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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
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78However, if a Perl script begins with the Unicode C<BOM> (UTF-16LE,
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
KW
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
51f494cc
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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
KW
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
KW
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
KW
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
KW
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
KW
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
KW
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
KW
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
KW
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
KW
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
TC
1071The following list of Unicode supported features for regular expressions describes
1072all features currently directly supported by core Perl. The references to "Level N"
8158862b 1073and the section numbers refer to the Unicode Technical Standard #18,
b19eb496 1074"Unicode Regular Expressions", version 13, from August 2008.
776f8809
JH
1075
1076=over 4
1077
1078=item *
1079
1080Level 1 - Basic Unicode Support
1081
755789c0
KW
1082 RL1.1 Hex Notation - done [1]
1083 RL1.2 Properties - done [2][3]
1084 RL1.2a Compatibility Properties - done [4]
9d1a5160 1085 RL1.3 Subtraction and Intersection - experimental [5]
755789c0
KW
1086 RL1.4 Simple Word Boundaries - done [6]
1087 RL1.5 Simple Loose Matches - done [7]
1088 RL1.6 Line Boundaries - MISSING [8][9]
1089 RL1.7 Supplementary Code Points - done [10]
1090
6f33e417
KW
1091=over 4
1092
a6a7eedc 1093=item [1] C<\N{U+...}> and C<\x{...}>
6f33e417 1094
a6a7eedc 1095=item [2] C<\p{...}> C<\P{...}>
6f33e417 1096
a6a7eedc
KW
1097=item [3] supports not only minimal list, but all Unicode character
1098properties (see Unicode Character Properties above)
6f33e417 1099
a6a7eedc
KW
1100=item [4] C<\d> C<\D> C<\s> C<\S> C<\w> C<\W> C<\X> C<[:I<prop>:]>
1101C<[:^I<prop>:]>
6f33e417 1102
a6a7eedc
KW
1103=item [5] The experimental feature starting in v5.18 C<"(?[...])"> accomplishes
1104this.
6f33e417 1105
a6a7eedc
KW
1106See L<perlre/(?[ ])>. If you don't want to use an experimental
1107feature, you can use one of the following:
6f33e417
KW
1108
1109=over 4
1110
a6a7eedc
KW
1111=item *
1112
f67a5002 1113Regular expression lookahead
6f33e417
KW
1114
1115You can mimic class subtraction using lookahead.
8158862b 1116For example, what UTS#18 might write as
29bdacb8 1117
209c9685 1118 [{Block=Greek}-[{UNASSIGNED}]]
dbe420b4
JH
1119
1120in Perl can be written as:
1121
209c9685
KW
1122 (?!\p{Unassigned})\p{Block=Greek}
1123 (?=\p{Assigned})\p{Block=Greek}
dbe420b4
JH
1124
1125But in this particular example, you probably really want
1126
209c9685 1127 \p{Greek}
dbe420b4
JH
1128
1129which will match assigned characters known to be part of the Greek script.
29bdacb8 1130
a6a7eedc
KW
1131=item *
1132
1133CPAN module C<L<Unicode::Regex::Set>>
8158862b 1134
6f33e417
KW
1135It does implement the full UTS#18 grouping, intersection, union, and
1136removal (subtraction) syntax.
8158862b 1137
a6a7eedc
KW
1138=item *
1139
1140L</"User-Defined Character Properties">
6f33e417 1141
a9130ea9 1142C<"+"> for union, C<"-"> for removal (set-difference), C<"&"> for intersection
6f33e417
KW
1143
1144=back
1145
a6a7eedc 1146=item [6] C<\b> C<\B>
6f33e417
KW
1147
1148=item [7]
a6a7eedc 1149Note that Perl does Full case-folding in matching, not Simple:
6f33e417 1150
a6a7eedc
KW
1151For example C<U+1F88> is equivalent to C<U+1F00 U+03B9>, instead of just
1152C<U+1F80>. This difference matters mainly for certain Greek capital
a9130ea9
KW
1153letters with certain modifiers: the Full case-folding decomposes the
1154letter, while the Simple case-folding would map it to a single
1155character.
6f33e417
KW
1156
1157=item [8]
a6a7eedc
KW
1158Perl treats C<\n> as the start- and end-line delimiter. Unicode
1159specifies more characters that should be so-interpreted.
6f33e417 1160
a6a7eedc 1161These are:
6f33e417 1162
a6a7eedc
KW
1163 VT U+000B (\v in C)
1164 FF U+000C (\f)
1165 CR U+000D (\r)
1166 NEL U+0085
1167 LS U+2028
1168 PS U+2029
6f33e417 1169
a6a7eedc
KW
1170C<^> and C<$> in regular expression patterns are supposed to match all
1171these, but don't.
1172These characters also don't, but should, affect C<< <> >> C<$.>, and
1173script line numbers.
6f33e417 1174
a6a7eedc
KW
1175Also, lines should not be split within C<CRLF> (i.e. there is no
1176empty line between C<\r> and C<\n>). For C<CRLF>, try the C<:crlf>
1177layer (see L<PerlIO>).
1178
6b659339 1179=item [9] But C<qr/\b{lb}/> and C<L<Unicode::LineBreak>> are available.
6f33e417 1180
6b659339
KW
1181L<C<qrE<sol>\b{lb}E<sol>>|perlrebackslash/\b{lb}> supplies default line
1182breaking conformant with
a6a7eedc
KW
1183L<UAX#14 "Unicode Line Breaking Algorithm"|http://www.unicode.org/reports/tr14>.
1184
6b659339
KW
1185And, the module C<L<Unicode::LineBreak>> also conformant with UAX#14,
1186provides customizable line breaking.
1187
a6a7eedc 1188=item [10]
a9130ea9
KW
1189UTF-8/UTF-EBDDIC used in Perl allows not only C<U+10000> to
1190C<U+10FFFF> but also beyond C<U+10FFFF>
6f33e417
KW
1191
1192=back
5ca1ac52 1193
776f8809
JH
1194=item *
1195
1196Level 2 - Extended Unicode Support
1197
755789c0
KW
1198 RL2.1 Canonical Equivalents - MISSING [10][11]
1199 RL2.2 Default Grapheme Clusters - MISSING [12]
ae3bb8ea 1200 RL2.3 Default Word Boundaries - DONE [14]
755789c0
KW
1201 RL2.4 Default Loose Matches - MISSING [15]
1202 RL2.5 Name Properties - DONE
1203 RL2.6 Wildcard Properties - MISSING
8158862b 1204
755789c0
KW
1205 [10] see UAX#15 "Unicode Normalization Forms"
1206 [11] have Unicode::Normalize but not integrated to regexes
64935bc6
KW
1207 [12] have \X and \b{gcb} but we don't have a "Grapheme Cluster
1208 Mode"
755789c0 1209 [14] see UAX#29, Word Boundaries
902b08d0 1210 [15] This is covered in Chapter 3.13 (in Unicode 6.0)
776f8809
JH
1211
1212=item *
1213
8158862b
ST
1214Level 3 - Tailored Support
1215
755789c0
KW
1216 RL3.1 Tailored Punctuation - MISSING
1217 RL3.2 Tailored Grapheme Clusters - MISSING [17][18]
1218 RL3.3 Tailored Word Boundaries - MISSING
1219 RL3.4 Tailored Loose Matches - MISSING
1220 RL3.5 Tailored Ranges - MISSING
1221 RL3.6 Context Matching - MISSING [19]
1222 RL3.7 Incremental Matches - MISSING
8158862b 1223 ( RL3.8 Unicode Set Sharing )
755789c0
KW
1224 RL3.9 Possible Match Sets - MISSING
1225 RL3.10 Folded Matching - MISSING [20]
1226 RL3.11 Submatchers - MISSING
1227
1228 [17] see UAX#10 "Unicode Collation Algorithms"
1229 [18] have Unicode::Collate but not integrated to regexes
f67a5002 1230 [19] have (?<=x) and (?=x), but lookaheads or lookbehinds
755789c0
KW
1231 should see outside of the target substring
1232 [20] need insensitive matching for linguistic features other
1233 than case; for example, hiragana to katakana, wide and
1234 narrow, simplified Han to traditional Han (see UTR#30
1235 "Character Foldings")
776f8809
JH
1236
1237=back
1238
c349b1b9
JH
1239=head2 Unicode Encodings
1240
376d9008
JB
1241Unicode characters are assigned to I<code points>, which are abstract
1242numbers. To use these numbers, various encodings are needed.
c349b1b9
JH
1243
1244=over 4
1245
c29a771d 1246=item *
5cb3728c
RB
1247
1248UTF-8
c349b1b9 1249
6d4f9cf2 1250UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
a6a7eedc
KW
1251encoding. In most of Perl's documentation, including elsewhere in this
1252document, the term "UTF-8" means also "UTF-EBCDIC". But in this section,
1253"UTF-8" refers only to the encoding used on ASCII platforms. It is a
1254superset of 7-bit US-ASCII, so anything encoded in ASCII has the
1255identical representation when encoded in UTF-8.
c349b1b9 1256
8c007b5a 1257The following table is from Unicode 3.2.
05632f9a 1258
755789c0 1259 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
05632f9a 1260
d88362ca 1261 U+0000..U+007F 00..7F
e1b711da 1262 U+0080..U+07FF * C2..DF 80..BF
d88362ca 1263 U+0800..U+0FFF E0 * A0..BF 80..BF
ec90690f
ST
1264 U+1000..U+CFFF E1..EC 80..BF 80..BF
1265 U+D000..U+D7FF ED 80..9F 80..BF
755789c0 1266 U+D800..U+DFFF +++++ utf16 surrogates, not legal utf8 +++++
ec90690f 1267 U+E000..U+FFFF EE..EF 80..BF 80..BF
d88362ca
KW
1268 U+10000..U+3FFFF F0 * 90..BF 80..BF 80..BF
1269 U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
1270 U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
e1b711da 1271
b19eb496 1272Note the gaps marked by "*" before several of the byte entries above. These are
e1b711da
KW
1273caused by legal UTF-8 avoiding non-shortest encodings: it is technically
1274possible to UTF-8-encode a single code point in different ways, but that is
1275explicitly forbidden, and the shortest possible encoding should always be used
1276(and that is what Perl does).
37361303 1277
376d9008 1278Another way to look at it is via bits:
05632f9a 1279
755789c0 1280 Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
05632f9a 1281
755789c0
KW
1282 0aaaaaaa 0aaaaaaa
1283 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
1284 ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
1285 00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
05632f9a 1286
a9130ea9 1287As you can see, the continuation bytes all begin with C<"10">, and the
e1b711da 1288leading bits of the start byte tell how many bytes there are in the
05632f9a
JH
1289encoded character.
1290
6d4f9cf2 1291The original UTF-8 specification allowed up to 6 bytes, to allow
a9130ea9 1292encoding of numbers up to C<0x7FFF_FFFF>. Perl continues to allow those,
6d4f9cf2
KW
1293and has extended that up to 13 bytes to encode code points up to what
1294can fit in a 64-bit word. However, Perl will warn if you output any of
b19eb496 1295these as being non-portable; and under strict UTF-8 input protocols,
760c7c2f
KW
1296they are forbidden. In addition, it is deprecated to use a code point
1297larger than what a signed integer variable on your system can hold. On
129832-bit ASCII systems, this means C<0x7FFF_FFFF> is the legal maximum
1299going forward (much higher on 64-bit systems).
6d4f9cf2 1300
c29a771d 1301=item *
5cb3728c
RB
1302
1303UTF-EBCDIC
dbe420b4 1304
b65e6125 1305Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
a6a7eedc
KW
1306This means that all the basic characters (which includes all
1307those that have ASCII equivalents (like C<"A">, C<"0">, C<"%">, I<etc.>)
1308are the same in both EBCDIC and UTF-EBCDIC.)
1309
c0236afe
KW
1310UTF-EBCDIC is used on EBCDIC platforms. It generally requires more
1311bytes to represent a given code point than UTF-8 does; the largest
1312Unicode code points take 5 bytes to represent (instead of 4 in UTF-8),
1313and, extended for 64-bit words, it uses 14 bytes instead of 13 bytes in
1314UTF-8.
dbe420b4 1315
c29a771d 1316=item *
5cb3728c 1317
b65e6125 1318UTF-16, UTF-16BE, UTF-16LE, Surrogates, and C<BOM>'s (Byte Order Marks)
c349b1b9 1319
1bfb14c4
JH
1320The followings items are mostly for reference and general Unicode
1321knowledge, Perl doesn't use these constructs internally.
dbe420b4 1322
b19eb496
TC
1323Like UTF-8, UTF-16 is a variable-width encoding, but where
1324UTF-8 uses 8-bit code units, UTF-16 uses 16-bit code units.
1325All code points occupy either 2 or 4 bytes in UTF-16: code points
1326C<U+0000..U+FFFF> are stored in a single 16-bit unit, and code
1bfb14c4 1327points C<U+10000..U+10FFFF> in two 16-bit units. The latter case is
c349b1b9
JH
1328using I<surrogates>, the first 16-bit unit being the I<high
1329surrogate>, and the second being the I<low surrogate>.
1330
376d9008 1331Surrogates are code points set aside to encode the C<U+10000..U+10FFFF>
c349b1b9 1332range of Unicode code points in pairs of 16-bit units. The I<high
9f815e24 1333surrogates> are the range C<U+D800..U+DBFF> and the I<low surrogates>
376d9008 1334are the range C<U+DC00..U+DFFF>. The surrogate encoding is
c349b1b9 1335
d88362ca
KW
1336 $hi = ($uni - 0x10000) / 0x400 + 0xD800;
1337 $lo = ($uni - 0x10000) % 0x400 + 0xDC00;
c349b1b9
JH
1338
1339and the decoding is
1340
d88362ca 1341 $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
c349b1b9 1342
376d9008 1343Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16
c349b1b9 1344itself can be used for in-memory computations, but if storage or
376d9008
JB
1345transfer is required either UTF-16BE (big-endian) or UTF-16LE
1346(little-endian) encodings must be chosen.
c349b1b9
JH
1347
1348This introduces another problem: what if you just know that your data
376d9008 1349is UTF-16, but you don't know which endianness? Byte Order Marks, or
b65e6125 1350C<BOM>'s, are a solution to this. A special character has been reserved
86bbd6d1 1351in Unicode to function as a byte order marker: the character with the
a9130ea9 1352code point C<U+FEFF> is the C<BOM>.
042da322 1353
a9130ea9 1354The trick is that if you read a C<BOM>, you will know the byte order,
376d9008
JB
1355since if it was written on a big-endian platform, you will read the
1356bytes C<0xFE 0xFF>, but if it was written on a little-endian platform,
1357you will read the bytes C<0xFF 0xFE>. (And if the originating platform
b65e6125
KW
1358was writing in ASCII platform UTF-8, you will read the bytes
1359C<0xEF 0xBB 0xBF>.)
042da322 1360
86bbd6d1 1361The way this trick works is that the character with the code point
6d4f9cf2 1362C<U+FFFE> is not supposed to be in input streams, so the
a9130ea9 1363sequence of bytes C<0xFF 0xFE> is unambiguously "C<BOM>, represented in
1bfb14c4 1364little-endian format" and cannot be C<U+FFFE>, represented in big-endian
6d4f9cf2
KW
1365format".
1366
1367Surrogates have no meaning in Unicode outside their use in pairs to
1368represent other code points. However, Perl allows them to be
1369represented individually internally, for example by saying
f651977e
TC
1370C<chr(0xD801)>, so that all code points, not just those valid for open
1371interchange, are
6d4f9cf2 1372representable. Unicode does define semantics for them, such as their
a9130ea9
KW
1373C<L</General_Category>> is C<"Cs">. But because their use is somewhat dangerous,
1374Perl will warn (using the warning category C<"surrogate">, which is a
1375sub-category of C<"utf8">) if an attempt is made
6d4f9cf2
KW
1376to do things like take the lower case of one, or match
1377case-insensitively, or to output them. (But don't try this on Perls
1378before 5.14.)
c349b1b9 1379
c29a771d 1380=item *
5cb3728c 1381
1e54db1a 1382UTF-32, UTF-32BE, UTF-32LE
c349b1b9 1383
b65e6125 1384The UTF-32 family is pretty much like the UTF-16 family, except that
042da322 1385the units are 32-bit, and therefore the surrogate scheme is not
a9130ea9 1386needed. UTF-32 is a fixed-width encoding. The C<BOM> signatures are
b19eb496 1387C<0x00 0x00 0xFE 0xFF> for BE and C<0xFF 0xFE 0x00 0x00> for LE.
c349b1b9 1388
c29a771d 1389=item *
5cb3728c
RB
1390
1391UCS-2, UCS-4
c349b1b9 1392
b19eb496 1393Legacy, fixed-width encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit
376d9008 1394encoding. Unlike UTF-16, UCS-2 is not extensible beyond C<U+FFFF>,
339cfa0e 1395because it does not use surrogates. UCS-4 is a 32-bit encoding,
b19eb496 1396functionally identical to UTF-32 (the difference being that
a9130ea9 1397UCS-4 forbids neither surrogates nor code points larger than C<0x10_FFFF>).
c349b1b9 1398
c29a771d 1399=item *
5cb3728c
RB
1400
1401UTF-7
c349b1b9 1402
376d9008
JB
1403A seven-bit safe (non-eight-bit) encoding, which is useful if the
1404transport or storage is not eight-bit safe. Defined by RFC 2152.
c349b1b9 1405
95a1a48b
JH
1406=back
1407
57e88091 1408=head2 Noncharacter code points
6d4f9cf2 1409
57e88091 141066 code points are set aside in Unicode as "noncharacter code points".
a9130ea9 1411These all have the C<Unassigned> (C<Cn>) C<L</General_Category>>, and
57e88091
KW
1412no character will ever be assigned to any of them. They are the 32 code
1413points between C<U+FDD0> and C<U+FDEF> inclusive, and the 34 code
1414points:
1415
1416 U+FFFE U+FFFF
1417 U+1FFFE U+1FFFF
1418 U+2FFFE U+2FFFF
1419 ...
1420 U+EFFFE U+EFFFF
1421 U+FFFFE U+FFFFF
1422 U+10FFFE U+10FFFF
1423
1424Until Unicode 7.0, the noncharacters were "B<forbidden> for use in open
1425interchange of Unicode text data", so that code that processed those
1426streams could use these code points as sentinels that could be mixed in
1427with character data, and would always be distinguishable from that data.
1428(Emphasis above and in the next paragraph are added in this document.)
1429
1430Unicode 7.0 changed the wording so that they are "B<not recommended> for
1431use in open interchange of Unicode text data". The 7.0 Standard goes on
1432to say:
1433
1434=over 4
1435
1436"If a noncharacter is received in open interchange, an application is
1437not required to interpret it in any way. It is good practice, however,
1438to recognize it as a noncharacter and to take appropriate action, such
1439as replacing it with C<U+FFFD> replacement character, to indicate the
1440problem in the text. It is not recommended to simply delete
1441noncharacter code points from such text, because of the potential
1442security issues caused by deleting uninterpreted characters. (See
1443conformance clause C7 in Section 3.2, Conformance Requirements, and
1444L<Unicode Technical Report #36, "Unicode Security
1445Considerations"|http://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences>)."
1446
1447=back
1448
1449This change was made because it was found that various commercial tools
1450like editors, or for things like source code control, had been written
1451so that they would not handle program files that used these code points,
1452effectively precluding their use almost entirely! And that was never
1453the intent. They've always been meant to be usable within an
1454application, or cooperating set of applications, at will.
1455
1456If you're writing code, such as an editor, that is supposed to be able
1457to handle any Unicode text data, then you shouldn't be using these code
1458points yourself, and instead allow them in the input. If you need
1459sentinels, they should instead be something that isn't legal Unicode.
1460For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
1461they never appear in well-formed UTF-8. (There are equivalents for
1462UTF-EBCDIC). You can also store your Unicode code points in integer
1463variables and use negative values as sentinels.
1464
1465If you're not writing such a tool, then whether you accept noncharacters
1466as input is up to you (though the Standard recommends that you not). If
1467you do strict input stream checking with Perl, these code points
1468continue to be forbidden. This is to maintain backward compatibility
1469(otherwise potential security holes could open up, as an unsuspecting
1470application that was written assuming the noncharacters would be
1471filtered out before getting to it, could now, without warning, start
1472getting them). To do strict checking, you can use the layer
1473C<:encoding('UTF-8')>.
1474
1475Perl continues to warn (using the warning category C<"nonchar">, which
1476is a sub-category of C<"utf8">) if an attempt is made to output
1477noncharacters.
42581d5d
KW
1478
1479=head2 Beyond Unicode code points
1480
a9130ea9
KW
1481The maximum Unicode code point is C<U+10FFFF>, and Unicode only defines
1482operations on code points up through that. But Perl works on code
42581d5d
KW
1483points up to the maximum permissible unsigned number available on the
1484platform. However, Perl will not accept these from input streams unless
1485lax rules are being used, and will warn (using the warning category
2d88a86a
KW
1486C<"non_unicode">, which is a sub-category of C<"utf8">) if any are output.
1487
1488Since Unicode rules are not defined on these code points, if a
1489Unicode-defined operation is done on them, Perl uses what we believe are
1490sensible rules, while generally warning, using the C<"non_unicode">
1491category. For example, C<uc("\x{11_0000}")> will generate such a
1492warning, returning the input parameter as its result, since Perl defines
1493the uppercase of every non-Unicode code point to be the code point
b65e6125
KW
1494itself. (All the case changing operations, not just uppercasing, work
1495this way.)
2d88a86a
KW
1496
1497The situation with matching Unicode properties in regular expressions,
1498the C<\p{}> and C<\P{}> constructs, against these code points is not as
1499clear cut, and how these are handled has changed as we've gained
1500experience.
1501
1502One possibility is to treat any match against these code points as
1503undefined. But since Perl doesn't have the concept of a match being
1504undefined, it converts this to failing or C<FALSE>. This is almost, but
1505not quite, what Perl did from v5.14 (when use of these code points
1506became generally reliable) through v5.18. The difference is that Perl
1507treated all C<\p{}> matches as failing, but all C<\P{}> matches as
1508succeeding.
1509
1510One problem with this is that it leads to unexpected, and confusting
1511results in some cases:
1512
1513 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Failed on <= v5.18
1514 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Failed! on <= v5.18
1515
1516That is, it treated both matches as undefined, and converted that to
1517false (raising a warning on each). The first case is the expected
1518result, but the second is likely counterintuitive: "How could both be
1519false when they are complements?" Another problem was that the
1520implementation optimized many Unicode property matches down to already
1521existing simpler, faster operations, which don't raise the warning. We
1522chose to not forgo those optimizations, which help the vast majority of
1523matches, just to generate a warning for the unlikely event that an
1524above-Unicode code point is being matched against.
1525
1526As a result of these problems, starting in v5.20, what Perl does is
1527to treat non-Unicode code points as just typical unassigned Unicode
1528characters, and matches accordingly. (Note: Unicode has atypical
57e88091 1529unassigned code points. For example, it has noncharacter code points,
2d88a86a
KW
1530and ones that, when they do get assigned, are destined to be written
1531Right-to-left, as Arabic and Hebrew are. Perl assumes that no
1532non-Unicode code point has any atypical properties.)
1533
1534Perl, in most cases, will raise a warning when matching an above-Unicode
1535code point against a Unicode property when the result is C<TRUE> for
1536C<\p{}>, and C<FALSE> for C<\P{}>. For example:
1537
1538 chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails, no warning
1539 chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Succeeds, with warning
1540
1541In both these examples, the character being matched is non-Unicode, so
1542Unicode doesn't define how it should match. It clearly isn't an ASCII
1543hex digit, so the first example clearly should fail, and so it does,
1544with no warning. But it is arguable that the second example should have
1545an undefined, hence C<FALSE>, result. So a warning is raised for it.
1546
1547Thus the warning is raised for many fewer cases than in earlier Perls,
1548and only when what the result is could be arguable. It turns out that
1549none of the optimizations made by Perl (or are ever likely to be made)
1550cause the warning to be skipped, so it solves both problems of Perl's
1551earlier approach. The most commonly used property that is affected by
1552this change is C<\p{Unassigned}> which is a short form for
1553C<\p{General_Category=Unassigned}>. Starting in v5.20, all non-Unicode
1554code points are considered C<Unassigned>. In earlier releases the
1555matches failed because the result was considered undefined.
1556
1557The only place where the warning is not raised when it might ought to
1558have been is if optimizations cause the whole pattern match to not even
1559be attempted. For example, Perl may figure out that for a string to
1560match a certain regular expression pattern, the string has to contain
1561the substring C<"foobar">. Before attempting the match, Perl may look
1562for that substring, and if not found, immediately fail the match without
1563actually trying it; so no warning gets generated even if the string
1564contains an above-Unicode code point.
1565
1566This behavior is more "Do what I mean" than in earlier Perls for most
1567applications. But it catches fewer issues for code that needs to be
1568strictly Unicode compliant. Therefore there is an additional mode of
1569operation available to accommodate such code. This mode is enabled if a
1570regular expression pattern is compiled within the lexical scope where
1571the C<"non_unicode"> warning class has been made fatal, say by:
1572
1573 use warnings FATAL => "non_unicode"
1574
44ecbbd8 1575(see L<warnings>). In this mode of operation, Perl will raise the
2d88a86a
KW
1576warning for all matches against a non-Unicode code point (not just the
1577arguable ones), and it skips the optimizations that might cause the
1578warning to not be output. (It currently still won't warn if the match
1579isn't even attempted, like in the C<"foobar"> example above.)
1580
1581In summary, Perl now normally treats non-Unicode code points as typical
1582Unicode unassigned code points for regular expression matches, raising a
1583warning only when it is arguable what the result should be. However, if
1584this warning has been made fatal, it isn't skipped.
1585
1586There is one exception to all this. C<\p{All}> looks like a Unicode
1587property, but it is a Perl extension that is defined to be true for all
1588possible code points, Unicode or not, so no warning is ever generated
1589when matching this against a non-Unicode code point. (Prior to v5.20,
1590it was an exact synonym for C<\p{Any}>, matching code points C<0>
1591through C<0x10FFFF>.)
6d4f9cf2 1592
0d7c09bb
JH
1593=head2 Security Implications of Unicode
1594
b65e6125
KW
1595First, read
1596L<Unicode Security Considerations|http://www.unicode.org/reports/tr36>.
1597
e1b711da
KW
1598Also, note the following:
1599
0d7c09bb
JH
1600=over 4
1601
1602=item *
1603
1604Malformed UTF-8
bf0fa0b2 1605
42581d5d 1606Unfortunately, the original specification of UTF-8 leaves some room for
bf0fa0b2 1607interpretation of how many bytes of encoded output one should generate
376d9008
JB
1608from one input Unicode character. Strictly speaking, the shortest
1609possible sequence of UTF-8 bytes should be generated,
1610because otherwise there is potential for an input buffer overflow at
feda178f 1611the receiving end of a UTF-8 connection. Perl always generates the
e1b711da 1612shortest length UTF-8, and with warnings on, Perl will warn about
376d9008 1613non-shortest length UTF-8 along with other malformations, such as the
b19eb496 1614surrogates, which are not Unicode code points valid for interchange.
bf0fa0b2 1615
0d7c09bb
JH
1616=item *
1617
68693f9e 1618Regular expression pattern matching may surprise you if you're not
b19eb496
TC
1619accustomed to Unicode. Starting in Perl 5.14, several pattern
1620modifiers are available to control this, called the character set
42581d5d
KW
1621modifiers. Details are given in L<perlre/Character set modifiers>.
1622
1623=back
0d7c09bb 1624
376d9008 1625As discussed elsewhere, Perl has one foot (two hooves?) planted in
a6a7eedc
KW
1626each of two worlds: the old world of ASCII and single-byte locales, and
1627the new world of Unicode, upgrading when necessary.
376d9008 1628If your legacy code does not explicitly use Unicode, no automatic
a6a7eedc 1629switch-over to Unicode should happen.
0d7c09bb 1630
c349b1b9
JH
1631=head2 Unicode in Perl on EBCDIC
1632
a6a7eedc
KW
1633Unicode is supported on EBCDIC platforms. See L<perlebcdic>.
1634
1635Unless ASCII vs. EBCDIC issues are specifically being discussed,
1636references to UTF-8 encoding in this document and elsewhere should be
1637read as meaning UTF-EBCDIC on EBCDIC platforms.
1638See L<perlebcdic/Unicode and UTF>.
1639
1640Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
1641hidden from you; S<C<use utf8>> (and NOT something like
1642S<C<use utfebcdic>>) declares the the script is in the platform's
1643"native" 8-bit encoding of Unicode. (Similarly for the C<":utf8">
1644layer.)
c349b1b9 1645
b310b053
JH
1646=head2 Locales
1647
42581d5d 1648See L<perllocale/Unicode and UTF-8>
b310b053 1649
1aad1664
JH
1650=head2 When Unicode Does Not Happen
1651
b65e6125
KW
1652There are still many places where Unicode (in some encoding or
1653another) could be given as arguments or received as results, or both in
1654Perl, but it is not, in spite of Perl having extensive ways to input and
1655output in Unicode, and a few other "entry points" like the C<@ARGV>
1656array (which can sometimes be interpreted as UTF-8).
1aad1664 1657
e1b711da
KW
1658The following are such interfaces. Also, see L</The "Unicode Bug">.
1659For all of these interfaces Perl
b9cedb1b 1660currently (as of v5.16.0) simply assumes byte strings both as arguments
b65e6125 1661and results, or UTF-8 strings if the (deprecated) C<encoding> pragma has been used.
1aad1664 1662
b19eb496
TC
1663One reason that Perl does not attempt to resolve the role of Unicode in
1664these situations is that the answers are highly dependent on the operating
1aad1664 1665system and the file system(s). For example, whether filenames can be
b19eb496
TC
1666in Unicode and in exactly what kind of encoding, is not exactly a
1667portable concept. Similarly for C<qx> and C<system>: how well will the
1668"command-line interface" (and which of them?) handle Unicode?
1aad1664
JH
1669
1670=over 4
1671
557a2462
RB
1672=item *
1673
a9130ea9
KW
1674C<chdir>, C<chmod>, C<chown>, C<chroot>, C<exec>, C<link>, C<lstat>, C<mkdir>,
1675C<rename>, C<rmdir>, C<stat>, C<symlink>, C<truncate>, C<unlink>, C<utime>, C<-X>
557a2462
RB
1676
1677=item *
1678
a9130ea9 1679C<%ENV>
557a2462
RB
1680
1681=item *
1682
a9130ea9 1683C<glob> (aka the C<E<lt>*E<gt>>)
557a2462
RB
1684
1685=item *
1aad1664 1686
a9130ea9 1687C<open>, C<opendir>, C<sysopen>
1aad1664 1688
557a2462 1689=item *
1aad1664 1690
a9130ea9 1691C<qx> (aka the backtick operator), C<system>
1aad1664 1692
557a2462 1693=item *
1aad1664 1694
a9130ea9 1695C<readdir>, C<readlink>
1aad1664
JH
1696
1697=back
1698
e1b711da
KW
1699=head2 The "Unicode Bug"
1700
a6a7eedc
KW
1701The term, "Unicode bug" has been applied to an inconsistency with the
1702code points in the C<Latin-1 Supplement> block, that is, between
1703128 and 255. Without a locale specified, unlike all other characters or
1704code points, these characters can have very different semantics
1705depending on the rules in effect. (Characters whose code points are
1706above 255 force Unicode rules; whereas the rules for ASCII characters
1707are the same under both ASCII and Unicode rules.)
1708
1709Under Unicode rules, these upper-Latin1 characters are interpreted as
1710Unicode code points, which means they have the same semantics as Latin-1
1711(ISO-8859-1) and C1 controls.
1712
1713As explained in L</ASCII Rules versus Unicode Rules>, under ASCII rules,
1714they are considered to be unassigned characters.
1715
1716This can lead to unexpected results. For example, a string's
1717semantics can suddenly change if a code point above 255 is appended to
1718it, which changes the rules from ASCII to Unicode. As an
1719example, consider the following program and its output:
1720
1721 $ perl -le'
1722 no feature 'unicode_strings';
1723 $s1 = "\xC2";
1724 $s2 = "\x{2660}";
1725 for ($s1, $s2, $s1.$s2) {
1726 print /\w/ || 0;
1727 }
1728 '
1729 0
1730 0
1731 1
1732
1733If there's no C<\w> in C<s1> nor in C<s2>, why does their concatenation
1734have one?
1735
1736This anomaly stems from Perl's attempt to not disturb older programs that
1737didn't use Unicode, along with Perl's desire to add Unicode support
1738seamlessly. But the result turned out to not be seamless. (By the way,
1739you can choose to be warned when things like this happen. See
1740C<L<encoding::warnings>>.)
1741
1742L<S<C<use feature 'unicode_strings'>>|feature/The 'unicode_strings' feature>
1743was added, starting in Perl v5.12, to address this problem. It affects
1744these things:
e1b711da
KW
1745
1746=over 4
1747
1748=item *
1749
1750Changing the case of a scalar, that is, using C<uc()>, C<ucfirst()>, C<lc()>,
2e2b2571
KW
1751and C<lcfirst()>, or C<\L>, C<\U>, C<\u> and C<\l> in double-quotish
1752contexts, such as regular expression substitutions.
a6a7eedc
KW
1753
1754Under C<unicode_strings> starting in Perl 5.12.0, Unicode rules are
2e2b2571
KW
1755generally used. See L<perlfunc/lc> for details on how this works
1756in combination with various other pragmas.
e1b711da
KW
1757
1758=item *
1759
2e2b2571 1760Using caseless (C</i>) regular expression matching.
a6a7eedc 1761
2e2b2571 1762Starting in Perl 5.14.0, regular expressions compiled within
a6a7eedc 1763the scope of C<unicode_strings> use Unicode rules
2e2b2571
KW
1764even when executed or compiled into larger
1765regular expressions outside the scope.
e1b711da
KW
1766
1767=item *
1768
a6a7eedc
KW
1769Matching any of several properties in regular expressions.
1770
1771These properties are C<\b> (without braces), C<\B> (without braces),
1772C<\s>, C<\S>, C<\w>, C<\W>, and all the Posix character classes
630d17dc 1773I<except> C<[[:ascii:]]>.
a6a7eedc 1774
2e2b2571 1775Starting in Perl 5.14.0, regular expressions compiled within
a6a7eedc 1776the scope of C<unicode_strings> use Unicode rules
2e2b2571
KW
1777even when executed or compiled into larger
1778regular expressions outside the scope.
e1b711da
KW
1779
1780=item *
1781
a6a7eedc
KW
1782In C<quotemeta> or its inline equivalent C<\Q>.
1783
2e2b2571
KW
1784Starting in Perl 5.16.0, consistent quoting rules are used within the
1785scope of C<unicode_strings>, as described in L<perlfunc/quotemeta>.
a6a7eedc
KW
1786Prior to that, or outside its scope, no code points above 127 are quoted
1787in UTF-8 encoded strings, but in byte encoded strings, code points
1788between 128-255 are always quoted.
eb88ed9e 1789
e1b711da
KW
1790=back
1791
a6a7eedc
KW
1792You can see from the above that the effect of C<unicode_strings>
1793increased over several Perl releases. (And Perl's support for Unicode
1794continues to improve; it's best to use the latest available release in
1795order to get the most complete and accurate results possible.) Note that
1796C<unicode_strings> is automatically chosen if you S<C<use 5.012>> or
1797higher.
e1b711da 1798
2e2b2571 1799For Perls earlier than those described above, or when a string is passed
a6a7eedc 1800to a function outside the scope of C<unicode_strings>, see the next section.
e1b711da 1801
1aad1664
JH
1802=head2 Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
1803
e1b711da
KW
1804Sometimes (see L</"When Unicode Does Not Happen"> or L</The "Unicode Bug">)
1805there are situations where you simply need to force a byte
a6a7eedc
KW
1806string into UTF-8, or vice versa. The standard module L<Encode> can be
1807used for this, or the low-level calls
a9130ea9 1808L<C<utf8::upgrade($bytestring)>|utf8/Utility functions> and
a6a7eedc 1809L<C<utf8::downgrade($utf8string[, FAIL_OK])>|utf8/Utility functions>.
1aad1664 1810
a9130ea9 1811Note that C<utf8::downgrade()> can fail if the string contains characters
2bbc8d55 1812that don't fit into a byte.
1aad1664 1813
e1b711da
KW
1814Calling either function on a string that already is in the desired state is a
1815no-op.
1816
a6a7eedc
KW
1817L</ASCII Rules versus Unicode Rules> gives all the ways that a string is
1818made to use Unicode rules.
95a1a48b 1819
37b3b608 1820=head2 Using Unicode in XS
c349b1b9 1821
37b3b608
KW
1822See L<perlguts/"Unicode Support"> for an introduction to Unicode at
1823the XS level, and L<perlapi/Unicode Support> for the API details.
95a1a48b 1824
e1b711da
KW
1825=head2 Hacking Perl to work on earlier Unicode versions (for very serious hackers only)
1826
a6a7eedc
KW
1827Perl by default comes with the latest supported Unicode version built-in, but
1828the goal is to allow you to change to use any earlier one. In Perls
1829v5.20 and v5.22, however, the earliest usable version is Unicode 5.1.
d2b457d7 1830Perl v5.18 and v5.24 are able to handle all earlier versions.
e1b711da 1831
42581d5d 1832Download the files in the desired version of Unicode from the Unicode web
e1b711da 1833site L<http://www.unicode.org>). These should replace the existing files in
b19eb496 1834F<lib/unicore> in the Perl source tree. Follow the instructions in
116693e8 1835F<README.perl> in that directory to change some of their names, and then build
26e391dd 1836perl (see L<INSTALL>).
116693e8 1837
c8d992ba
A
1838=head2 Porting code from perl-5.6.X
1839
a6a7eedc
KW
1840Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6 the
1841programmer was required to use the C<utf8> pragma to declare that a
1842given scope expected to deal with Unicode data and had to make sure that
1843only Unicode data were reaching that scope. If you have code that is
c8d992ba 1844working with 5.6, you will need some of the following adjustments to
a6a7eedc
KW
1845your code. The examples are written such that the code will continue to
1846work under 5.6, so you should be safe to try them out.
c8d992ba 1847
755789c0 1848=over 3
c8d992ba
A
1849
1850=item *
1851
1852A filehandle that should read or write UTF-8
1853
b9cedb1b 1854 if ($] > 5.008) {
740d4bb2 1855 binmode $fh, ":encoding(utf8)";
c8d992ba
A
1856 }
1857
1858=item *
1859
1860A scalar that is going to be passed to some extension
1861
a9130ea9 1862Be it C<Compress::Zlib>, C<Apache::Request> or any extension that has no
c8d992ba 1863mention of Unicode in the manpage, you need to make sure that the
2575c402 1864UTF8 flag is stripped off. Note that at the time of this writing
b9cedb1b 1865(January 2012) the mentioned modules are not UTF-8-aware. Please
c8d992ba
A
1866check the documentation to verify if this is still true.
1867
b9cedb1b 1868 if ($] > 5.008) {
c8d992ba
A
1869 require Encode;
1870 $val = Encode::encode_utf8($val); # make octets
1871 }
1872
1873=item *
1874
1875A scalar we got back from an extension
1876
1877If you believe the scalar comes back as UTF-8, you will most likely
2575c402 1878want the UTF8 flag restored:
c8d992ba 1879
b9cedb1b 1880 if ($] > 5.008) {
c8d992ba
A
1881 require Encode;
1882 $val = Encode::decode_utf8($val);
1883 }
1884
1885=item *
1886
1887Same thing, if you are really sure it is UTF-8
1888
b9cedb1b 1889 if ($] > 5.008) {
c8d992ba
A
1890 require Encode;
1891 Encode::_utf8_on($val);
1892 }
1893
1894=item *
1895
a9130ea9 1896A wrapper for L<DBI> C<fetchrow_array> and C<fetchrow_hashref>
c8d992ba
A
1897
1898When the database contains only UTF-8, a wrapper function or method is
a9130ea9
KW
1899a convenient way to replace all your C<fetchrow_array> and
1900C<fetchrow_hashref> calls. A wrapper function will also make it easier to
c8d992ba 1901adapt to future enhancements in your database driver. Note that at the
b9cedb1b 1902time of this writing (January 2012), the DBI has no standardized way
a9130ea9 1903to deal with UTF-8 data. Please check the L<DBI documentation|DBI> to verify if
c8d992ba
A
1904that is still true.
1905
1906 sub fetchrow {
d88362ca
KW
1907 # $what is one of fetchrow_{array,hashref}
1908 my($self, $sth, $what) = @_;
b9cedb1b 1909 if ($] < 5.008) {
c8d992ba
A
1910 return $sth->$what;
1911 } else {
1912 require Encode;
1913 if (wantarray) {
1914 my @arr = $sth->$what;
1915 for (@arr) {
1916 defined && /[^\000-\177]/ && Encode::_utf8_on($_);
1917 }
1918 return @arr;
1919 } else {
1920 my $ret = $sth->$what;
1921 if (ref $ret) {
1922 for my $k (keys %$ret) {
d88362ca
KW
1923 defined
1924 && /[^\000-\177]/
1925 && Encode::_utf8_on($_) for $ret->{$k};
c8d992ba
A
1926 }
1927 return $ret;
1928 } else {
1929 defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
1930 return $ret;
1931 }
1932 }
1933 }
1934 }
1935
1936
1937=item *
1938
1939A large scalar that you know can only contain ASCII
1940
1941Scalars that contain only ASCII and are marked as UTF-8 are sometimes
1942a drag to your program. If you recognize such a situation, just remove
2575c402 1943the UTF8 flag:
c8d992ba 1944
b9cedb1b 1945 utf8::downgrade($val) if $] > 5.008;
c8d992ba
A
1946
1947=back
1948
a6a7eedc
KW
1949=head1 BUGS
1950
1951See also L</The "Unicode Bug"> above.
1952
1953=head2 Interaction with Extensions
1954
1955When Perl exchanges data with an extension, the extension should be
1956able to understand the UTF8 flag and act accordingly. If the
1957extension doesn't recognize that flag, it's likely that the extension
1958will return incorrectly-flagged data.
1959
1960So if you're working with Unicode data, consult the documentation of
1961every module you're using if there are any issues with Unicode data
1962exchange. If the documentation does not talk about Unicode at all,
1963suspect the worst and probably look at the source to learn how the
1964module is implemented. Modules written completely in Perl shouldn't
1965cause problems. Modules that directly or indirectly access code written
1966in other programming languages are at risk.
1967
1968For affected functions, the simple strategy to avoid data corruption is
1969to always make the encoding of the exchanged data explicit. Choose an
1970encoding that you know the extension can handle. Convert arguments passed
1971to the extensions to that encoding and convert results back from that
1972encoding. Write wrapper functions that do the conversions for you, so
1973you can later change the functions when the extension catches up.
1974
1975To provide an example, let's say the popular C<Foo::Bar::escape_html>
1976function doesn't deal with Unicode data yet. The wrapper function
1977would convert the argument to raw UTF-8 and convert the result back to
1978Perl's internal representation like so:
1979
1980 sub my_escape_html ($) {
1981 my($what) = shift;
1982 return unless defined $what;
1983 Encode::decode_utf8(Foo::Bar::escape_html(
1984 Encode::encode_utf8($what)));
1985 }
1986
1987Sometimes, when the extension does not convert data but just stores
1988and retrieves it, you will be able to use the otherwise
1989dangerous L<C<Encode::_utf8_on()>|Encode/_utf8_on> function. Let's say
1990the popular C<Foo::Bar> extension, written in C, provides a C<param>
1991method that lets you store and retrieve data according to these prototypes:
1992
1993 $self->param($name, $value); # set a scalar
1994 $value = $self->param($name); # retrieve a scalar
1995
1996If it does not yet provide support for any encoding, one could write a
1997derived class with such a C<param> method:
1998
1999 sub param {
2000 my($self,$name,$value) = @_;
2001 utf8::upgrade($name); # make sure it is UTF-8 encoded
2002 if (defined $value) {
2003 utf8::upgrade($value); # make sure it is UTF-8 encoded
2004 return $self->SUPER::param($name,$value);
2005 } else {
2006 my $ret = $self->SUPER::param($name);
2007 Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
2008 return $ret;
2009 }
2010 }
2011
2012Some extensions provide filters on data entry/exit points, such as
2013C<DB_File::filter_store_key> and family. Look out for such filters in
2014the documentation of your extensions; they can make the transition to
2015Unicode data much easier.
2016
2017=head2 Speed
2018
2019Some functions are slower when working on UTF-8 encoded strings than
2020on byte encoded strings. All functions that need to hop over
2021characters such as C<length()>, C<substr()> or C<index()>, or matching
2022regular expressions can work B<much> faster when the underlying data are
2023byte-encoded.
2024
2025In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
2026a caching scheme was introduced which improved the situation. In general,
2027operations with UTF-8 encoded strings are still slower. As an example,
2028the Unicode properties (character classes) like C<\p{Nd}> are known to
2029be quite a bit slower (5-20 times) than their simpler counterparts
2030like C<[0-9]> (then again, there are hundreds of Unicode characters matching
2031C<Nd> compared with the 10 ASCII characters matching C<[0-9]>).
2032
393fec97
GS
2033=head1 SEE ALSO
2034
51f494cc 2035L<perlunitut>, L<perluniintro>, L<perluniprops>, L<Encode>, L<open>, L<utf8>, L<bytes>,
b65e6125 2036L<perlretut>, L<perlvar/"${^UNICODE}">,
51f494cc 2037L<http://www.unicode.org/reports/tr44>).
393fec97
GS
2038
2039=cut