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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 | |
34 | One particular condition will enable character semantics on the entire |
35 | program, bypassing the compatibility mode: if the C<$^U> global flag is |
36 | set to C<1>, nearly all operations will use character semantics by |
37 | default. As an added convenience, if the C<utf8> pragma is used in the |
38 | C<main> package, C<$^U> is enabled automatically. [XXX: Should there |
39 | be a -C switch to enable $^U?] |
40 | |
41 | Regardless of the above, the C<byte> pragma can always be used to force |
42 | byte semantics in a particular lexical scope. See L<byte>. |
43 | |
44 | The C<utf8> pragma is primarily a compatibility device that enables |
45 | recognition of UTF-8 in literals encountered by the parser. It is also |
46 | used for enabling some of the more experimental Unicode support features. |
47 | Note that this pragma is only required until a future version of Perl |
48 | in which character semantics will become the default. This pragma may |
49 | then become a no-op. See L<utf8>. |
50 | |
51 | Unless mentioned otherwise, Perl operators will use character semantics |
52 | when they are dealing with Unicode data, and byte semantics otherwise. |
53 | Thus, character semantics for these operations apply transparently; if |
54 | the input data came from a Unicode source (for example, by adding a |
55 | character encoding discipline to the filehandle whence it came, or a |
56 | literal UTF-8 string constant in the program), character semantics |
57 | apply; otherwise, byte semantics are in effect. To force byte semantics |
58 | on Unicode data, the C<byte> pragma should be used. |
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59 | |
60 | Under character semantics, many operations that formerly operated on |
61 | bytes change to operating on characters. For ASCII data this makes |
62 | no difference, because UTF-8 stores ASCII in single bytes, but for |
63 | any character greater than C<chr(127)>, the character is stored in |
64 | a sequence of two or more bytes, all of which have the high bit set. |
65 | But by and large, the user need not worry about this, because Perl |
66 | hides it from the user. A character in Perl is logically just a number |
67 | ranging from 0 to 2**32 or so. Larger characters encode to longer |
68 | sequences of bytes internally, but again, this is just an internal |
69 | detail which is hidden at the Perl level. |
70 | |
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71 | =head2 Effects of character semantics |
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72 | |
73 | Character semantics have the following effects: |
74 | |
75 | =over 4 |
76 | |
77 | =item * |
78 | |
79 | Strings and patterns may contain characters that have an ordinal value |
80 | larger than 255. In Perl v5.6, this is only enabled if the lexical |
81 | scope has a C<use utf8> declaration (due to compatibility needs) but |
82 | future versions may enable this by default. |
83 | |
84 | Presuming you use a Unicode editor to edit your program, such characters |
85 | will typically occur directly within the literal strings as UTF-8 |
86 | characters, but you can also specify a particular character with an |
87 | extension of the C<\x> notation. UTF-8 characters are specified by |
88 | putting the hexadecimal code within curlies after the C<\x>. For instance, |
89 | a Unicode smiley face is C<\x{263A}>. A character in the Latin-1 range |
90 | (128..255) should be written C<\x{ab}> rather than C<\xab>, since the |
91 | former will turn into a two-byte UTF-8 code, while the latter will |
92 | continue to be interpreted as generating a 8-bit byte rather than a |
93 | character. In fact, if C<-w> is turned on, it will produce a warning |
94 | that you might be generating invalid UTF-8. |
95 | |
96 | =item * |
97 | |
98 | Identifiers within the Perl script may contain Unicode alphanumeric |
99 | characters, including ideographs. (You are currently on your own when |
100 | it comes to using the canonical forms of characters--Perl doesn't (yet) |
101 | attempt to canonicalize variable names for you.) |
102 | |
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103 | This also needs C<use utf8> currently. [XXX: Why?!? High-bit chars were |
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104 | syntax errors when they occurred within identifiers in previous versions, |
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105 | so this should probably be enabled by default.] |
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106 | |
107 | =item * |
108 | |
109 | Regular expressions match characters instead of bytes. For instance, |
110 | "." matches a character instead of a byte. (However, the C<\C> pattern |
111 | is provided to force a match a single byte ("C<char>" in C, hence |
112 | C<\C>).) |
113 | |
114 | Unicode support in regular expressions needs C<use utf8> currently. |
115 | [XXX: Because the SWASH routines need to be loaded. And the RE engine |
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116 | appears to need an overhaul to dynamically match Unicode anyway--the |
117 | current RE compiler creates different nodes with and without C<use utf8>.] |
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118 | |
119 | =item * |
120 | |
121 | Character classes in regular expressions match characters instead of |
122 | bytes, and match against the character properties specified in the |
123 | Unicode properties database. So C<\w> can be used to match an ideograph, |
124 | for instance. |
125 | |
126 | C<use utf8> is needed to enable this. See above. |
127 | |
128 | =item * |
129 | |
130 | Named Unicode properties and block ranges make be used as character |
131 | classes via the new C<\p{}> (matches property) and C<\P{}> (doesn't |
132 | match property) constructs. For instance, C<\p{Lu}> matches any |
133 | character with the Unicode uppercase property, while C<\p{M}> matches |
134 | any mark character. Single letter properties may omit the brackets, so |
135 | that can be written C<\pM> also. Many predefined character classes are |
136 | available, such as C<\p{IsMirrored}> and C<\p{InTibetan}>. |
137 | |
138 | C<use utf8> is needed to enable this. See above. |
139 | |
140 | =item * |
141 | |
142 | The special pattern C<\X> match matches any extended Unicode sequence |
143 | (a "combining character sequence" in Standardese), where the first |
144 | character is a base character and subsequent characters are mark |
145 | characters that apply to the base character. It is equivalent to |
146 | C<(?:\PM\pM*)>. |
147 | |
148 | C<use utf8> is needed to enable this. See above. |
149 | |
150 | =item * |
151 | |
152 | The C<tr///> operator translates characters instead of bytes. It can also |
153 | be forced to translate between 8-bit codes and UTF-8 regardless of the |
154 | surrounding utf8 state. For instance, if you know your input in Latin-1, |
155 | you can say: |
156 | |
157 | use utf8; |
158 | while (<>) { |
159 | tr/\0-\xff//CU; # latin1 char to utf8 |
160 | ... |
161 | } |
162 | |
163 | Similarly you could translate your output with |
164 | |
165 | tr/\0-\x{ff}//UC; # utf8 to latin1 char |
166 | |
167 | No, C<s///> doesn't take /U or /C (yet?). |
168 | |
169 | C<use utf8> is needed to enable this. See above. |
170 | |
171 | =item * |
172 | |
173 | Case translation operators use the Unicode case translation tables |
174 | when provided character input. Note that C<uc()> translates to |
175 | uppercase, while C<ucfirst> translates to titlecase (for languages |
176 | that make the distinction). Naturally the corresponding backslash |
177 | sequences have the same semantics. |
178 | |
179 | =item * |
180 | |
181 | Most operators that deal with positions or lengths in the string will |
182 | automatically switch to using character positions, including C<chop()>, |
183 | C<substr()>, C<pos()>, C<index()>, C<rindex()>, C<sprintf()>, |
184 | C<write()>, and C<length()>. Operators that specifically don't switch |
185 | include C<vec()>, C<pack()>, and C<unpack()>. Operators that really |
186 | don't care include C<chomp()>, as well as any other operator that |
187 | treats a string as a bucket of bits, such as C<sort()>, and the |
188 | operators dealing with filenames. |
189 | |
190 | =item * |
191 | |
192 | The C<pack()>/C<unpack()> letters "C<c>" and "C<C>" do I<not> change, |
193 | since they're often used for byte-oriented formats. (Again, think |
194 | "C<char>" in the C language.) However, there is a new "C<U>" specifier |
195 | that will convert between UTF-8 characters and integers. (It works |
196 | outside of the utf8 pragma too.) |
197 | |
198 | =item * |
199 | |
200 | The C<chr()> and C<ord()> functions work on characters. This is like |
201 | C<pack("U")> and C<unpack("U")>, not like C<pack("C")> and |
202 | C<unpack("C")>. In fact, the latter are how you now emulate |
203 | byte-oriented C<chr()> and C<ord()> under utf8. |
204 | |
205 | =item * |
206 | |
207 | And finally, C<scalar reverse()> reverses by character rather than by byte. |
208 | |
209 | =back |
210 | |
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211 | =head2 Character encodings for input and output |
212 | |
213 | [XXX: This feature is not yet implemented.] |
214 | |
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215 | =head1 CAVEATS |
216 | |
217 | As of yet, there is no method for automatically coercing input and |
218 | output to some encoding other than UTF-8. This is planned in the near |
219 | future, however. |
220 | |
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221 | Whether an arbitrary piece of data will be treated as "characters" or |
222 | "bytes" by internal operations cannot be divined at the current time. |
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223 | |
224 | Use of locales with utf8 may lead to odd results. Currently there is |
225 | some attempt to apply 8-bit locale info to characters in the range |
226 | 0..255, but this is demonstrably incorrect for locales that use |
227 | characters above that range (when mapped into Unicode). It will also |
228 | tend to run slower. Avoidance of locales is strongly encouraged. |
229 | |
230 | =head1 SEE ALSO |
231 | |
232 | L<byte>, L<utf8>, L<perlvar/"$^U"> |
233 | |
234 | =cut |