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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
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39Also, the use of Unicode may present security issues that aren't obvious.
40Read L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
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
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63=item You should convert your non-ASCII, non-UTF-8 Perl scripts to be
64UTF-8.
21bad921 65
a6a7eedc 66See L<encoding>.
21bad921 67
a6a7eedc 68=item C<use utf8> still needed to enable L<UTF-8|/Unicode Encodings> in scripts
21bad921 69
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70If your Perl script is itself encoded in L<UTF-8|/Unicode Encodings>,
71the S<C<use utf8>> pragma must be explicitly included to enable
72recognition of that (in string or regular expression literals, or in
73identifier names). B<This is the only time when an explicit S<C<use
74utf8>> is needed.> (See L<utf8>).
7aa207d6 75
a6a7eedc 76=item C<BOM>-marked scripts and L<UTF-16|/Unicode Encodings> scripts autodetected
7aa207d6 77
fea12a3e 78If a Perl script begins with the Unicode C<BOM> (UTF-16LE,
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79UTF16-BE, or UTF-8), or if the script looks like non-C<BOM>-marked
80UTF-16 of either endianness, Perl will correctly read in the script as
81the appropriate Unicode encoding. (C<BOM>-less UTF-8 cannot be
82effectively recognized or differentiated from ISO 8859-1 or other
83eight-bit encodings.)
990e18f7 84
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85=back
86
376d9008 87=head2 Byte and Character Semantics
393fec97 88
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89Before Unicode, most encodings used 8 bits (a single byte) to encode
90each character. Thus a character was a byte, and a byte was a
91character, and there could be only 256 or fewer possible characters.
92"Byte Semantics" in the title of this section refers to
93this behavior. There was no need to distinguish between "Byte" and
94"Character".
95
96Then along comes Unicode which has room for over a million characters
97(and Perl allows for even more). This means that a character may
98require more than a single byte to represent it, and so the two terms
99are no longer equivalent. What matter are the characters as whole
100entities, and not usually the bytes that comprise them. That's what the
101term "Character Semantics" in the title of this section refers to.
102
103Perl had to change internally to decouple "bytes" from "characters".
104It is important that you too change your ideas, if you haven't already,
105so that "byte" and "character" no longer mean the same thing in your
106mind.
107
108The basic building block of Perl strings has always been a "character".
109The changes basically come down to that the implementation no longer
110thinks that a character is always just a single byte.
111
112There are various things to note:
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113
114=over 4
115
116=item *
117
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118String handling functions, for the most part, continue to operate in
119terms of characters. C<length()>, for example, returns the number of
120characters in a string, just as before. But that number no longer is
121necessarily the same as the number of bytes in the string (there may be
122more bytes than characters). The other such functions include
123C<chop()>, C<chomp()>, C<substr()>, C<pos()>, C<index()>, C<rindex()>,
124C<sort()>, C<sprintf()>, and C<write()>.
125
126The exceptions are:
127
128=over 4
129
130=item *
131
132the bit-oriented C<vec>
133
134E<nbsp>
135
136=item *
137
138the byte-oriented C<pack>/C<unpack> C<"C"> format
139
140However, the C<W> specifier does operate on whole characters, as does the
141C<U> specifier.
142
143=item *
144
145some operators that interact with the platform's operating system
146
147Operators dealing with filenames are examples.
148
149=item *
150
151when the functions are called from within the scope of the
152S<C<L<use bytes|bytes>>> pragma
153
154Likely, you should use this only for debugging anyway.
155
156=back
157
158=item *
159
376d9008 160Strings--including hash keys--and regular expression patterns may
b65e6125 161contain characters that have ordinal values larger than 255.
393fec97 162
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163If you use a Unicode editor to edit your program, Unicode characters may
164occur directly within the literal strings in UTF-8 encoding, or UTF-16.
a9130ea9 165(The former requires a C<BOM> or C<use utf8>, the latter requires a C<BOM>.)
3e4dbfed 166
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167L<perluniintro/Creating Unicode> gives other ways to place non-ASCII
168characters in your strings.
6f335b04 169
a6a7eedc 170=item *
fbb93542 171
a6a7eedc 172The C<chr()> and C<ord()> functions work on whole characters.
376d9008 173
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174=item *
175
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176Regular expressions match whole characters. For example, C<"."> matches
177a whole character instead of only a single byte.
393fec97 178
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179=item *
180
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181The C<tr///> operator translates whole characters. (Note that the
182C<tr///CU> functionality has been removed. For similar functionality to
183that, see C<pack('U0', ...)> and C<pack('C0', ...)>).
393fec97 184
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185=item *
186
a6a7eedc 187C<scalar reverse()> reverses by character rather than by byte.
393fec97 188
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189=item *
190
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191The bit string operators, C<& | ^ ~> and (starting in v5.22)
192C<&. |. ^. ~.> can operate on characters that don't fit into a byte.
193However, the current behavior is likely to change. You should not use
194these operators on strings that are encoded in UTF-8. If you're not
195sure about the encoding of a string, downgrade it before using any of
196these operators; you can use
197L<C<utf8::utf8_downgrade()>|utf8/Utility functions>.
822502e5 198
a6a7eedc 199=back
822502e5 200
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201The bottom line is that Perl has always practiced "Character Semantics",
202but with the advent of Unicode, that is now different than "Byte
203Semantics".
204
205=head2 ASCII Rules versus Unicode Rules
206
207Before Unicode, when a character was a byte was a character,
208Perl knew only about the 128 characters defined by ASCII, code points 0
209through 127 (except for under S<C<use locale>>). That left the code
210points 128 to 255 as unassigned, and available for whatever use a
211program might want. The only semantics they have is their ordinal
212numbers, and that they are members of none of the non-negative character
213classes. None are considered to match C<\w> for example, but all match
214C<\W>.
822502e5 215
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216Unicode, of course, assigns each of those code points a particular
217meaning (along with ones above 255). To preserve backward
218compatibility, Perl only uses the Unicode meanings when there is some
219indication that Unicode is what is intended; otherwise the non-ASCII
220code points remain treated as if they are unassigned.
221
222Here are the ways that Perl knows that a string should be treated as
223Unicode:
224
225=over
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226
227=item *
228
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229Within the scope of S<C<use utf8>>
230
231If the whole program is Unicode (signified by using 8-bit B<U>nicode
232B<T>ransformation B<F>ormat), then all strings within it must be
233Unicode.
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234
235=item *
236
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237Within the scope of
238L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
239
240This pragma was created so you can explicitly tell Perl that operations
241executed within its scope are to use Unicode rules. More operations are
242affected with newer perls. See L</The "Unicode Bug">.
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243
244=item *
245
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246Within the scope of S<C<use 5.012>> or higher
247
248This implicitly turns on S<C<use feature 'unicode_strings'>>.
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249
250=item *
251
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252Within the scope of
253L<S<C<use locale 'not_characters'>>|perllocale/Unicode and UTF-8>,
254or L<S<C<use locale>>|perllocale> and the current
255locale is a UTF-8 locale.
822502e5 256
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257The former is defined to imply Unicode handling; and the latter
258indicates a Unicode locale, hence a Unicode interpretation of all
259strings within it.
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260
261=item *
262
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263When the string contains a Unicode-only code point
264
265Perl has never accepted code points above 255 without them being
266Unicode, so their use implies Unicode for the whole string.
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267
268=item *
269
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270When the string contains a Unicode named code point C<\N{...}>
271
272The C<\N{...}> construct explicitly refers to a Unicode code point,
273even if it is one that is also in ASCII. Therefore the string
274containing it must be Unicode.
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275
276=item *
277
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278When the string has come from an external source marked as
279Unicode
280
281The L<C<-C>|perlrun/-C [numberE<sol>list]> command line option can
282specify that certain inputs to the program are Unicode, and the values
283of this can be read by your Perl code, see L<perlvar/"${^UNICODE}">.
284
285=item * When the string has been upgraded to UTF-8
286
287The function L<C<utf8::utf8_upgrade()>|utf8/Utility functions>
288can be explicitly used to permanently (unless a subsequent
289C<utf8::utf8_downgrade()> is called) cause a string to be treated as
290Unicode.
291
292=item * There are additional methods for regular expression patterns
293
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294A pattern that is compiled with the C<< /u >> or C<< /a >> modifiers is
295treated as Unicode (though there are some restrictions with C<< /a >>).
296Under the C<< /d >> and C<< /l >> modifiers, there are several other
297indications for Unicode; see L<perlre/Character set modifiers>.
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298
299=back
300
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301Note that all of the above are overridden within the scope of
302C<L<use bytes|bytes>>; but you should be using this pragma only for
303debugging.
304
305Note also that some interactions with the platform's operating system
306never use Unicode rules.
307
308When Unicode rules are in effect:
309
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310=over 4
311
312=item *
313
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314Case translation operators use the Unicode case translation tables.
315
316Note that C<uc()>, or C<\U> in interpolated strings, translates to
317uppercase, while C<ucfirst>, or C<\u> in interpolated strings,
318translates to titlecase in languages that make the distinction (which is
319equivalent to uppercase in languages without the distinction).
320
321There is a CPAN module, C<L<Unicode::Casing>>, which allows you to
322define your own mappings to be used in C<lc()>, C<lcfirst()>, C<uc()>,
323C<ucfirst()>, and C<fc> (or their double-quoted string inlined versions
324such as C<\U>). (Prior to Perl 5.16, this functionality was partially
325provided in the Perl core, but suffered from a number of insurmountable
326drawbacks, so the CPAN module was written instead.)
327
328=item *
329
330Character classes in regular expressions match based on the character
331properties specified in the Unicode properties database.
332
333C<\w> can be used to match a Japanese ideograph, for instance; and
334C<[[:digit:]]> a Bengali number.
335
336=item *
337
338Named Unicode properties, scripts, and block ranges may be used (like
339bracketed character classes) by using the C<\p{}> "matches property"
340construct and the C<\P{}> negation, "doesn't match property".
341
342See L</"Unicode Character Properties"> for more details.
343
344You can define your own character properties and use them
345in the regular expression with the C<\p{}> or C<\P{}> construct.
346See L</"User-Defined Character Properties"> for more details.
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347
348=back
349
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350=head2 Extended Grapheme Clusters (Logical characters)
351
352Consider a character, say C<H>. It could appear with various marks around it,
353such as an acute accent, or a circumflex, or various hooks, circles, arrows,
354I<etc.>, above, below, to one side or the other, I<etc>. There are many
355possibilities among the world's languages. The number of combinations is
356astronomical, and if there were a character for each combination, it would
357soon exhaust Unicode's more than a million possible characters. So Unicode
358took a different approach: there is a character for the base C<H>, and a
359character for each of the possible marks, and these can be variously combined
360to get a final logical character. So a logical character--what appears to be a
361single character--can be a sequence of more than one individual characters.
362The Unicode standard calls these "extended grapheme clusters" (which
363is an improved version of the no-longer much used "grapheme cluster");
364Perl furnishes the C<\X> regular expression construct to match such
365sequences in their entirety.
366
367But Unicode's intent is to unify the existing character set standards and
368practices, and several pre-existing standards have single characters that
369mean the same thing as some of these combinations, like ISO-8859-1,
370which has quite a few of them. For example, C<"LATIN CAPITAL LETTER E
371WITH ACUTE"> was already in this standard when Unicode came along.
372Unicode therefore added it to its repertoire as that single character.
373But this character is considered by Unicode to be equivalent to the
374sequence consisting of the character C<"LATIN CAPITAL LETTER E">
375followed by the character C<"COMBINING ACUTE ACCENT">.
376
377C<"LATIN CAPITAL LETTER E WITH ACUTE"> is called a "pre-composed"
378character, and its equivalence with the "E" and the "COMBINING ACCENT"
379sequence is called canonical equivalence. All pre-composed characters
380are said to have a decomposition (into the equivalent sequence), and the
381decomposition type is also called canonical. A string may be comprised
382as much as possible of precomposed characters, or it may be comprised of
383entirely decomposed characters. Unicode calls these respectively,
384"Normalization Form Composed" (NFC) and "Normalization Form Decomposed".
385The C<L<Unicode::Normalize>> module contains functions that convert
386between the two. A string may also have both composed characters and
387decomposed characters; this module can be used to make it all one or the
388other.
389
390You may be presented with strings in any of these equivalent forms.
391There is currently nothing in Perl 5 that ignores the differences. So
392you'll have to specially hanlde it. The usual advice is to convert your
393inputs to C<NFD> before processing further.
394
395For more detailed information, see L<http://unicode.org/reports/tr15/>.
396
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397=head2 Unicode Character Properties
398
ee88f7b6 399(The only time that Perl considers a sequence of individual code
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400points as a single logical character is in the C<\X> construct, already
401mentioned above. Therefore "character" in this discussion means a single
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402Unicode code point.)
403
404Very nearly all Unicode character properties are accessible through
405regular expressions by using the C<\p{}> "matches property" construct
406and the C<\P{}> "doesn't match property" for its negation.
51f494cc 407
9d1c51c1 408For instance, C<\p{Uppercase}> matches any single character with the Unicode
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409C<"Uppercase"> property, while C<\p{L}> matches any character with a
410C<General_Category> of C<"L"> (letter) property (see
411L</General_Category> below). Brackets are not
9d1c51c1 412required for single letter property names, so C<\p{L}> is equivalent to C<\pL>.
51f494cc 413
9d1c51c1 414More formally, C<\p{Uppercase}> matches any single character whose Unicode
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415C<Uppercase> property value is C<True>, and C<\P{Uppercase}> matches any character
416whose C<Uppercase> property value is C<False>, and they could have been written as
9d1c51c1 417C<\p{Uppercase=True}> and C<\p{Uppercase=False}>, respectively.
51f494cc 418
b19eb496 419This formality is needed when properties are not binary; that is, if they can
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420take on more values than just C<True> and C<False>. For example, the
421C<Bidi_Class> property (see L</"Bidirectional Character Types"> below),
422can take on several different
423values, such as C<Left>, C<Right>, C<Whitespace>, and others. To match these, one needs
424to specify both the property name (C<Bidi_Class>), AND the value being
5bff2035 425matched against
b65e6125 426(C<Left>, C<Right>, I<etc.>). This is done, as in the examples above, by having the
9f815e24 427two components separated by an equal sign (or interchangeably, a colon), like
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428C<\p{Bidi_Class: Left}>.
429
430All Unicode-defined character properties may be written in these compound forms
a9130ea9 431of C<\p{I<property>=I<value>}> or C<\p{I<property>:I<value>}>, but Perl provides some
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432additional properties that are written only in the single form, as well as
433single-form short-cuts for all binary properties and certain others described
434below, in which you may omit the property name and the equals or colon
435separator.
436
437Most Unicode character properties have at least two synonyms (or aliases if you
b19eb496 438prefer): a short one that is easier to type and a longer one that is more
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439descriptive and hence easier to understand. Thus the C<"L"> and
440C<"Letter"> properties above are equivalent and can be used
441interchangeably. Likewise, C<"Upper"> is a synonym for C<"Uppercase">,
442and we could have written C<\p{Uppercase}> equivalently as C<\p{Upper}>.
443Also, there are typically various synonyms for the values the property
444can be. For binary properties, C<"True"> has 3 synonyms: C<"T">,
445C<"Yes">, and C<"Y">; and C<"False"> has correspondingly C<"F">,
446C<"No">, and C<"N">. But be careful. A short form of a value for one
447property may not mean the same thing as the same short form for another.
448Thus, for the C<L</General_Category>> property, C<"L"> means
449C<"Letter">, but for the L<C<Bidi_Class>|/Bidirectional Character Types>
450property, C<"L"> means C<"Left">. A complete list of properties and
451synonyms is in L<perluniprops>.
51f494cc 452
b19eb496 453Upper/lower case differences in property names and values are irrelevant;
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454thus C<\p{Upper}> means the same thing as C<\p{upper}> or even C<\p{UpPeR}>.
455Similarly, you can add or subtract underscores anywhere in the middle of a
456word, so that these are also equivalent to C<\p{U_p_p_e_r}>. And white space
457is irrelevant adjacent to non-word characters, such as the braces and the equals
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458or colon separators, so C<\p{ Upper }> and C<\p{ Upper_case : Y }> are
459equivalent to these as well. In fact, white space and even
460hyphens can usually be added or deleted anywhere. So even C<\p{ Up-per case = Yes}> is
51f494cc 461equivalent. All this is called "loose-matching" by Unicode. The few places
b19eb496 462where stricter matching is used is in the middle of numbers, and in the Perl
51f494cc 463extension properties that begin or end with an underscore. Stricter matching
b19eb496 464cares about white space (except adjacent to non-word characters),
51f494cc 465hyphens, and non-interior underscores.
4193bef7 466
376d9008 467You can also use negation in both C<\p{}> and C<\P{}> by introducing a caret
a9130ea9 468(C<^>) between the first brace and the property name: C<\p{^Tamil}> is
eb0cc9e3 469equal to C<\P{Tamil}>.
4193bef7 470
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471Almost all properties are immune to case-insensitive matching. That is,
472adding a C</i> regular expression modifier does not change what they
473match. There are two sets that are affected.
474The first set is
475C<Uppercase_Letter>,
476C<Lowercase_Letter>,
477and C<Titlecase_Letter>,
478all of which match C<Cased_Letter> under C</i> matching.
479And the second set is
480C<Uppercase>,
481C<Lowercase>,
482and C<Titlecase>,
483all of which match C<Cased> under C</i> matching.
484This set also includes its subsets C<PosixUpper> and C<PosixLower> both
a9130ea9 485of which under C</i> match C<PosixAlpha>.
56ca34ca 486(The difference between these sets is that some things, such as Roman
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487numerals, come in both upper and lower case so they are C<Cased>, but
488aren't considered letters, so they aren't C<Cased_Letter>'s.)
56ca34ca 489
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490See L</Beyond Unicode code points> for special considerations when
491matching Unicode properties against non-Unicode code points.
94b42e47 492
51f494cc 493=head3 B<General_Category>
14bb0a9a 494
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495Every Unicode character is assigned a general category, which is the "most
496usual categorization of a character" (from
497L<http://www.unicode.org/reports/tr44>).
822502e5 498
9f815e24 499The compound way of writing these is like C<\p{General_Category=Number}>
b65e6125 500(short: C<\p{gc:n}>). But Perl furnishes shortcuts in which everything up
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501through the equal or colon separator is omitted. So you can instead just write
502C<\pN>.
822502e5 503
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504Here are the short and long forms of the values the C<General Category> property
505can have:
393fec97 506
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507 Short Long
508
509 L Letter
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510 LC, L& Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
511 Lu Uppercase_Letter
512 Ll Lowercase_Letter
513 Lt Titlecase_Letter
514 Lm Modifier_Letter
515 Lo Other_Letter
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516
517 M Mark
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518 Mn Nonspacing_Mark
519 Mc Spacing_Mark
520 Me Enclosing_Mark
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521
522 N Number
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523 Nd Decimal_Number (also Digit)
524 Nl Letter_Number
525 No Other_Number
526
527 P Punctuation (also Punct)
528 Pc Connector_Punctuation
529 Pd Dash_Punctuation
530 Ps Open_Punctuation
531 Pe Close_Punctuation
532 Pi Initial_Punctuation
d73e5302 533 (may behave like Ps or Pe depending on usage)
51f494cc 534 Pf Final_Punctuation
d73e5302 535 (may behave like Ps or Pe depending on usage)
51f494cc 536 Po Other_Punctuation
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537
538 S Symbol
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539 Sm Math_Symbol
540 Sc Currency_Symbol
541 Sk Modifier_Symbol
542 So Other_Symbol
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543
544 Z Separator
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545 Zs Space_Separator
546 Zl Line_Separator
547 Zp Paragraph_Separator
d73e5302
JH
548
549 C Other
d88362ca 550 Cc Control (also Cntrl)
e150c829 551 Cf Format
6d4f9cf2 552 Cs Surrogate
51f494cc 553 Co Private_Use
e150c829 554 Cn Unassigned
1ac13f9a 555
376d9008 556Single-letter properties match all characters in any of the
3e4dbfed 557two-letter sub-properties starting with the same letter.
b19eb496 558C<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 559
51f494cc 560=head3 B<Bidirectional Character Types>
822502e5 561
b19eb496 562Because scripts differ in their directionality (Hebrew and Arabic are
a9130ea9 563written right to left, for example) Unicode supplies a C<Bidi_Class> property.
1850f57f 564Some of the values this property can have are:
32293815 565
88af3b93 566 Value Meaning
92e830a9 567
12ac2576
JP
568 L Left-to-Right
569 LRE Left-to-Right Embedding
570 LRO Left-to-Right Override
571 R Right-to-Left
51f494cc 572 AL Arabic Letter
12ac2576
JP
573 RLE Right-to-Left Embedding
574 RLO Right-to-Left Override
575 PDF Pop Directional Format
576 EN European Number
51f494cc
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577 ES European Separator
578 ET European Terminator
12ac2576 579 AN Arabic Number
51f494cc 580 CS Common Separator
12ac2576
JP
581 NSM Non-Spacing Mark
582 BN Boundary Neutral
583 B Paragraph Separator
584 S Segment Separator
585 WS Whitespace
586 ON Other Neutrals
587
51f494cc
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588This property is always written in the compound form.
589For example, C<\p{Bidi_Class:R}> matches characters that are normally
1850f57f 590written right to left. Unlike the
a9130ea9 591C<L</General_Category>> property, this
1850f57f
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592property can have more values added in a future Unicode release. Those
593listed above comprised the complete set for many Unicode releases, but
594others were added in Unicode 6.3; you can always find what the
20ada7da 595current ones are in L<perluniprops>. And
1850f57f 596L<http://www.unicode.org/reports/tr9/> describes how to use them.
eb0cc9e3 597
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598=head3 B<Scripts>
599
b19eb496 600The world's languages are written in many different scripts. This sentence
e1b711da 601(unless you're reading it in translation) is written in Latin, while Russian is
c69ca1d4 602written in Cyrillic, and Greek is written in, well, Greek; Japanese mainly in
e1b711da 603Hiragana or Katakana. There are many more.
51f494cc 604
b65e6125 605The Unicode C<Script> and C<Script_Extensions> properties give what script a
82aed44a
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606given character is in. Either property can be specified with the
607compound form like
608C<\p{Script=Hebrew}> (short: C<\p{sc=hebr}>), or
609C<\p{Script_Extensions=Javanese}> (short: C<\p{scx=java}>).
610In addition, Perl furnishes shortcuts for all
611C<Script> property names. You can omit everything up through the equals
612(or colon), and simply write C<\p{Latin}> or C<\P{Cyrillic}>.
613(This is not true for C<Script_Extensions>, which is required to be
614written in the compound form.)
615
616The difference between these two properties involves characters that are
617used in multiple scripts. For example the digits '0' through '9' are
618used in many parts of the world. These are placed in a script named
619C<Common>. Other characters are used in just a few scripts. For
a9130ea9 620example, the C<"KATAKANA-HIRAGANA DOUBLE HYPHEN"> is used in both Japanese
82aed44a
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621scripts, Katakana and Hiragana, but nowhere else. The C<Script>
622property places all characters that are used in multiple scripts in the
623C<Common> script, while the C<Script_Extensions> property places those
624that are used in only a few scripts into each of those scripts; while
625still using C<Common> for those used in many scripts. Thus both these
626match:
627
628 "0" =~ /\p{sc=Common}/ # Matches
629 "0" =~ /\p{scx=Common}/ # Matches
630
631and only the first of these match:
632
633 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common} # Matches
634 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match
635
636And only the last two of these match:
637
638 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana} # No match
639 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana} # No match
640 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
641 "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches
642
643C<Script_Extensions> is thus an improved C<Script>, in which there are
644fewer characters in the C<Common> script, and correspondingly more in
645other scripts. It is new in Unicode version 6.0, and its data are likely
646to change significantly in later releases, as things get sorted out.
b65e6125
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647New code should probably be using C<Script_Extensions> and not plain
648C<Script>.
82aed44a
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649
650(Actually, besides C<Common>, the C<Inherited> script, contains
651characters that are used in multiple scripts. These are modifier
b65e6125 652characters which inherit the script value
82aed44a
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653of the controlling character. Some of these are used in many scripts,
654and so go into C<Inherited> in both C<Script> and C<Script_Extensions>.
655Others are used in just a few scripts, so are in C<Inherited> in
656C<Script>, but not in C<Script_Extensions>.)
657
658It is worth stressing that there are several different sets of digits in
659Unicode that are equivalent to 0-9 and are matchable by C<\d> in a
660regular expression. If they are used in a single language only, they
661are in that language's C<Script> and C<Script_Extension>. If they are
662used in more than one script, they will be in C<sc=Common>, but only
663if they are used in many scripts should they be in C<scx=Common>.
51f494cc
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664
665A complete list of scripts and their shortcuts is in L<perluniprops>.
666
a9130ea9 667=head3 B<Use of the C<"Is"> Prefix>
822502e5 668
b65e6125
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669For backward compatibility (with Perl 5.6), all properties writable
670without using the compound form mentioned
51f494cc
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671so far may have C<Is> or C<Is_> prepended to their name, so C<\P{Is_Lu}>, for
672example, is equal to C<\P{Lu}>, and C<\p{IsScript:Arabic}> is equal to
673C<\p{Arabic}>.
eb0cc9e3 674
51f494cc 675=head3 B<Blocks>
2796c109 676
1bfb14c4
JH
677In addition to B<scripts>, Unicode also defines B<blocks> of
678characters. The difference between scripts and blocks is that the
679concept of scripts is closer to natural languages, while the concept
51f494cc 680of blocks is more of an artificial grouping based on groups of Unicode
a9130ea9 681characters with consecutive ordinal values. For example, the C<"Basic Latin">
b65e6125 682block is all the characters whose ordinals are between 0 and 127, inclusive; in
a9130ea9
KW
683other words, the ASCII characters. The C<"Latin"> script contains some letters
684from this as well as several other blocks, like C<"Latin-1 Supplement">,
b65e6125 685C<"Latin Extended-A">, I<etc.>, but it does not contain all the characters from
7be67b37 686those blocks. It does not, for example, contain the digits 0-9, because
82aed44a
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687those digits are shared across many scripts, and hence are in the
688C<Common> script.
51f494cc
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689
690For more about scripts versus blocks, see UAX#24 "Unicode Script Property":
691L<http://www.unicode.org/reports/tr24>
692
82aed44a
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693The C<Script> or C<Script_Extensions> properties are likely to be the
694ones you want to use when processing
a9130ea9 695natural language; the C<Block> property may occasionally be useful in working
b19eb496 696with the nuts and bolts of Unicode.
51f494cc
KW
697
698Block names are matched in the compound form, like C<\p{Block: Arrows}> or
b19eb496 699C<\p{Blk=Hebrew}>. Unlike most other properties, only a few block names have a
6b5cf123
KW
700Unicode-defined short name.
701
702Perl also defines single form synonyms for the block property in cases
703where these do not conflict with something else. But don't use any of
704these, because they are unstable. Since these are Perl extensions, they
705are subordinate to official Unicode property names; Unicode doesn't know
706nor care about Perl's extensions. It may happen that a name that
707currently means the Perl extension will later be changed without warning
708to mean a different Unicode property in a future version of the perl
709interpreter that uses a later Unicode release, and your code would no
710longer work. The extensions are mentioned here for completeness: Take
711the block name and prefix it with one of: C<In> (for example
712C<\p{Blk=Arrows}> can currently be written as C<\p{In_Arrows}>); or
713sometimes C<Is> (like C<\p{Is_Arrows}>); or sometimes no prefix at all
714(C<\p{Arrows}>). As of this writing (Unicode 8.0) there are no
715conflicts with using the C<In_> prefix, but there are plenty with the
716other two forms. For example, C<\p{Is_Hebrew}> and C<\p{Hebrew}> mean
717C<\p{Script=Hebrew}> which is NOT the same thing as C<\p{Blk=Hebrew}>. Our
718advice used to be to use the C<In_> prefix as a single form way of
719specifying a block. But Unicode 8.0 added properties whose names begin
720with C<In>, and it's now clear that it's only luck that's so far
721prevented a conflict. Using C<In> is only marginally less typing than
722C<Blk:>, and the latter's meaning is clearer anyway, and guaranteed to
723never conflict. So don't take chances. Use C<\p{Blk=foo}> for new
724code. And be sure that block is what you really really want to do. In
725most cases scripts are what you want instead.
726
727A complete list of blocks is in L<perluniprops>.
51f494cc 728
9f815e24
KW
729=head3 B<Other Properties>
730
731There are many more properties than the very basic ones described here.
732A complete list is in L<perluniprops>.
733
734Unicode defines all its properties in the compound form, so all single-form
b19eb496
TC
735properties are Perl extensions. Most of these are just synonyms for the
736Unicode ones, but some are genuine extensions, including several that are in
9f815e24
KW
737the compound form. And quite a few of these are actually recommended by Unicode
738(in L<http://www.unicode.org/reports/tr18>).
739
5bff2035
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740This section gives some details on all extensions that aren't just
741synonyms for compound-form Unicode properties
742(for those properties, you'll have to refer to the
9f815e24
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743L<Unicode Standard|http://www.unicode.org/reports/tr44>.
744
745=over
746
747=item B<C<\p{All}>>
748
2d88a86a
KW
749This matches every possible code point. It is equivalent to C<qr/./s>.
750Unlike all the other non-user-defined C<\p{}> property matches, no
751warning is ever generated if this is property is matched against a
752non-Unicode code point (see L</Beyond Unicode code points> below).
9f815e24
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753
754=item B<C<\p{Alnum}>>
755
756This matches any C<\p{Alphabetic}> or C<\p{Decimal_Number}> character.
757
758=item B<C<\p{Any}>>
759
2d88a86a
KW
760This matches any of the 1_114_112 Unicode code points. It is a synonym
761for C<\p{Unicode}>.
9f815e24 762
42581d5d
KW
763=item B<C<\p{ASCII}>>
764
765This matches any of the 128 characters in the US-ASCII character set,
766which is a subset of Unicode.
767
9f815e24
KW
768=item B<C<\p{Assigned}>>
769
a9130ea9
KW
770This matches any assigned code point; that is, any code point whose L<general
771category|/General_Category> is not C<Unassigned> (or equivalently, not C<Cn>).
9f815e24
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772
773=item B<C<\p{Blank}>>
774
775This is the same as C<\h> and C<\p{HorizSpace}>: A character that changes the
776spacing horizontally.
777
778=item B<C<\p{Decomposition_Type: Non_Canonical}>> (Short: C<\p{Dt=NonCanon}>)
779
780Matches a character that has a non-canonical decomposition.
781
a6a7eedc
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782The L</Extended Grapheme Clusters (Logical characters)> section above
783talked about canonical decompositions. However, many more characters
784have a different type of decomposition, a "compatible" or
785"non-canonical" decomposition. The sequences that form these
786decompositions are not considered canonically equivalent to the
787pre-composed character. An example is the C<"SUPERSCRIPT ONE">. It is
788somewhat like a regular digit 1, but not exactly; its decomposition into
789the digit 1 is called a "compatible" decomposition, specifically a
9f815e24 790"super" decomposition. There are several such compatibility
b65e6125
KW
791decompositions (see L<http://www.unicode.org/reports/tr44>), including
792one called "compat", which means some miscellaneous type of
793decomposition that doesn't fit into the other decomposition categories
794that Unicode has chosen.
9f815e24
KW
795
796Note that most Unicode characters don't have a decomposition, so their
a9130ea9 797decomposition type is C<"None">.
9f815e24 798
b19eb496
TC
799For your convenience, Perl has added the C<Non_Canonical> decomposition
800type to mean any of the several compatibility decompositions.
9f815e24
KW
801
802=item B<C<\p{Graph}>>
803
804Matches any character that is graphic. Theoretically, this means a character
805that on a printer would cause ink to be used.
806
807=item B<C<\p{HorizSpace}>>
808
b19eb496 809This is the same as C<\h> and C<\p{Blank}>: a character that changes the
9f815e24
KW
810spacing horizontally.
811
42581d5d 812=item B<C<\p{In=*}>>
9f815e24
KW
813
814This is a synonym for C<\p{Present_In=*}>
815
816=item B<C<\p{PerlSpace}>>
817
d28d8023 818This is the same as C<\s>, restricted to ASCII, namely C<S<[ \f\n\r\t]>>
779cf272 819and starting in Perl v5.18, a vertical tab.
9f815e24
KW
820
821Mnemonic: Perl's (original) space
822
823=item B<C<\p{PerlWord}>>
824
825This is the same as C<\w>, restricted to ASCII, namely C<[A-Za-z0-9_]>
826
827Mnemonic: Perl's (original) word.
828
42581d5d 829=item B<C<\p{Posix...}>>
9f815e24 830
b65e6125
KW
831There are several of these, which are equivalents, using the C<\p{}>
832notation, for Posix classes and are described in
42581d5d 833L<perlrecharclass/POSIX Character Classes>.
9f815e24
KW
834
835=item B<C<\p{Present_In: *}>> (Short: C<\p{In=*}>)
836
837This property is used when you need to know in what Unicode version(s) a
838character is.
839
840The "*" above stands for some two digit Unicode version number, such as
841C<1.1> or C<4.0>; or the "*" can also be C<Unassigned>. This property will
842match the code points whose final disposition has been settled as of the
843Unicode release given by the version number; C<\p{Present_In: Unassigned}>
844will match those code points whose meaning has yet to be assigned.
845
a9130ea9 846For example, C<U+0041> C<"LATIN CAPITAL LETTER A"> was present in the very first
9f815e24
KW
847Unicode release available, which is C<1.1>, so this property is true for all
848valid "*" versions. On the other hand, C<U+1EFF> was not assigned until version
a9130ea9 8495.1 when it became C<"LATIN SMALL LETTER Y WITH LOOP">, so the only "*" that
9f815e24
KW
850would match it are 5.1, 5.2, and later.
851
852Unicode furnishes the C<Age> property from which this is derived. The problem
853with Age is that a strict interpretation of it (which Perl takes) has it
854matching the precise release a code point's meaning is introduced in. Thus
855C<U+0041> would match only 1.1; and C<U+1EFF> only 5.1. This is not usually what
856you want.
857
858Some non-Perl implementations of the Age property may change its meaning to be
a9130ea9 859the same as the Perl C<Present_In> property; just be aware of that.
9f815e24
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860
861Another confusion with both these properties is that the definition is not
b19eb496
TC
862that the code point has been I<assigned>, but that the meaning of the code point
863has been I<determined>. This is because 66 code points will always be
a9130ea9 864unassigned, and so the C<Age> for them is the Unicode version in which the decision
b19eb496 865to make them so was made. For example, C<U+FDD0> is to be permanently
9f815e24 866unassigned to a character, and the decision to do that was made in version 3.1,
b19eb496 867so C<\p{Age=3.1}> matches this character, as also does C<\p{Present_In: 3.1}> and up.
9f815e24
KW
868
869=item B<C<\p{Print}>>
870
ae5b72c8 871This matches any character that is graphical or blank, except controls.
9f815e24
KW
872
873=item B<C<\p{SpacePerl}>>
874
875This is the same as C<\s>, including beyond ASCII.
876
4d4acfba 877Mnemonic: Space, as modified by Perl. (It doesn't include the vertical tab
779cf272 878until v5.18, which both the Posix standard and Unicode consider white space.)
9f815e24 879
4364919a
KW
880=item B<C<\p{Title}>> and B<C<\p{Titlecase}>>
881
882Under case-sensitive matching, these both match the same code points as
883C<\p{General Category=Titlecase_Letter}> (C<\p{gc=lt}>). The difference
884is that under C</i> caseless matching, these match the same as
885C<\p{Cased}>, whereas C<\p{gc=lt}> matches C<\p{Cased_Letter>).
886
2d88a86a
KW
887=item B<C<\p{Unicode}>>
888
889This matches any of the 1_114_112 Unicode code points.
890C<\p{Any}>.
891
9f815e24
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892=item B<C<\p{VertSpace}>>
893
894This is the same as C<\v>: A character that changes the spacing vertically.
895
896=item B<C<\p{Word}>>
897
b19eb496 898This is the same as C<\w>, including over 100_000 characters beyond ASCII.
9f815e24 899
42581d5d
KW
900=item B<C<\p{XPosix...}>>
901
b19eb496 902There are several of these, which are the standard Posix classes
42581d5d
KW
903extended to the full Unicode range. They are described in
904L<perlrecharclass/POSIX Character Classes>.
905
9f815e24
KW
906=back
907
a9130ea9 908
376d9008 909=head2 User-Defined Character Properties
491fd90a 910
51f494cc 911You can define your own binary character properties by defining subroutines
a9130ea9 912whose names begin with C<"In"> or C<"Is">. (The experimental feature
9d1a5160
KW
913L<perlre/(?[ ])> provides an alternative which allows more complex
914definitions.) The subroutines can be defined in any
51f494cc 915package. The user-defined properties can be used in the regular expression
a9130ea9 916C<\p{}> and C<\P{}> constructs; if you are using a user-defined property from a
51f494cc 917package other than the one you are in, you must specify its package in the
a9130ea9 918C<\p{}> or C<\P{}> construct.
bac0b425 919
51f494cc 920 # assuming property Is_Foreign defined in Lang::
bac0b425
JP
921 package main; # property package name required
922 if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
923
924 package Lang; # property package name not required
925 if ($txt =~ /\p{IsForeign}+/) { ... }
926
927
928Note that the effect is compile-time and immutable once defined.
b19eb496
TC
929However, the subroutines are passed a single parameter, which is 0 if
930case-sensitive matching is in effect and non-zero if caseless matching
56ca34ca
KW
931is in effect. The subroutine may return different values depending on
932the value of the flag, and one set of values will immutably be in effect
b19eb496 933for all case-sensitive matches, and the other set for all case-insensitive
56ca34ca 934matches.
491fd90a 935
b19eb496 936Note that if the regular expression is tainted, then Perl will die rather
a9130ea9 937than calling the subroutine when the name of the subroutine is
0e9be77f
DM
938determined by the tainted data.
939
376d9008
JB
940The subroutines must return a specially-formatted string, with one
941or more newline-separated lines. Each line must be one of the following:
491fd90a
JH
942
943=over 4
944
945=item *
946
df9e1087 947A single hexadecimal number denoting a code point to include.
510254c9
A
948
949=item *
950
99a6b1f0 951Two hexadecimal numbers separated by horizontal whitespace (space or
df9e1087 952tabular characters) denoting a range of code points to include.
491fd90a
JH
953
954=item *
955
a9130ea9
KW
956Something to include, prefixed by C<"+">: a built-in character
957property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 958name) user-defined character property,
bac0b425
JP
959to represent all the characters in that property; two hexadecimal code
960points for a range; or a single hexadecimal code point.
491fd90a
JH
961
962=item *
963
a9130ea9
KW
964Something to exclude, prefixed by C<"-">: an existing character
965property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 966name) user-defined character property,
bac0b425
JP
967to represent all the characters in that property; two hexadecimal code
968points for a range; or a single hexadecimal code point.
491fd90a
JH
969
970=item *
971
a9130ea9
KW
972Something to negate, prefixed C<"!">: an existing character
973property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 974name) user-defined character property,
bac0b425
JP
975to represent all the characters in that property; two hexadecimal code
976points for a range; or a single hexadecimal code point.
977
978=item *
979
a9130ea9
KW
980Something to intersect with, prefixed by C<"&">: an existing character
981property (prefixed by C<"utf8::">) or a fully qualified (including package
830137a2 982name) user-defined character property,
bac0b425
JP
983for all the characters except the characters in the property; two
984hexadecimal code points for a range; or a single hexadecimal code point.
491fd90a
JH
985
986=back
987
988For example, to define a property that covers both the Japanese
989syllabaries (hiragana and katakana), you can define
990
991 sub InKana {
d88362ca 992 return <<END;
d5822f25
A
993 3040\t309F
994 30A0\t30FF
491fd90a
JH
995 END
996 }
997
d5822f25
A
998Imagine that the here-doc end marker is at the beginning of the line.
999Now you can use C<\p{InKana}> and C<\P{InKana}>.
491fd90a
JH
1000
1001You could also have used the existing block property names:
1002
1003 sub InKana {
d88362ca 1004 return <<'END';
491fd90a
JH
1005 +utf8::InHiragana
1006 +utf8::InKatakana
1007 END
1008 }
1009
1010Suppose you wanted to match only the allocated characters,
d5822f25 1011not the raw block ranges: in other words, you want to remove
b65e6125 1012the unassigned characters:
491fd90a
JH
1013
1014 sub InKana {
d88362ca 1015 return <<'END';
491fd90a
JH
1016 +utf8::InHiragana
1017 +utf8::InKatakana
1018 -utf8::IsCn
1019 END
1020 }
1021
1022The negation is useful for defining (surprise!) negated classes.
1023
1024 sub InNotKana {
d88362ca 1025 return <<'END';
491fd90a
JH
1026 !utf8::InHiragana
1027 -utf8::InKatakana
1028 +utf8::IsCn
1029 END
1030 }
1031
461020ad
KW
1032This will match all non-Unicode code points, since every one of them is
1033not in Kana. You can use intersection to exclude these, if desired, as
1034this modified example shows:
bac0b425 1035
461020ad 1036 sub InNotKana {
bac0b425 1037 return <<'END';
461020ad
KW
1038 !utf8::InHiragana
1039 -utf8::InKatakana
1040 +utf8::IsCn
1041 &utf8::Any
bac0b425
JP
1042 END
1043 }
1044
461020ad
KW
1045C<&utf8::Any> must be the last line in the definition.
1046
1047Intersection is used generally for getting the common characters matched
a9130ea9 1048by two (or more) classes. It's important to remember not to use C<"&"> for
461020ad
KW
1049the first set; that would be intersecting with nothing, resulting in an
1050empty set.
1051
2d88a86a
KW
1052Unlike non-user-defined C<\p{}> property matches, no warning is ever
1053generated if these properties are matched against a non-Unicode code
1054point (see L</Beyond Unicode code points> below).
bac0b425 1055
68585b5e 1056=head2 User-Defined Case Mappings (for serious hackers only)
822502e5 1057
5d1892be 1058B<This feature has been removed as of Perl 5.16.>
a9130ea9 1059The CPAN module C<L<Unicode::Casing>> provides better functionality without
5d1892be
KW
1060the drawbacks that this feature had. If you are using a Perl earlier
1061than 5.16, this feature was most fully documented in the 5.14 version of
1062this pod:
1063L<http://perldoc.perl.org/5.14.0/perlunicode.html#User-Defined-Case-Mappings-%28for-serious-hackers-only%29>
3a2263fe 1064
376d9008 1065=head2 Character Encodings for Input and Output
8cbd9a7a 1066
7221edc9 1067See L<Encode>.
8cbd9a7a 1068
c29a771d 1069=head2 Unicode Regular Expression Support Level
776f8809 1070
b19eb496 1071The following list of Unicode supported features for regular expressions describes
fea12a3e
KW
1072all features currently directly supported by core Perl. The references
1073to "Level I<N>" and the section numbers refer to
1074L<UTS#18 "Unicode Regular Expressions"|http://www.unicode.org/reports/tr18>,
1075version 13, November 2013.
1076
1077=head3 Level 1 - Basic Unicode Support
1078
1079 RL1.1 Hex Notation - Done [1]
1080 RL1.2 Properties - Done [2]
1081 RL1.2a Compatibility Properties - Done [3]
1082 RL1.3 Subtraction and Intersection - Experimental [4]
1083 RL1.4 Simple Word Boundaries - Done [5]
1084 RL1.5 Simple Loose Matches - Done [6]
1085 RL1.6 Line Boundaries - Partial [7]
1086 RL1.7 Supplementary Code Points - Done [8]
755789c0 1087
6f33e417
KW
1088=over 4
1089
a6a7eedc 1090=item [1] C<\N{U+...}> and C<\x{...}>
6f33e417 1091
fea12a3e
KW
1092=item [2]
1093C<\p{...}> C<\P{...}>. This requirement is for a minimal list of
1094properties. Perl supports these and all other Unicode character
1095properties, as R2.7 asks (see L</"Unicode Character Properties"> above).
6f33e417 1096
fea12a3e
KW
1097=item [3]
1098Perl has C<\d> C<\D> C<\s> C<\S> C<\w> C<\W> C<\X> C<[:I<prop>:]>
1099C<[:^I<prop>:]>, plus all the properties specified by
1100L<http://www.unicode.org/reports/tr18/#Compatibility_Properties>. These
1101are described above in L</Other Properties>
6f33e417 1102
fea12a3e 1103=item [4]
6f33e417 1104
fea12a3e 1105The experimental feature C<"(?[...])"> starting in v5.18 accomplishes
a6a7eedc 1106this.
6f33e417 1107
a6a7eedc
KW
1108See L<perlre/(?[ ])>. If you don't want to use an experimental
1109feature, you can use one of the following:
6f33e417
KW
1110
1111=over 4
1112
a6a7eedc 1113=item *
f67a5002 1114Regular expression lookahead
6f33e417
KW
1115
1116You can mimic class subtraction using lookahead.
8158862b 1117For example, what UTS#18 might write as
29bdacb8 1118
209c9685 1119 [{Block=Greek}-[{UNASSIGNED}]]
dbe420b4
JH
1120
1121in Perl can be written as:
1122
209c9685
KW
1123 (?!\p{Unassigned})\p{Block=Greek}
1124 (?=\p{Assigned})\p{Block=Greek}
dbe420b4
JH
1125
1126But in this particular example, you probably really want
1127
209c9685 1128 \p{Greek}
dbe420b4
JH
1129
1130which will match assigned characters known to be part of the Greek script.
29bdacb8 1131
a6a7eedc
KW
1132=item *
1133
1134CPAN module C<L<Unicode::Regex::Set>>
8158862b 1135
6f33e417
KW
1136It does implement the full UTS#18 grouping, intersection, union, and
1137removal (subtraction) syntax.
8158862b 1138
a6a7eedc
KW
1139=item *
1140
1141L</"User-Defined Character Properties">
6f33e417 1142
a9130ea9 1143C<"+"> for union, C<"-"> for removal (set-difference), C<"&"> for intersection
6f33e417
KW
1144
1145=back
1146
fea12a3e
KW
1147=item [5]
1148C<\b> C<\B> meet most, but not all, the details of this requirement, but
1149C<\b{wb}> and C<\B{wb}> do, as well as the stricter R2.3.
1150
1151=item [6]
6f33e417 1152
a6a7eedc 1153Note that Perl does Full case-folding in matching, not Simple:
6f33e417 1154
a6a7eedc
KW
1155For example C<U+1F88> is equivalent to C<U+1F00 U+03B9>, instead of just
1156C<U+1F80>. This difference matters mainly for certain Greek capital
a9130ea9
KW
1157letters with certain modifiers: the Full case-folding decomposes the
1158letter, while the Simple case-folding would map it to a single
1159character.
6f33e417 1160
fea12a3e
KW
1161=item [7]
1162
1163The reason this is considered to be only partially implemented is that
1164Perl has L<C<qrE<sol>\b{lb}E<sol>>|perlrebackslash/\b{lb}> and
1165C<L<Unicode::LineBreak>> that are conformant with
1166L<UAX#14 "Unicode Line Breaking Algorithm"|http://www.unicode.org/reports/tr14>.
1167The regular expression construct provides default behavior, while the
1168heavier-weight module provides customizable line breaking.
1169
1170But Perl treats C<\n> as the start- and end-line
1171delimiter, whereas Unicode specifies more characters that should be
1172so-interpreted.
6f33e417 1173
a6a7eedc 1174These are:
6f33e417 1175
a6a7eedc
KW
1176 VT U+000B (\v in C)
1177 FF U+000C (\f)
1178 CR U+000D (\r)
1179 NEL U+0085
1180 LS U+2028
1181 PS U+2029
6f33e417 1182
a6a7eedc
KW
1183C<^> and C<$> in regular expression patterns are supposed to match all
1184these, but don't.
1185These characters also don't, but should, affect C<< <> >> C<$.>, and
1186script line numbers.
6f33e417 1187
a6a7eedc
KW
1188Also, lines should not be split within C<CRLF> (i.e. there is no
1189empty line between C<\r> and C<\n>). For C<CRLF>, try the C<:crlf>
1190layer (see L<PerlIO>).
1191
fea12a3e 1192=item [8]
a9130ea9
KW
1193UTF-8/UTF-EBDDIC used in Perl allows not only C<U+10000> to
1194C<U+10FFFF> but also beyond C<U+10FFFF>
6f33e417
KW
1195
1196=back
5ca1ac52 1197
fea12a3e 1198=head3 Level 2 - Extended Unicode Support
776f8809 1199
fea12a3e
KW
1200 RL2.1 Canonical Equivalents - Retracted [9]
1201 by Unicode
1202 RL2.2 Extended Grapheme Clusters - Partial [10]
1203 RL2.3 Default Word Boundaries - Done [11]
1204 RL2.4 Default Case Conversion - Done
1205 RL2.5 Name Properties - Done
1206 RL2.6 Wildcard Properties - Missing
1207 RL2.7 Full Properties - Done
776f8809 1208
fea12a3e 1209=over 4
8158862b 1210
fea12a3e
KW
1211=item [9]
1212Unicode has rewritten this portion of UTS#18 to say that getting
1213canonical equivalence (see UAX#15
1214L<"Unicode Normalization Forms"|http://www.unicode.org/reports/tr15>)
1215is basically to be done at the programmer level. Use NFD to write
1216both your regular expressions and text to match them against (you
1217can use L<Unicode::Normalize>).
776f8809 1218
fea12a3e
KW
1219=item [10]
1220Perl has C<\X> and C<\b{gcb}> but we don't have a "Grapheme Cluster Mode".
1221
1222=item [11] see
1223L<UAX#29 "Unicode Text Segmentation"|http://www.unicode.org/reports/tr29>,
1224
1225=back
1226
1227=head3 Level 3 - Tailored Support
1228
1229 RL3.1 Tailored Punctuation - Missing
1230 RL3.2 Tailored Grapheme Clusters - Missing [12]
1231 RL3.3 Tailored Word Boundaries - Missing
1232 RL3.4 Tailored Loose Matches - Retracted by Unicode
1233 RL3.5 Tailored Ranges - Retracted by Unicode
1234 RL3.6 Context Matching - Missing [13]
1235 RL3.7 Incremental Matches - Missing
1236 RL3.8 Unicode Set Sharing - Unicode is proposing
1237 to retract this
1238 RL3.9 Possible Match Sets - Missing
1239 RL3.10 Folded Matching - Retracted by Unicode
1240 RL3.11 Submatchers - Missing
1241
1242=over 4
1243
1244=item [12]
1245Perl has L<Unicode::Collate>, but it isn't integrated with regular
1246expressions. See
1247L<UTS#10 "Unicode Collation Algorithms"|http://www.unicode.org/reports/tr10>.
776f8809 1248
fea12a3e
KW
1249=item [13]
1250Perl has C<(?<=x)> and C<(?=x)>, but lookaheads or lookbehinds should
1251see outside of the target substring
776f8809
JH
1252
1253=back
1254
c349b1b9
JH
1255=head2 Unicode Encodings
1256
376d9008
JB
1257Unicode characters are assigned to I<code points>, which are abstract
1258numbers. To use these numbers, various encodings are needed.
c349b1b9
JH
1259
1260=over 4
1261
c29a771d 1262=item *
5cb3728c
RB
1263
1264UTF-8
c349b1b9 1265
6d4f9cf2 1266UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
a6a7eedc
KW
1267encoding. In most of Perl's documentation, including elsewhere in this
1268document, the term "UTF-8" means also "UTF-EBCDIC". But in this section,
1269"UTF-8" refers only to the encoding used on ASCII platforms. It is a
1270superset of 7-bit US-ASCII, so anything encoded in ASCII has the
1271identical representation when encoded in UTF-8.
c349b1b9 1272
8c007b5a 1273The following table is from Unicode 3.2.
05632f9a 1274
755789c0 1275 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
05632f9a 1276
d88362ca 1277 U+0000..U+007F 00..7F
e1b711da 1278 U+0080..U+07FF * C2..DF 80..BF
d88362ca 1279 U+0800..U+0FFF E0 * A0..BF 80..BF
ec90690f
ST
1280 U+1000..U+CFFF E1..EC 80..BF 80..BF
1281 U+D000..U+D7FF ED 80..9F 80..BF
755789c0 1282 U+D800..U+DFFF +++++ utf16 surrogates, not legal utf8 +++++
ec90690f 1283 U+E000..U+FFFF EE..EF 80..BF 80..BF
d88362ca
KW
1284 U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF
1285 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
1286 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
e1b711da 1287
b19eb496 1288Note the gaps marked by "*" before several of the byte entries above. These are
e1b711da
KW
1289caused by legal UTF-8 avoiding non-shortest encodings: it is technically
1290possible to UTF-8-encode a single code point in different ways, but that is
1291explicitly forbidden, and the shortest possible encoding should always be used
1292(and that is what Perl does).
37361303 1293
376d9008 1294Another way to look at it is via bits:
05632f9a 1295
755789c0 1296 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
05632f9a 1297
755789c0
KW
1298 0aaaaaaa 0aaaaaaa
1299 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
1300 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
1301 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
05632f9a 1302
a9130ea9 1303As you can see, the continuation bytes all begin with C<"10">, and the
e1b711da 1304leading bits of the start byte tell how many bytes there are in the
05632f9a
JH
1305encoded character.
1306
6d4f9cf2 1307The original UTF-8 specification allowed up to 6 bytes, to allow
a9130ea9 1308encoding of numbers up to C<0x7FFF_FFFF>. Perl continues to allow those,
6d4f9cf2
KW
1309and has extended that up to 13 bytes to encode code points up to what
1310can fit in a 64-bit word. However, Perl will warn if you output any of
b19eb496 1311these as being non-portable; and under strict UTF-8 input protocols,
760c7c2f
KW
1312they are forbidden. In addition, it is deprecated to use a code point
1313larger than what a signed integer variable on your system can hold. On
131432-bit ASCII systems, this means C<0x7FFF_FFFF> is the legal maximum
1315going forward (much higher on 64-bit systems).
6d4f9cf2 1316
c29a771d 1317=item *
5cb3728c
RB
1318
1319UTF-EBCDIC
dbe420b4 1320
b65e6125 1321Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
a6a7eedc
KW
1322This means that all the basic characters (which includes all
1323those that have ASCII equivalents (like C<"A">, C<"0">, C<"%">, I<etc.>)
1324are the same in both EBCDIC and UTF-EBCDIC.)
1325
c0236afe
KW
1326UTF-EBCDIC is used on EBCDIC platforms. It generally requires more
1327bytes to represent a given code point than UTF-8 does; the largest
1328Unicode code points take 5 bytes to represent (instead of 4 in UTF-8),
1329and, extended for 64-bit words, it uses 14 bytes instead of 13 bytes in
1330UTF-8.
dbe420b4 1331
c29a771d 1332=item *
5cb3728c 1333
b65e6125 1334UTF-16, UTF-16BE, UTF-16LE, Surrogates, and C<BOM>'s (Byte Order Marks)
c349b1b9 1335
1bfb14c4
JH
1336The followings items are mostly for reference and general Unicode
1337knowledge, Perl doesn't use these constructs internally.
dbe420b4 1338
b19eb496
TC
1339Like UTF-8, UTF-16 is a variable-width encoding, but where
1340UTF-8 uses 8-bit code units, UTF-16 uses 16-bit code units.
1341All code points occupy either 2 or 4 bytes in UTF-16: code points
1342C<U+0000..U+FFFF> are stored in a single 16-bit unit, and code
1bfb14c4 1343points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is
c349b1b9
JH
1344using I<surrogates>, the first 16-bit unit being the I<high
1345surrogate>, and the second being the I<low surrogate>.
1346
376d9008 1347Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
c349b1b9 1348range of Unicode code points in pairs of 16-bit units. The I<high
9f815e24 1349surrogates> are the range C<U+D800..U+DBFF> and the I<low surrogates>
376d9008 1350are the range C<U+DC00..U+DFFF>. The surrogate encoding is
c349b1b9 1351
d88362ca
KW
1352 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1353 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
c349b1b9
JH
1354
1355and the decoding is
1356
d88362ca 1357 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
c349b1b9 1358
376d9008 1359Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
c349b1b9 1360itself can be used for in-memory computations, but if storage or
376d9008
JB
1361transfer is required either UTF-16BE (big-endian) or UTF-16LE
1362(little-endian) encodings must be chosen.
c349b1b9
JH
1363
1364This introduces another problem: what if you just know that your data
376d9008 1365is UTF-16, but you don't know which endianness? Byte Order Marks, or
b65e6125 1366C<BOM>'s, are a solution to this. A special character has been reserved
86bbd6d1 1367in Unicode to function as a byte order marker: the character with the
a9130ea9 1368code point C<U+FEFF> is the C<BOM>.
042da322 1369
a9130ea9 1370The trick is that if you read a C<BOM>, you will know the byte order,
376d9008
JB
1371since if it was written on a big-endian platform, you will read the
1372bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
1373you will read the bytes C<0xFF 0xFE>. (And if the originating platform
b65e6125
KW
1374was writing in ASCII platform UTF-8, you will read the bytes
1375C<0xEF 0xBB 0xBF>.)
042da322 1376
86bbd6d1 1377The way this trick works is that the character with the code point
6d4f9cf2 1378C<U+FFFE> is not supposed to be in input streams, so the
a9130ea9 1379sequence of bytes C<0xFF 0xFE> is unambiguously "C<BOM>, represented in
1bfb14c4 1380little-endian format" and cannot be C<U+FFFE>, represented in big-endian
6d4f9cf2
KW
1381format".
1382
1383Surrogates have no meaning in Unicode outside their use in pairs to
1384represent other code points. However, Perl allows them to be
1385represented individually internally, for example by saying
f651977e
TC
1386C<chr(0xD801)>, so that all code points, not just those valid for open
1387interchange, are
6d4f9cf2 1388representable. Unicode does define semantics for them, such as their
a9130ea9
KW
1389C<L</General_Category>> is C<"Cs">. But because their use is somewhat dangerous,
1390Perl will warn (using the warning category C<"surrogate">, which is a
1391sub-category of C<"utf8">) if an attempt is made
6d4f9cf2
KW
1392to do things like take the lower case of one, or match
1393case-insensitively, or to output them. (But don't try this on Perls
1394before 5.14.)
c349b1b9 1395
c29a771d 1396=item *
5cb3728c 1397
1e54db1a 1398UTF-32, UTF-32BE, UTF-32LE
c349b1b9 1399
b65e6125 1400The UTF-32 family is pretty much like the UTF-16 family, except that
042da322 1401the units are 32-bit, and therefore the surrogate scheme is not
a9130ea9 1402needed. UTF-32 is a fixed-width encoding. The C<BOM> signatures are
b19eb496 1403C<0x00 0x00 0xFE 0xFF> for BE and C<0xFF 0xFE 0x00 0x00> for LE.
c349b1b9 1404
c29a771d 1405=item *
5cb3728c
RB
1406
1407UCS-2, UCS-4
c349b1b9 1408
b19eb496 1409Legacy, fixed-width encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
376d9008 1410encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
339cfa0e 1411because it does not use surrogates. UCS-4 is a 32-bit encoding,
b19eb496 1412functionally identical to UTF-32 (the difference being that
a9130ea9 1413UCS-4 forbids neither surrogates nor code points larger than C<0x10_FFFF>).
c349b1b9 1414
c29a771d 1415=item *
5cb3728c
RB
1416
1417UTF-7
c349b1b9 1418
376d9008
JB
1419A seven-bit safe (non-eight-bit) encoding, which is useful if the
1420transport or storage is not eight-bit safe. Defined by RFC 2152.
c349b1b9 1421
95a1a48b
JH
1422=back
1423
57e88091 1424=head2 Noncharacter code points
6d4f9cf2 1425
57e88091 142666 code points are set aside in Unicode as "noncharacter code points".
a9130ea9 1427These all have the C<Unassigned> (C<Cn>) C<L</General_Category>>, and
57e88091
KW
1428no character will ever be assigned to any of them. They are the 32 code
1429points between C<U+FDD0> and C<U+FDEF> inclusive, and the 34 code
1430points:
1431
1432 U+FFFE U+FFFF
1433 U+1FFFE U+1FFFF
1434 U+2FFFE U+2FFFF
1435 ...
1436 U+EFFFE U+EFFFF
1437 U+FFFFE U+FFFFF
1438 U+10FFFE U+10FFFF
1439
1440Until Unicode 7.0, the noncharacters were "B<forbidden> for use in open
1441interchange of Unicode text data", so that code that processed those
1442streams could use these code points as sentinels that could be mixed in
1443with character data, and would always be distinguishable from that data.
1444(Emphasis above and in the next paragraph are added in this document.)
1445
1446Unicode 7.0 changed the wording so that they are "B<not recommended> for
1447use in open interchange of Unicode text data". The 7.0 Standard goes on
1448to say:
1449
1450=over 4
1451
1452"If a noncharacter is received in open interchange, an application is
1453not required to interpret it in any way. It is good practice, however,
1454to recognize it as a noncharacter and to take appropriate action, such
1455as replacing it with C<U+FFFD> replacement character, to indicate the
1456problem in the text. It is not recommended to simply delete
1457noncharacter code points from such text, because of the potential
1458security issues caused by deleting uninterpreted characters. (See
1459conformance clause C7 in Section 3.2, Conformance Requirements, and
1460L<Unicode Technical Report #36, "Unicode Security
1461Considerations"|http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
1462
1463=back
1464
1465This change was made because it was found that various commercial tools
1466like editors, or for things like source code control, had been written
1467so that they would not handle program files that used these code points,
1468effectively precluding their use almost entirely! And that was never
1469the intent. They've always been meant to be usable within an
1470application, or cooperating set of applications, at will.
1471
1472If you're writing code, such as an editor, that is supposed to be able
1473to handle any Unicode text data, then you shouldn't be using these code
1474points yourself, and instead allow them in the input. If you need
1475sentinels, they should instead be something that isn't legal Unicode.
1476For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
1477they never appear in well-formed UTF-8. (There are equivalents for
1478UTF-EBCDIC). You can also store your Unicode code points in integer
1479variables and use negative values as sentinels.
1480
1481If you're not writing such a tool, then whether you accept noncharacters
1482as input is up to you (though the Standard recommends that you not). If
1483you do strict input stream checking with Perl, these code points
1484continue to be forbidden. This is to maintain backward compatibility
1485(otherwise potential security holes could open up, as an unsuspecting
1486application that was written assuming the noncharacters would be
1487filtered out before getting to it, could now, without warning, start
1488getting them). To do strict checking, you can use the layer
1489C<:encoding('UTF-8')>.
1490
1491Perl continues to warn (using the warning category C<"nonchar">, which
1492is a sub-category of C<"utf8">) if an attempt is made to output
1493noncharacters.
42581d5d
KW
1494
1495=head2 Beyond Unicode code points
1496
a9130ea9
KW
1497The maximum Unicode code point is C<U+10FFFF>, and Unicode only defines
1498operations on code points up through that. But Perl works on code
42581d5d
KW
1499points up to the maximum permissible unsigned number available on the
1500platform. However, Perl will not accept these from input streams unless
1501lax rules are being used, and will warn (using the warning category
2d88a86a
KW
1502C<"non_unicode">, which is a sub-category of C<"utf8">) if any are output.
1503
1504Since Unicode rules are not defined on these code points, if a
1505Unicode-defined operation is done on them, Perl uses what we believe are
1506sensible rules, while generally warning, using the C<"non_unicode">
1507category. For example, C<uc("\x{11_0000}")> will generate such a
1508warning, returning the input parameter as its result, since Perl defines
1509the uppercase of every non-Unicode code point to be the code point
b65e6125
KW
1510itself. (All the case changing operations, not just uppercasing, work
1511this way.)
2d88a86a
KW
1512
1513The situation with matching Unicode properties in regular expressions,
1514the C<\p{}> and C<\P{}> constructs, against these code points is not as
1515clear cut, and how these are handled has changed as we've gained
1516experience.
1517
1518One possibility is to treat any match against these code points as
1519undefined. But since Perl doesn't have the concept of a match being
1520undefined, it converts this to failing or C<FALSE>. This is almost, but
1521not quite, what Perl did from v5.14 (when use of these code points
1522became generally reliable) through v5.18. The difference is that Perl
1523treated all C<\p{}> matches as failing, but all C<\P{}> matches as
1524succeeding.
1525
1526One problem with this is that it leads to unexpected, and confusting
1527results in some cases:
1528
1529 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Failed on <= v5.18
1530 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Failed! on <= v5.18
1531
1532That is, it treated both matches as undefined, and converted that to
1533false (raising a warning on each). The first case is the expected
1534result, but the second is likely counterintuitive: "How could both be
1535false when they are complements?" Another problem was that the
1536implementation optimized many Unicode property matches down to already
1537existing simpler, faster operations, which don't raise the warning. We
1538chose to not forgo those optimizations, which help the vast majority of
1539matches, just to generate a warning for the unlikely event that an
1540above-Unicode code point is being matched against.
1541
1542As a result of these problems, starting in v5.20, what Perl does is
1543to treat non-Unicode code points as just typical unassigned Unicode
1544characters, and matches accordingly. (Note: Unicode has atypical
57e88091 1545unassigned code points. For example, it has noncharacter code points,
2d88a86a
KW
1546and ones that, when they do get assigned, are destined to be written
1547Right-to-left, as Arabic and Hebrew are. Perl assumes that no
1548non-Unicode code point has any atypical properties.)
1549
1550Perl, in most cases, will raise a warning when matching an above-Unicode
1551code point against a Unicode property when the result is C<TRUE> for
1552C<\p{}>, and C<FALSE> for C<\P{}>. For example:
1553
1554 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails, no warning
1555 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Succeeds, with warning
1556
1557In both these examples, the character being matched is non-Unicode, so
1558Unicode doesn't define how it should match. It clearly isn't an ASCII
1559hex digit, so the first example clearly should fail, and so it does,
1560with no warning. But it is arguable that the second example should have
1561an undefined, hence C<FALSE>, result. So a warning is raised for it.
1562
1563Thus the warning is raised for many fewer cases than in earlier Perls,
1564and only when what the result is could be arguable. It turns out that
1565none of the optimizations made by Perl (or are ever likely to be made)
1566cause the warning to be skipped, so it solves both problems of Perl's
1567earlier approach. The most commonly used property that is affected by
1568this change is C<\p{Unassigned}> which is a short form for
1569C<\p{General_Category=Unassigned}>. Starting in v5.20, all non-Unicode
1570code points are considered C<Unassigned>. In earlier releases the
1571matches failed because the result was considered undefined.
1572
1573The only place where the warning is not raised when it might ought to
1574have been is if optimizations cause the whole pattern match to not even
1575be attempted. For example, Perl may figure out that for a string to
1576match a certain regular expression pattern, the string has to contain
1577the substring C<"foobar">. Before attempting the match, Perl may look
1578for that substring, and if not found, immediately fail the match without
1579actually trying it; so no warning gets generated even if the string
1580contains an above-Unicode code point.
1581
1582This behavior is more "Do what I mean" than in earlier Perls for most
1583applications. But it catches fewer issues for code that needs to be
1584strictly Unicode compliant. Therefore there is an additional mode of
1585operation available to accommodate such code. This mode is enabled if a
1586regular expression pattern is compiled within the lexical scope where
1587the C<"non_unicode"> warning class has been made fatal, say by:
1588
1589 use warnings FATAL => "non_unicode"
1590
44ecbbd8 1591(see L<warnings>). In this mode of operation, Perl will raise the
2d88a86a
KW
1592warning for all matches against a non-Unicode code point (not just the
1593arguable ones), and it skips the optimizations that might cause the
1594warning to not be output. (It currently still won't warn if the match
1595isn't even attempted, like in the C<"foobar"> example above.)
1596
1597In summary, Perl now normally treats non-Unicode code points as typical
1598Unicode unassigned code points for regular expression matches, raising a
1599warning only when it is arguable what the result should be. However, if
1600this warning has been made fatal, it isn't skipped.
1601
1602There is one exception to all this. C<\p{All}> looks like a Unicode
1603property, but it is a Perl extension that is defined to be true for all
1604possible code points, Unicode or not, so no warning is ever generated
1605when matching this against a non-Unicode code point. (Prior to v5.20,
1606it was an exact synonym for C<\p{Any}>, matching code points C<0>
1607through C<0x10FFFF>.)
6d4f9cf2 1608
0d7c09bb
JH
1609=head2 Security Implications of Unicode
1610
b65e6125
KW
1611First, read
1612L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
1613
e1b711da
KW
1614Also, note the following:
1615
0d7c09bb
JH
1616=over 4
1617
1618=item *
1619
1620Malformed UTF-8
bf0fa0b2 1621
42581d5d 1622Unfortunately, the original specification of UTF-8 leaves some room for
bf0fa0b2 1623interpretation of how many bytes of encoded output one should generate
376d9008
JB
1624from one input Unicode character. Strictly speaking, the shortest
1625possible sequence of UTF-8 bytes should be generated,
1626because otherwise there is potential for an input buffer overflow at
feda178f 1627the receiving end of a UTF-8 connection. Perl always generates the
e1b711da 1628shortest length UTF-8, and with warnings on, Perl will warn about
376d9008 1629non-shortest length UTF-8 along with other malformations, such as the
b19eb496 1630surrogates, which are not Unicode code points valid for interchange.
bf0fa0b2 1631
0d7c09bb
JH
1632=item *
1633
68693f9e 1634Regular expression pattern matching may surprise you if you're not
b19eb496
TC
1635accustomed to Unicode. Starting in Perl 5.14, several pattern
1636modifiers are available to control this, called the character set
42581d5d
KW
1637modifiers. Details are given in L<perlre/Character set modifiers>.
1638
1639=back
0d7c09bb 1640
376d9008 1641As discussed elsewhere, Perl has one foot (two hooves?) planted in
a6a7eedc
KW
1642each of two worlds: the old world of ASCII and single-byte locales, and
1643the new world of Unicode, upgrading when necessary.
376d9008 1644If your legacy code does not explicitly use Unicode, no automatic
a6a7eedc 1645switch-over to Unicode should happen.
0d7c09bb 1646
c349b1b9
JH
1647=head2 Unicode in Perl on EBCDIC
1648
a6a7eedc
KW
1649Unicode is supported on EBCDIC platforms. See L<perlebcdic>.
1650
1651Unless ASCII vs. EBCDIC issues are specifically being discussed,
1652references to UTF-8 encoding in this document and elsewhere should be
1653read as meaning UTF-EBCDIC on EBCDIC platforms.
1654See L<perlebcdic/Unicode and UTF>.
1655
1656Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
1657hidden from you; S<C<use utf8>> (and NOT something like
1658S<C<use utfebcdic>>) declares the the script is in the platform's
1659"native" 8-bit encoding of Unicode. (Similarly for the C<":utf8">
1660layer.)
c349b1b9 1661
b310b053
JH
1662=head2 Locales
1663
42581d5d 1664See L<perllocale/Unicode and UTF-8>
b310b053 1665
1aad1664
JH
1666=head2 When Unicode Does Not Happen
1667
b65e6125
KW
1668There are still many places where Unicode (in some encoding or
1669another) could be given as arguments or received as results, or both in
1670Perl, but it is not, in spite of Perl having extensive ways to input and
1671output in Unicode, and a few other "entry points" like the C<@ARGV>
1672array (which can sometimes be interpreted as UTF-8).
1aad1664 1673
e1b711da
KW
1674The following are such interfaces. Also, see L</The "Unicode Bug">.
1675For all of these interfaces Perl
b9cedb1b 1676currently (as of v5.16.0) simply assumes byte strings both as arguments
b65e6125 1677and results, or UTF-8 strings if the (deprecated) C<encoding> pragma has been used.
1aad1664 1678
b19eb496
TC
1679One reason that Perl does not attempt to resolve the role of Unicode in
1680these situations is that the answers are highly dependent on the operating
1aad1664 1681system and the file system(s). For example, whether filenames can be
b19eb496
TC
1682in Unicode and in exactly what kind of encoding, is not exactly a
1683portable concept. Similarly for C<qx> and C<system>: how well will the
1684"command-line interface" (and which of them?) handle Unicode?
1aad1664
JH
1685
1686=over 4
1687
557a2462
RB
1688=item *
1689
a9130ea9
KW
1690C<chdir>, C<chmod>, C<chown>, C<chroot>, C<exec>, C<link>, C<lstat>, C<mkdir>,
1691C<rename>, C<rmdir>, C<stat>, C<symlink>, C<truncate>, C<unlink>, C<utime>, C<-X>
557a2462
RB
1692
1693=item *
1694
a9130ea9 1695C<%ENV>
557a2462
RB
1696
1697=item *
1698
a9130ea9 1699C<glob> (aka the C<E<lt>*E<gt>>)
557a2462
RB
1700
1701=item *
1aad1664 1702
a9130ea9 1703C<open>, C<opendir>, C<sysopen>
1aad1664 1704
557a2462 1705=item *
1aad1664 1706
a9130ea9 1707C<qx> (aka the backtick operator), C<system>
1aad1664 1708
557a2462 1709=item *
1aad1664 1710
a9130ea9 1711C<readdir>, C<readlink>
1aad1664
JH
1712
1713=back
1714
e1b711da
KW
1715=head2 The "Unicode Bug"
1716
a6a7eedc
KW
1717The term, "Unicode bug" has been applied to an inconsistency with the
1718code points in the C<Latin-1 Supplement> block, that is, between
1719128 and 255. Without a locale specified, unlike all other characters or
1720code points, these characters can have very different semantics
1721depending on the rules in effect. (Characters whose code points are
1722above 255 force Unicode rules; whereas the rules for ASCII characters
1723are the same under both ASCII and Unicode rules.)
1724
1725Under Unicode rules, these upper-Latin1 characters are interpreted as
1726Unicode code points, which means they have the same semantics as Latin-1
1727(ISO-8859-1) and C1 controls.
1728
1729As explained in L</ASCII Rules versus Unicode Rules>, under ASCII rules,
1730they are considered to be unassigned characters.
1731
1732This can lead to unexpected results. For example, a string's
1733semantics can suddenly change if a code point above 255 is appended to
1734it, which changes the rules from ASCII to Unicode. As an
1735example, consider the following program and its output:
1736
1737 $ perl -le'
1738 no feature 'unicode_strings';
1739 $s1 = "\xC2";
1740 $s2 = "\x{2660}";
1741 for ($s1, $s2, $s1.$s2) {
1742 print /\w/ || 0;
1743 }
1744 '
1745 0
1746 0
1747 1
1748
1749If there's no C<\w> in C<s1> nor in C<s2>, why does their concatenation
1750have one?
1751
1752This anomaly stems from Perl's attempt to not disturb older programs that
1753didn't use Unicode, along with Perl's desire to add Unicode support
1754seamlessly. But the result turned out to not be seamless. (By the way,
1755you can choose to be warned when things like this happen. See
1756C<L<encoding::warnings>>.)
1757
1758L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
1759was added, starting in Perl v5.12, to address this problem. It affects
1760these things:
e1b711da
KW
1761
1762=over 4
1763
1764=item *
1765
1766Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
2e2b2571
KW
1767and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in double-quotish
1768contexts, such as regular expression substitutions.
a6a7eedc
KW
1769
1770Under C<unicode_strings> starting in Perl 5.12.0, Unicode rules are
2e2b2571
KW
1771generally used. See L<perlfunc/lc> for details on how this works
1772in combination with various other pragmas.
e1b711da
KW
1773
1774=item *
1775
2e2b2571 1776Using caseless (C</i>) regular expression matching.
a6a7eedc 1777
2e2b2571 1778Starting in Perl 5.14.0, regular expressions compiled within
a6a7eedc 1779the scope of C<unicode_strings> use Unicode rules
2e2b2571
KW
1780even when executed or compiled into larger
1781regular expressions outside the scope.
e1b711da
KW
1782
1783=item *
1784
a6a7eedc
KW
1785Matching any of several properties in regular expressions.
1786
1787These properties are C<\b> (without braces), C<\B> (without braces),
1788C<\s>, C<\S>, C<\w>, C<\W>, and all the Posix character classes
630d17dc 1789I<except> C<[[:ascii:]]>.
a6a7eedc 1790
2e2b2571 1791Starting in Perl 5.14.0, regular expressions compiled within
a6a7eedc 1792the scope of C<unicode_strings> use Unicode rules
2e2b2571
KW
1793even when executed or compiled into larger
1794regular expressions outside the scope.
e1b711da
KW
1795
1796=item *
1797
a6a7eedc
KW
1798In C<quotemeta> or its inline equivalent C<\Q>.
1799
2e2b2571
KW
1800Starting in Perl 5.16.0, consistent quoting rules are used within the
1801scope of C<unicode_strings>, as described in L<perlfunc/quotemeta>.
a6a7eedc
KW
1802Prior to that, or outside its scope, no code points above 127 are quoted
1803in UTF-8 encoded strings, but in byte encoded strings, code points
1804between 128-255 are always quoted.
eb88ed9e 1805
e1b711da
KW
1806=back
1807
a6a7eedc
KW
1808You can see from the above that the effect of C<unicode_strings>
1809increased over several Perl releases. (And Perl's support for Unicode
1810continues to improve; it's best to use the latest available release in
1811order to get the most complete and accurate results possible.) Note that
1812C<unicode_strings> is automatically chosen if you S<C<use 5.012>> or
1813higher.
e1b711da 1814
2e2b2571 1815For Perls earlier than those described above, or when a string is passed
a6a7eedc 1816to a function outside the scope of C<unicode_strings>, see the next section.
e1b711da 1817
1aad1664
JH
1818=head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
1819
e1b711da
KW
1820Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
1821there are situations where you simply need to force a byte
a6a7eedc
KW
1822string into UTF-8, or vice versa. The standard module L<Encode> can be
1823used for this, or the low-level calls
a9130ea9 1824L<C<utf8::upgrade($bytestring)>|utf8/Utility functions> and
a6a7eedc 1825L<C<utf8::downgrade($utf8string[, FAIL_OK])>|utf8/Utility functions>.
1aad1664 1826
a9130ea9 1827Note that C<utf8::downgrade()> can fail if the string contains characters
2bbc8d55 1828that don't fit into a byte.
1aad1664 1829
e1b711da
KW
1830Calling either function on a string that already is in the desired state is a
1831no-op.
1832
a6a7eedc
KW
1833L</ASCII Rules versus Unicode Rules> gives all the ways that a string is
1834made to use Unicode rules.
95a1a48b 1835
37b3b608 1836=head2 Using Unicode in XS
c349b1b9 1837
37b3b608
KW
1838See L<perlguts/"Unicode Support"> for an introduction to Unicode at
1839the XS level, and L<perlapi/Unicode Support> for the API details.
95a1a48b 1840
e1b711da
KW
1841=head2 Hacking Perl to work on earlier Unicode versions (for very serious hackers only)
1842
a6a7eedc
KW
1843Perl by default comes with the latest supported Unicode version built-in, but
1844the goal is to allow you to change to use any earlier one. In Perls
1845v5.20 and v5.22, however, the earliest usable version is Unicode 5.1.
c55dd03d 1846Perl v5.18 and v5.24 are able to handle all earlier versions.
e1b711da 1847
42581d5d 1848Download the files in the desired version of Unicode from the Unicode web
e1b711da 1849site L<http://www.unicode.org>). These should replace the existing files in
b19eb496 1850F<lib/unicore> in the Perl source tree. Follow the instructions in
116693e8 1851F<README.perl> in that directory to change some of their names, and then build
26e391dd 1852perl (see L<INSTALL>).
116693e8 1853
c8d992ba
A
1854=head2 Porting code from perl-5.6.X
1855
a6a7eedc
KW
1856Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6 the
1857programmer was required to use the C<utf8> pragma to declare that a
1858given scope expected to deal with Unicode data and had to make sure that
1859only Unicode data were reaching that scope. If you have code that is
c8d992ba 1860working with 5.6, you will need some of the following adjustments to
a6a7eedc
KW
1861your code. The examples are written such that the code will continue to
1862work under 5.6, so you should be safe to try them out.
c8d992ba 1863
755789c0 1864=over 3
c8d992ba
A
1865
1866=item *
1867
1868A filehandle that should read or write UTF-8
1869
b9cedb1b 1870 if ($] > 5.008) {
740d4bb2 1871 binmode $fh, ":encoding(utf8)";
c8d992ba
A
1872 }
1873
1874=item *
1875
1876A scalar that is going to be passed to some extension
1877
a9130ea9 1878Be it C<Compress::Zlib>, C<Apache::Request> or any extension that has no
c8d992ba 1879mention of Unicode in the manpage, you need to make sure that the
2575c402 1880UTF8 flag is stripped off. Note that at the time of this writing
b9cedb1b 1881(January 2012) the mentioned modules are not UTF-8-aware. Please
c8d992ba
A
1882check the documentation to verify if this is still true.
1883
b9cedb1b 1884 if ($] > 5.008) {
c8d992ba
A
1885 require Encode;
1886 $val = Encode::encode_utf8($val); # make octets
1887 }
1888
1889=item *
1890
1891A scalar we got back from an extension
1892
1893If you believe the scalar comes back as UTF-8, you will most likely
2575c402 1894want the UTF8 flag restored:
c8d992ba 1895
b9cedb1b 1896 if ($] > 5.008) {
c8d992ba
A
1897 require Encode;
1898 $val = Encode::decode_utf8($val);
1899 }
1900
1901=item *
1902
1903Same thing, if you are really sure it is UTF-8
1904
b9cedb1b 1905 if ($] > 5.008) {
c8d992ba
A
1906 require Encode;
1907 Encode::_utf8_on($val);
1908 }
1909
1910=item *
1911
a9130ea9 1912A wrapper for L<DBI> C<fetchrow_array> and C<fetchrow_hashref>
c8d992ba
A
1913
1914When the database contains only UTF-8, a wrapper function or method is
a9130ea9
KW
1915a convenient way to replace all your C<fetchrow_array> and
1916C<fetchrow_hashref> calls. A wrapper function will also make it easier to
c8d992ba 1917adapt to future enhancements in your database driver. Note that at the
b9cedb1b 1918time of this writing (January 2012), the DBI has no standardized way
a9130ea9 1919to deal with UTF-8 data. Please check the L<DBI documentation|DBI> to verify if
c8d992ba
A
1920that is still true.
1921
1922 sub fetchrow {
d88362ca
KW
1923 # $what is one of fetchrow_{array,hashref}
1924 my($self, $sth, $what) = @_;
b9cedb1b 1925 if ($] < 5.008) {
c8d992ba
A
1926 return $sth->$what;
1927 } else {
1928 require Encode;
1929 if (wantarray) {
1930 my @arr = $sth->$what;
1931 for (@arr) {
1932 defined && /[^\000-\177]/ && Encode::_utf8_on($_);
1933 }
1934 return @arr;
1935 } else {
1936 my $ret = $sth->$what;
1937 if (ref $ret) {
1938 for my $k (keys %$ret) {
d88362ca
KW
1939 defined
1940 && /[^\000-\177]/
1941 && Encode::_utf8_on($_) for $ret->{$k};
c8d992ba
A
1942 }
1943 return $ret;
1944 } else {
1945 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
1946 return $ret;
1947 }
1948 }
1949 }
1950 }
1951
1952
1953=item *
1954
1955A large scalar that you know can only contain ASCII
1956
1957Scalars that contain only ASCII and are marked as UTF-8 are sometimes
1958a drag to your program. If you recognize such a situation, just remove
2575c402 1959the UTF8 flag:
c8d992ba 1960
b9cedb1b 1961 utf8::downgrade($val) if $] > 5.008;
c8d992ba
A
1962
1963=back
1964
a6a7eedc
KW
1965=head1 BUGS
1966
1967See also L</The "Unicode Bug"> above.
1968
1969=head2 Interaction with Extensions
1970
1971When Perl exchanges data with an extension, the extension should be
1972able to understand the UTF8 flag and act accordingly. If the
1973extension doesn't recognize that flag, it's likely that the extension
1974will return incorrectly-flagged data.
1975
1976So if you're working with Unicode data, consult the documentation of
1977every module you're using if there are any issues with Unicode data
1978exchange. If the documentation does not talk about Unicode at all,
1979suspect the worst and probably look at the source to learn how the
1980module is implemented. Modules written completely in Perl shouldn't
1981cause problems. Modules that directly or indirectly access code written
1982in other programming languages are at risk.
1983
1984For affected functions, the simple strategy to avoid data corruption is
1985to always make the encoding of the exchanged data explicit. Choose an
1986encoding that you know the extension can handle. Convert arguments passed
1987to the extensions to that encoding and convert results back from that
1988encoding. Write wrapper functions that do the conversions for you, so
1989you can later change the functions when the extension catches up.
1990
1991To provide an example, let's say the popular C<Foo::Bar::escape_html>
1992function doesn't deal with Unicode data yet. The wrapper function
1993would convert the argument to raw UTF-8 and convert the result back to
1994Perl's internal representation like so:
1995
1996 sub my_escape_html ($) {
1997 my($what) = shift;
1998 return unless defined $what;
1999 Encode::decode_utf8(Foo::Bar::escape_html(
2000 Encode::encode_utf8($what)));
2001 }
2002
2003Sometimes, when the extension does not convert data but just stores
2004and retrieves it, you will be able to use the otherwise
2005dangerous L<C<Encode::_utf8_on()>|Encode/_utf8_on> function. Let's say
2006the popular C<Foo::Bar> extension, written in C, provides a C<param>
2007method that lets you store and retrieve data according to these prototypes:
2008
2009 $self->param($name, $value); # set a scalar
2010 $value = $self->param($name); # retrieve a scalar
2011
2012If it does not yet provide support for any encoding, one could write a
2013derived class with such a C<param> method:
2014
2015 sub param {
2016 my($self,$name,$value) = @_;
2017 utf8::upgrade($name); # make sure it is UTF-8 encoded
2018 if (defined $value) {
2019 utf8::upgrade($value); # make sure it is UTF-8 encoded
2020 return $self->SUPER::param($name,$value);
2021 } else {
2022 my $ret = $self->SUPER::param($name);
2023 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
2024 return $ret;
2025 }
2026 }
2027
2028Some extensions provide filters on data entry/exit points, such as
2029C<DB_File::filter_store_key> and family. Look out for such filters in
2030the documentation of your extensions; they can make the transition to
2031Unicode data much easier.
2032
2033=head2 Speed
2034
2035Some functions are slower when working on UTF-8 encoded strings than
2036on byte encoded strings. All functions that need to hop over
2037characters such as C<length()>, C<substr()> or C<index()>, or matching
2038regular expressions can work B<much> faster when the underlying data are
2039byte-encoded.
2040
2041In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
2042a caching scheme was introduced which improved the situation. In general,
2043operations with UTF-8 encoded strings are still slower. As an example,
2044the Unicode properties (character classes) like C<\p{Nd}> are known to
2045be quite a bit slower (5-20 times) than their simpler counterparts
2046like C<[0-9]> (then again, there are hundreds of Unicode characters matching
2047C<Nd> compared with the 10 ASCII characters matching C<[0-9]>).
2048
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GS
2049=head1 SEE ALSO
2050
51f494cc 2051L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>,
b65e6125 2052L<perlretut>, L<perlvar/"${^UNICODE}">,
51f494cc 2053L<http://www.unicode.org/reports/tr44>).
393fec97
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2054
2055=cut