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1 | =head1 NAME |
2 | |
3 | perlunicode - Unicode support in Perl |
4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | WARNING: The implementation of Unicode support in Perl is incomplete. |
8 | Expect sudden and unannounced changes! |
9 | |
10 | Beginning with version 5.6, Perl uses logically wide characters to |
11 | represent strings internally. This internal representation of strings |
12 | uses the UTF-8 encoding. |
13 | |
14 | In future, Perl-level operations will expect to work with characters |
15 | rather than bytes, in general. |
16 | |
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17 | However, as strictly an interim compatibility measure, Perl v5.6 aims to |
18 | provide a safe migration path from byte semantics to character semantics |
19 | for programs. For operations where Perl can unambiguously decide that the |
20 | input data is characters, Perl now switches to character semantics. |
21 | For operations where this determination cannot be made without additional |
22 | information from the user, Perl decides in favor of compatibility, and |
23 | chooses to use byte semantics. |
24 | |
25 | This behavior preserves compatibility with earlier versions of Perl, |
26 | which allowed byte semantics in Perl operations, but only as long as |
27 | none of the program's inputs are marked as being as source of Unicode |
28 | character data. Such data may come from filehandles, from calls to |
29 | external programs, from information provided by the system (such as %ENV), |
30 | or from literals and constants in the source text. Later, in |
31 | L</Character encodings for input and output>, we'll see how such |
32 | inputs may be marked as being Unicode character data sources. |
33 | |
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34 | If the C<-C> command line switch is used, (or the ${^WIDE_SYSTEM_CALLS} |
35 | global flag is set to C<1>), all system calls will use the |
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36 | corresponding wide character APIs. This is currently only implemented |
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37 | on Windows. |
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38 | |
39 | Regardless of the above, the C<byte> pragma can always be used to force |
40 | byte semantics in a particular lexical scope. See L<byte>. |
41 | |
42 | The C<utf8> pragma is primarily a compatibility device that enables |
43 | recognition of UTF-8 in literals encountered by the parser. It is also |
44 | used for enabling some of the more experimental Unicode support features. |
45 | Note that this pragma is only required until a future version of Perl |
46 | in which character semantics will become the default. This pragma may |
47 | then become a no-op. See L<utf8>. |
48 | |
49 | Unless mentioned otherwise, Perl operators will use character semantics |
50 | when they are dealing with Unicode data, and byte semantics otherwise. |
51 | Thus, character semantics for these operations apply transparently; if |
52 | the input data came from a Unicode source (for example, by adding a |
53 | character encoding discipline to the filehandle whence it came, or a |
54 | literal UTF-8 string constant in the program), character semantics |
55 | apply; otherwise, byte semantics are in effect. To force byte semantics |
56 | on Unicode data, the C<byte> pragma should be used. |
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57 | |
58 | Under character semantics, many operations that formerly operated on |
59 | bytes change to operating on characters. For ASCII data this makes |
60 | no difference, because UTF-8 stores ASCII in single bytes, but for |
61 | any character greater than C<chr(127)>, the character is stored in |
62 | a sequence of two or more bytes, all of which have the high bit set. |
63 | But by and large, the user need not worry about this, because Perl |
64 | hides it from the user. A character in Perl is logically just a number |
65 | ranging from 0 to 2**32 or so. Larger characters encode to longer |
66 | sequences of bytes internally, but again, this is just an internal |
67 | detail which is hidden at the Perl level. |
68 | |
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69 | =head2 Effects of character semantics |
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70 | |
71 | Character semantics have the following effects: |
72 | |
73 | =over 4 |
74 | |
75 | =item * |
76 | |
77 | Strings and patterns may contain characters that have an ordinal value |
78 | larger than 255. In Perl v5.6, this is only enabled if the lexical |
79 | scope has a C<use utf8> declaration (due to compatibility needs) but |
80 | future versions may enable this by default. |
81 | |
82 | Presuming you use a Unicode editor to edit your program, such characters |
83 | will typically occur directly within the literal strings as UTF-8 |
84 | characters, but you can also specify a particular character with an |
85 | extension of the C<\x> notation. UTF-8 characters are specified by |
86 | putting the hexadecimal code within curlies after the C<\x>. For instance, |
87 | a Unicode smiley face is C<\x{263A}>. A character in the Latin-1 range |
88 | (128..255) should be written C<\x{ab}> rather than C<\xab>, since the |
89 | former will turn into a two-byte UTF-8 code, while the latter will |
90 | continue to be interpreted as generating a 8-bit byte rather than a |
91 | character. In fact, if C<-w> is turned on, it will produce a warning |
92 | that you might be generating invalid UTF-8. |
93 | |
94 | =item * |
95 | |
96 | Identifiers within the Perl script may contain Unicode alphanumeric |
97 | characters, including ideographs. (You are currently on your own when |
98 | it comes to using the canonical forms of characters--Perl doesn't (yet) |
99 | attempt to canonicalize variable names for you.) |
100 | |
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101 | This also needs C<use utf8> currently. [XXX: Why?!? High-bit chars were |
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102 | syntax errors when they occurred within identifiers in previous versions, |
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103 | so this should probably be enabled by default.] |
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104 | |
105 | =item * |
106 | |
107 | Regular expressions match characters instead of bytes. For instance, |
108 | "." matches a character instead of a byte. (However, the C<\C> pattern |
109 | is provided to force a match a single byte ("C<char>" in C, hence |
110 | C<\C>).) |
111 | |
112 | Unicode support in regular expressions needs C<use utf8> currently. |
113 | [XXX: Because the SWASH routines need to be loaded. And the RE engine |
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114 | appears to need an overhaul to dynamically match Unicode anyway--the |
115 | current RE compiler creates different nodes with and without C<use utf8>.] |
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116 | |
117 | =item * |
118 | |
119 | Character classes in regular expressions match characters instead of |
120 | bytes, and match against the character properties specified in the |
121 | Unicode properties database. So C<\w> can be used to match an ideograph, |
122 | for instance. |
123 | |
124 | C<use utf8> is needed to enable this. See above. |
125 | |
126 | =item * |
127 | |
128 | Named Unicode properties and block ranges make be used as character |
129 | classes via the new C<\p{}> (matches property) and C<\P{}> (doesn't |
130 | match property) constructs. For instance, C<\p{Lu}> matches any |
131 | character with the Unicode uppercase property, while C<\p{M}> matches |
132 | any mark character. Single letter properties may omit the brackets, so |
133 | that can be written C<\pM> also. Many predefined character classes are |
134 | available, such as C<\p{IsMirrored}> and C<\p{InTibetan}>. |
135 | |
136 | C<use utf8> is needed to enable this. See above. |
137 | |
138 | =item * |
139 | |
140 | The special pattern C<\X> match matches any extended Unicode sequence |
141 | (a "combining character sequence" in Standardese), where the first |
142 | character is a base character and subsequent characters are mark |
143 | characters that apply to the base character. It is equivalent to |
144 | C<(?:\PM\pM*)>. |
145 | |
146 | C<use utf8> is needed to enable this. See above. |
147 | |
148 | =item * |
149 | |
150 | The C<tr///> operator translates characters instead of bytes. It can also |
151 | be forced to translate between 8-bit codes and UTF-8 regardless of the |
152 | surrounding utf8 state. For instance, if you know your input in Latin-1, |
153 | you can say: |
154 | |
155 | use utf8; |
156 | while (<>) { |
157 | tr/\0-\xff//CU; # latin1 char to utf8 |
158 | ... |
159 | } |
160 | |
161 | Similarly you could translate your output with |
162 | |
163 | tr/\0-\x{ff}//UC; # utf8 to latin1 char |
164 | |
165 | No, C<s///> doesn't take /U or /C (yet?). |
166 | |
167 | C<use utf8> is needed to enable this. See above. |
168 | |
169 | =item * |
170 | |
171 | Case translation operators use the Unicode case translation tables |
172 | when provided character input. Note that C<uc()> translates to |
173 | uppercase, while C<ucfirst> translates to titlecase (for languages |
174 | that make the distinction). Naturally the corresponding backslash |
175 | sequences have the same semantics. |
176 | |
177 | =item * |
178 | |
179 | Most operators that deal with positions or lengths in the string will |
180 | automatically switch to using character positions, including C<chop()>, |
181 | C<substr()>, C<pos()>, C<index()>, C<rindex()>, C<sprintf()>, |
182 | C<write()>, and C<length()>. Operators that specifically don't switch |
183 | include C<vec()>, C<pack()>, and C<unpack()>. Operators that really |
184 | don't care include C<chomp()>, as well as any other operator that |
185 | treats a string as a bucket of bits, such as C<sort()>, and the |
186 | operators dealing with filenames. |
187 | |
188 | =item * |
189 | |
190 | The C<pack()>/C<unpack()> letters "C<c>" and "C<C>" do I<not> change, |
191 | since they're often used for byte-oriented formats. (Again, think |
192 | "C<char>" in the C language.) However, there is a new "C<U>" specifier |
193 | that will convert between UTF-8 characters and integers. (It works |
194 | outside of the utf8 pragma too.) |
195 | |
196 | =item * |
197 | |
198 | The C<chr()> and C<ord()> functions work on characters. This is like |
199 | C<pack("U")> and C<unpack("U")>, not like C<pack("C")> and |
200 | C<unpack("C")>. In fact, the latter are how you now emulate |
201 | byte-oriented C<chr()> and C<ord()> under utf8. |
202 | |
203 | =item * |
204 | |
205 | And finally, C<scalar reverse()> reverses by character rather than by byte. |
206 | |
207 | =back |
208 | |
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209 | =head2 Character encodings for input and output |
210 | |
211 | [XXX: This feature is not yet implemented.] |
212 | |
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213 | =head1 CAVEATS |
214 | |
215 | As of yet, there is no method for automatically coercing input and |
216 | output to some encoding other than UTF-8. This is planned in the near |
217 | future, however. |
218 | |
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219 | Whether an arbitrary piece of data will be treated as "characters" or |
220 | "bytes" by internal operations cannot be divined at the current time. |
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221 | |
222 | Use of locales with utf8 may lead to odd results. Currently there is |
223 | some attempt to apply 8-bit locale info to characters in the range |
224 | 0..255, but this is demonstrably incorrect for locales that use |
225 | characters above that range (when mapped into Unicode). It will also |
226 | tend to run slower. Avoidance of locales is strongly encouraged. |
227 | |
228 | =head1 SEE ALSO |
229 | |
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230 | L<byte>, L<utf8>, L<perlvar/"${^WIDE_SYSTEM_CALLS}"> |
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231 | |
232 | =cut |