030So if you're working with Unicode data, consult the documentation of
2031every module you're using if there are any issues with Unicode data
2032exchange. If the documentation does not talk about Unicode at all,
2033suspect the worst and probably look at the source to learn how the
2034module is implemented. Modules written completely in Perl shouldn't
2035cause problems. Modules that directly or indirectly access code written
2036in other programming languages are at risk.
2037
2038For affected functions, the simple strategy to avoid data corruption is
2039to always make the encoding of the exchanged data explicit. Choose an
2040encoding that you know the extension can handle. Convert arguments passed
2041to the extensions to that encoding and convert results back from that
2042encoding. Write wrapper functions that do the conversions for you, so
2043you can later change the functions when the extension catches up.
2044
2045To provide an example, let's say the popular C<Foo::Bar::escape_html>
2046function doesn't deal with Unicode data yet. The wrapper function
2047would convert the argument to raw UTF-8 and convert the result back to
2048Perl's internal representation like so:
2049
2050 sub my_escape_html ($) {
2051 my($what) = shift;
2052 return unless defined $what;
8e179dd8
P
2053 Encode::decode("UTF-8", Foo::Bar::escape_html(
2054 Encode::encode("UTF-8", $what)));
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KW
2055 }
2056
2057Sometimes, when the extension does not convert data but just stores
2058and retrieves it, you will be able to use the otherwise
2059dangerous L<C<Encode::_utf8_on()>|Encode/_utf8_on> function. Let's say
2060the popular C<Foo::Bar> extension, written in C, provides a C<param>
2061method that lets you store and retrieve data according to these prototypes:
2062
2063 $self->param($name, $value); # set a scalar
2064 $value = $self->param($name); # retrieve a scalar
2065
2066If it does not yet provide support for any encoding, one could write a
2067derived class with such a C<param> method:
2068
2069 sub param {
2070 my($self,$name,$value) = @_;
2071 utf8::upgrade($name); # make sure it is UTF-8 encoded
2072 if (defined $value) {
2073 utf8::upgrade($value); # make sure it is UTF-8 encoded
2074 return $self->SUPER::param($name,$value);
2075 } else {
2076 my $ret = $self->SUPER::param($name);
2077 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
2078 return $ret;
2079 }
2080 }
2081
2082Some extensions provide filters on data entry/exit points, such as
2083C<DB_File::filter_store_key> and family. Look out for such filters in
2084the documentation of your extensions; they can make the transition to
2085Unicode data much easier.
2086
2087=head2 Speed
2088
2089Some functions are slower when working on UTF-8 encoded strings than
2090on byte encoded strings. All functions that need to hop over
2091characters such as C<length()>, C<substr()> or C<index()>, or matching
2092regular expressions can work B<much> faster when the underlying data are
2093byte-encoded.
2094
2095In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
2096a caching scheme was introduced which improved the situation. In general,
2097operations with UTF-8 encoded strings are still slower. As an example,
2098the Unicode properties (character classes) like C<\p{Nd}> are known to
2099be quite a bit slower (5-20 times) than their simpler counterparts
2100like C<[0-9]> (then again, there are hundreds of Unicode characters matching
2101C<Nd> compared with the 10 ASCII characters matching C<[0-9]>).
2102
393fec97
GS
2103=head1 SEE ALSO
2104
51f494cc 2105L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>,
b65e6125 2106L<perlretut>, L<perlvar/"${^UNICODE}">,
51f494cc 2107L<http://www.unicode.org/reports/tr44>).
393fec97
GS
2108
2109=cut