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1 | =head1 NAME |
2 | |
3 | perlunicode - Unicode support in Perl |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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7 | =head2 Important Caveats |
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8 | |
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9 | WARNING: While the implementation of Unicode support in Perl is now fairly |
10 | complete it is still evolving to some extent. |
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11 | |
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12 | In particular the way Unicode is handled on EBCDIC platforms is still rather |
13 | experimental. On such a platform references to UTF-8 encoding in this |
14 | document and elsewhere should be read as meaning UTF-EBCDIC as specified |
15 | in Unicode Technical Report 16 unless ASCII vs EBCDIC issues are specifically |
16 | discussed. There is no C<utfebcdic> pragma or ":utfebcdic" layer, rather |
17 | "utf8" and ":utf8" are re-used to mean platform's "natural" 8-bit encoding |
18 | of Unicode. See L<perlebcdic> for more discussion of the issues. |
19 | |
20 | The following areas are still under development. |
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21 | |
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22 | =over 4 |
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23 | |
24 | =item Input and Output Disciplines |
25 | |
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26 | A filehandle can be marked as containing perl's internal Unicode encoding |
27 | (UTF-8 or UTF-EBCDIC) by opening it with the ":utf8" layer. |
28 | Other encodings can be converted to perl's encoding on input, or from |
29 | perl's encoding on output by use of the ":encoding()" layer. |
30 | There is not yet a clean way to mark the perl source itself as being |
31 | in an particular encoding. |
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32 | |
33 | =item Regular Expressions |
34 | |
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35 | The regular expression compiler does now attempt to produce |
36 | polymorphic opcodes. That is the pattern should now adapt to the data |
37 | and automatically switch to the Unicode character scheme when presented |
38 | with Unicode data, or a traditional byte scheme when presented with |
39 | byte data. The implementation is still new and (particularly on |
40 | EBCDIC platforms) may need further work. |
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41 | |
42 | =item C<use utf8> still needed to enable a few features |
43 | |
44 | The C<utf8> pragma implements the tables used for Unicode support. These |
45 | tables are automatically loaded on demand, so the C<utf8> pragma need not |
46 | normally be used. |
47 | |
48 | However, as a compatibility measure, this pragma must be explicitly used |
49 | to enable recognition of UTF-8 encoded literals and identifiers in the |
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50 | source text on ASCII based machines or recognize UTF-EBCDIC encoded literals |
51 | and identifiers on EBCDIC based machines. |
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52 | |
53 | =back |
54 | |
55 | =head2 Byte and Character semantics |
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56 | |
57 | Beginning with version 5.6, Perl uses logically wide characters to |
58 | represent strings internally. This internal representation of strings |
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59 | uses either the UTF-8 or the UTF-EBCDIC encoding. |
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60 | |
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61 | In future, Perl-level operations can be expected to work with characters |
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62 | rather than bytes, in general. |
63 | |
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64 | However, as strictly an interim compatibility measure, Perl v5.6 aims to |
65 | provide a safe migration path from byte semantics to character semantics |
66 | for programs. For operations where Perl can unambiguously decide that the |
67 | input data is characters, Perl now switches to character semantics. |
68 | For operations where this determination cannot be made without additional |
69 | information from the user, Perl decides in favor of compatibility, and |
70 | chooses to use byte semantics. |
71 | |
72 | This behavior preserves compatibility with earlier versions of Perl, |
73 | which allowed byte semantics in Perl operations, but only as long as |
74 | none of the program's inputs are marked as being as source of Unicode |
75 | character data. Such data may come from filehandles, from calls to |
76 | external programs, from information provided by the system (such as %ENV), |
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77 | or from literals and constants in the source text. |
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78 | |
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79 | If the C<-C> command line switch is used, (or the ${^WIDE_SYSTEM_CALLS} |
80 | global flag is set to C<1>), all system calls will use the |
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81 | corresponding wide character APIs. This is currently only implemented |
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82 | on Windows since UNIXes lack API standard on this area. |
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83 | |
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84 | Regardless of the above, the C<bytes> pragma can always be used to force |
85 | byte semantics in a particular lexical scope. See L<bytes>. |
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86 | |
87 | The C<utf8> pragma is primarily a compatibility device that enables |
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88 | recognition of UTF-(8|EBCDIC) in literals encountered by the parser. It may also |
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89 | be used for enabling some of the more experimental Unicode support features. |
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90 | Note that this pragma is only required until a future version of Perl |
91 | in which character semantics will become the default. This pragma may |
92 | then become a no-op. See L<utf8>. |
93 | |
94 | Unless mentioned otherwise, Perl operators will use character semantics |
95 | when they are dealing with Unicode data, and byte semantics otherwise. |
96 | Thus, character semantics for these operations apply transparently; if |
97 | the input data came from a Unicode source (for example, by adding a |
98 | character encoding discipline to the filehandle whence it came, or a |
99 | literal UTF-8 string constant in the program), character semantics |
100 | apply; otherwise, byte semantics are in effect. To force byte semantics |
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101 | on Unicode data, the C<bytes> pragma should be used. |
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102 | |
103 | Under character semantics, many operations that formerly operated on |
104 | bytes change to operating on characters. For ASCII data this makes |
105 | no difference, because UTF-8 stores ASCII in single bytes, but for |
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106 | any character greater than C<chr(127)>, the character may be stored in |
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107 | a sequence of two or more bytes, all of which have the high bit set. |
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108 | For C1 controls or Latin 1 characters on an EBCDIC platform the character |
109 | may be stored in a UTF-EBCDIC multi byte sequence. |
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110 | But by and large, the user need not worry about this, because Perl |
111 | hides it from the user. A character in Perl is logically just a number |
112 | ranging from 0 to 2**32 or so. Larger characters encode to longer |
113 | sequences of bytes internally, but again, this is just an internal |
114 | detail which is hidden at the Perl level. |
115 | |
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116 | =head2 Effects of character semantics |
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117 | |
118 | Character semantics have the following effects: |
119 | |
120 | =over 4 |
121 | |
122 | =item * |
123 | |
124 | Strings and patterns may contain characters that have an ordinal value |
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125 | larger than 255. |
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126 | |
127 | Presuming you use a Unicode editor to edit your program, such characters |
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128 | will typically occur directly within the literal strings as UTF-(8|EBCDIC) |
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129 | characters, but you can also specify a particular character with an |
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130 | extension of the C<\x> notation. UTF-X characters are specified by |
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131 | putting the hexadecimal code within curlies after the C<\x>. For instance, |
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132 | a Unicode smiley face is C<\x{263A}>. |
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133 | |
134 | =item * |
135 | |
136 | Identifiers within the Perl script may contain Unicode alphanumeric |
137 | characters, including ideographs. (You are currently on your own when |
138 | it comes to using the canonical forms of characters--Perl doesn't (yet) |
139 | attempt to canonicalize variable names for you.) |
140 | |
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141 | =item * |
142 | |
143 | Regular expressions match characters instead of bytes. For instance, |
144 | "." matches a character instead of a byte. (However, the C<\C> pattern |
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145 | is provided to force a match a single byte ("C<char>" in C, hence |
146 | C<\C>).) |
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147 | |
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148 | =item * |
149 | |
150 | Character classes in regular expressions match characters instead of |
151 | bytes, and match against the character properties specified in the |
152 | Unicode properties database. So C<\w> can be used to match an ideograph, |
153 | for instance. |
154 | |
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155 | =item * |
156 | |
157 | Named Unicode properties and block ranges make be used as character |
158 | classes via the new C<\p{}> (matches property) and C<\P{}> (doesn't |
159 | match property) constructs. For instance, C<\p{Lu}> matches any |
160 | character with the Unicode uppercase property, while C<\p{M}> matches |
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161 | any mark character. Single letter properties may omit the brackets, |
162 | so that can be written C<\pM> also. Many predefined character classes |
163 | are available, such as C<\p{IsMirrored}> and C<\p{InTibetan}>. The |
164 | names of the C<In> classes are the official Unicode block names but |
165 | with all non-alphanumeric characters removed, for example the block |
166 | name C<"Latin-1 Supplement"> becomes C<\p{InLatin1Supplement}>. |
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167 | |
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168 | =item * |
169 | |
170 | The special pattern C<\X> match matches any extended Unicode sequence |
171 | (a "combining character sequence" in Standardese), where the first |
172 | character is a base character and subsequent characters are mark |
173 | characters that apply to the base character. It is equivalent to |
174 | C<(?:\PM\pM*)>. |
175 | |
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176 | =item * |
177 | |
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178 | The C<tr///> operator translates characters instead of bytes. Note |
179 | that the C<tr///CU> functionality has been removed, as the interface |
180 | was a mistake. For similar functionality see pack('U0', ...) and |
181 | pack('C0', ...). |
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182 | |
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183 | =item * |
184 | |
185 | Case translation operators use the Unicode case translation tables |
186 | when provided character input. Note that C<uc()> translates to |
187 | uppercase, while C<ucfirst> translates to titlecase (for languages |
188 | that make the distinction). Naturally the corresponding backslash |
189 | sequences have the same semantics. |
190 | |
191 | =item * |
192 | |
193 | Most operators that deal with positions or lengths in the string will |
194 | automatically switch to using character positions, including C<chop()>, |
195 | C<substr()>, C<pos()>, C<index()>, C<rindex()>, C<sprintf()>, |
196 | C<write()>, and C<length()>. Operators that specifically don't switch |
197 | include C<vec()>, C<pack()>, and C<unpack()>. Operators that really |
198 | don't care include C<chomp()>, as well as any other operator that |
199 | treats a string as a bucket of bits, such as C<sort()>, and the |
200 | operators dealing with filenames. |
201 | |
202 | =item * |
203 | |
204 | The C<pack()>/C<unpack()> letters "C<c>" and "C<C>" do I<not> change, |
205 | since they're often used for byte-oriented formats. (Again, think |
206 | "C<char>" in the C language.) However, there is a new "C<U>" specifier |
207 | that will convert between UTF-8 characters and integers. (It works |
208 | outside of the utf8 pragma too.) |
209 | |
210 | =item * |
211 | |
212 | The C<chr()> and C<ord()> functions work on characters. This is like |
213 | C<pack("U")> and C<unpack("U")>, not like C<pack("C")> and |
214 | C<unpack("C")>. In fact, the latter are how you now emulate |
215 | byte-oriented C<chr()> and C<ord()> under utf8. |
216 | |
217 | =item * |
218 | |
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219 | The bit string operators C<& | ^ ~> can operate on character data. |
220 | However, for backward compatibility reasons (bit string operations |
221 | when the characters all are less than 256 in ordinal value) one cannot |
222 | mix C<~> (the bit complement) and characters both less than 256 and |
223 | equal or greater than 256. Most importantly, the DeMorgan's laws |
224 | (C<~($x|$y) eq ~$x&~$y>, C<~($x&$y) eq ~$x|~$y>) won't hold. |
225 | Another way to look at this is that the complement cannot return |
226 | B<both> the 8-bit (byte) wide bit complement, and the full character |
227 | wide bit complement. |
228 | |
229 | =item * |
230 | |
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231 | And finally, C<scalar reverse()> reverses by character rather than by byte. |
232 | |
233 | =back |
234 | |
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235 | =head2 Character encodings for input and output |
236 | |
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237 | See L<Encode>. |
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238 | |
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239 | =head1 CAVEATS |
240 | |
241 | As of yet, there is no method for automatically coercing input and |
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242 | output to some encoding other than UTF-8 or UTF-EBCDIC. This is planned |
243 | in the near future, however. |
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244 | |
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245 | Whether an arbitrary piece of data will be treated as "characters" or |
246 | "bytes" by internal operations cannot be divined at the current time. |
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247 | |
248 | Use of locales with utf8 may lead to odd results. Currently there is |
249 | some attempt to apply 8-bit locale info to characters in the range |
250 | 0..255, but this is demonstrably incorrect for locales that use |
251 | characters above that range (when mapped into Unicode). It will also |
252 | tend to run slower. Avoidance of locales is strongly encouraged. |
253 | |
254 | =head1 SEE ALSO |
255 | |
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256 | L<bytes>, L<utf8>, L<perlvar/"${^WIDE_SYSTEM_CALLS}"> |
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257 | |
258 | =cut |