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393fec97 GS |
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 | ||
8cbd9a7a GS |
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 | ||
3969a896 GS |
34 | If the C<$^U> global flag is set to C<1>, all system calls will use the |
35 | corresponding wide character APIs. This is currently only implemented | |
36 | on Windows. [XXX: Should there be a -C switch to enable $^U?] | |
8cbd9a7a GS |
37 | |
38 | Regardless of the above, the C<byte> pragma can always be used to force | |
39 | byte semantics in a particular lexical scope. See L<byte>. | |
40 | ||
41 | The C<utf8> pragma is primarily a compatibility device that enables | |
42 | recognition of UTF-8 in literals encountered by the parser. It is also | |
43 | used for enabling some of the more experimental Unicode support features. | |
44 | Note that this pragma is only required until a future version of Perl | |
45 | in which character semantics will become the default. This pragma may | |
46 | then become a no-op. See L<utf8>. | |
47 | ||
48 | Unless mentioned otherwise, Perl operators will use character semantics | |
49 | when they are dealing with Unicode data, and byte semantics otherwise. | |
50 | Thus, character semantics for these operations apply transparently; if | |
51 | the input data came from a Unicode source (for example, by adding a | |
52 | character encoding discipline to the filehandle whence it came, or a | |
53 | literal UTF-8 string constant in the program), character semantics | |
54 | apply; otherwise, byte semantics are in effect. To force byte semantics | |
55 | on Unicode data, the C<byte> pragma should be used. | |
393fec97 GS |
56 | |
57 | Under character semantics, many operations that formerly operated on | |
58 | bytes change to operating on characters. For ASCII data this makes | |
59 | no difference, because UTF-8 stores ASCII in single bytes, but for | |
60 | any character greater than C<chr(127)>, the character is stored in | |
61 | a sequence of two or more bytes, all of which have the high bit set. | |
62 | But by and large, the user need not worry about this, because Perl | |
63 | hides it from the user. A character in Perl is logically just a number | |
64 | ranging from 0 to 2**32 or so. Larger characters encode to longer | |
65 | sequences of bytes internally, but again, this is just an internal | |
66 | detail which is hidden at the Perl level. | |
67 | ||
8cbd9a7a | 68 | =head2 Effects of character semantics |
393fec97 GS |
69 | |
70 | Character semantics have the following effects: | |
71 | ||
72 | =over 4 | |
73 | ||
74 | =item * | |
75 | ||
76 | Strings and patterns may contain characters that have an ordinal value | |
77 | larger than 255. In Perl v5.6, this is only enabled if the lexical | |
78 | scope has a C<use utf8> declaration (due to compatibility needs) but | |
79 | future versions may enable this by default. | |
80 | ||
81 | Presuming you use a Unicode editor to edit your program, such characters | |
82 | will typically occur directly within the literal strings as UTF-8 | |
83 | characters, but you can also specify a particular character with an | |
84 | extension of the C<\x> notation. UTF-8 characters are specified by | |
85 | putting the hexadecimal code within curlies after the C<\x>. For instance, | |
86 | a Unicode smiley face is C<\x{263A}>. A character in the Latin-1 range | |
87 | (128..255) should be written C<\x{ab}> rather than C<\xab>, since the | |
88 | former will turn into a two-byte UTF-8 code, while the latter will | |
89 | continue to be interpreted as generating a 8-bit byte rather than a | |
90 | character. In fact, if C<-w> is turned on, it will produce a warning | |
91 | that you might be generating invalid UTF-8. | |
92 | ||
93 | =item * | |
94 | ||
95 | Identifiers within the Perl script may contain Unicode alphanumeric | |
96 | characters, including ideographs. (You are currently on your own when | |
97 | it comes to using the canonical forms of characters--Perl doesn't (yet) | |
98 | attempt to canonicalize variable names for you.) | |
99 | ||
8cbd9a7a | 100 | This also needs C<use utf8> currently. [XXX: Why?!? High-bit chars were |
393fec97 | 101 | syntax errors when they occurred within identifiers in previous versions, |
8cbd9a7a | 102 | so this should probably be enabled by default.] |
393fec97 GS |
103 | |
104 | =item * | |
105 | ||
106 | Regular expressions match characters instead of bytes. For instance, | |
107 | "." matches a character instead of a byte. (However, the C<\C> pattern | |
108 | is provided to force a match a single byte ("C<char>" in C, hence | |
109 | C<\C>).) | |
110 | ||
111 | Unicode support in regular expressions needs C<use utf8> currently. | |
112 | [XXX: Because the SWASH routines need to be loaded. And the RE engine | |
8cbd9a7a GS |
113 | appears to need an overhaul to dynamically match Unicode anyway--the |
114 | current RE compiler creates different nodes with and without C<use utf8>.] | |
393fec97 GS |
115 | |
116 | =item * | |
117 | ||
118 | Character classes in regular expressions match characters instead of | |
119 | bytes, and match against the character properties specified in the | |
120 | Unicode properties database. So C<\w> can be used to match an ideograph, | |
121 | for instance. | |
122 | ||
123 | C<use utf8> is needed to enable this. See above. | |
124 | ||
125 | =item * | |
126 | ||
127 | Named Unicode properties and block ranges make be used as character | |
128 | classes via the new C<\p{}> (matches property) and C<\P{}> (doesn't | |
129 | match property) constructs. For instance, C<\p{Lu}> matches any | |
130 | character with the Unicode uppercase property, while C<\p{M}> matches | |
131 | any mark character. Single letter properties may omit the brackets, so | |
132 | that can be written C<\pM> also. Many predefined character classes are | |
133 | available, such as C<\p{IsMirrored}> and C<\p{InTibetan}>. | |
134 | ||
135 | C<use utf8> is needed to enable this. See above. | |
136 | ||
137 | =item * | |
138 | ||
139 | The special pattern C<\X> match matches any extended Unicode sequence | |
140 | (a "combining character sequence" in Standardese), where the first | |
141 | character is a base character and subsequent characters are mark | |
142 | characters that apply to the base character. It is equivalent to | |
143 | C<(?:\PM\pM*)>. | |
144 | ||
145 | C<use utf8> is needed to enable this. See above. | |
146 | ||
147 | =item * | |
148 | ||
149 | The C<tr///> operator translates characters instead of bytes. It can also | |
150 | be forced to translate between 8-bit codes and UTF-8 regardless of the | |
151 | surrounding utf8 state. For instance, if you know your input in Latin-1, | |
152 | you can say: | |
153 | ||
154 | use utf8; | |
155 | while (<>) { | |
156 | tr/\0-\xff//CU; # latin1 char to utf8 | |
157 | ... | |
158 | } | |
159 | ||
160 | Similarly you could translate your output with | |
161 | ||
162 | tr/\0-\x{ff}//UC; # utf8 to latin1 char | |
163 | ||
164 | No, C<s///> doesn't take /U or /C (yet?). | |
165 | ||
166 | C<use utf8> is needed to enable this. See above. | |
167 | ||
168 | =item * | |
169 | ||
170 | Case translation operators use the Unicode case translation tables | |
171 | when provided character input. Note that C<uc()> translates to | |
172 | uppercase, while C<ucfirst> translates to titlecase (for languages | |
173 | that make the distinction). Naturally the corresponding backslash | |
174 | sequences have the same semantics. | |
175 | ||
176 | =item * | |
177 | ||
178 | Most operators that deal with positions or lengths in the string will | |
179 | automatically switch to using character positions, including C<chop()>, | |
180 | C<substr()>, C<pos()>, C<index()>, C<rindex()>, C<sprintf()>, | |
181 | C<write()>, and C<length()>. Operators that specifically don't switch | |
182 | include C<vec()>, C<pack()>, and C<unpack()>. Operators that really | |
183 | don't care include C<chomp()>, as well as any other operator that | |
184 | treats a string as a bucket of bits, such as C<sort()>, and the | |
185 | operators dealing with filenames. | |
186 | ||
187 | =item * | |
188 | ||
189 | The C<pack()>/C<unpack()> letters "C<c>" and "C<C>" do I<not> change, | |
190 | since they're often used for byte-oriented formats. (Again, think | |
191 | "C<char>" in the C language.) However, there is a new "C<U>" specifier | |
192 | that will convert between UTF-8 characters and integers. (It works | |
193 | outside of the utf8 pragma too.) | |
194 | ||
195 | =item * | |
196 | ||
197 | The C<chr()> and C<ord()> functions work on characters. This is like | |
198 | C<pack("U")> and C<unpack("U")>, not like C<pack("C")> and | |
199 | C<unpack("C")>. In fact, the latter are how you now emulate | |
200 | byte-oriented C<chr()> and C<ord()> under utf8. | |
201 | ||
202 | =item * | |
203 | ||
204 | And finally, C<scalar reverse()> reverses by character rather than by byte. | |
205 | ||
206 | =back | |
207 | ||
8cbd9a7a GS |
208 | =head2 Character encodings for input and output |
209 | ||
210 | [XXX: This feature is not yet implemented.] | |
211 | ||
393fec97 GS |
212 | =head1 CAVEATS |
213 | ||
214 | As of yet, there is no method for automatically coercing input and | |
215 | output to some encoding other than UTF-8. This is planned in the near | |
216 | future, however. | |
217 | ||
8cbd9a7a GS |
218 | Whether an arbitrary piece of data will be treated as "characters" or |
219 | "bytes" by internal operations cannot be divined at the current time. | |
393fec97 GS |
220 | |
221 | Use of locales with utf8 may lead to odd results. Currently there is | |
222 | some attempt to apply 8-bit locale info to characters in the range | |
223 | 0..255, but this is demonstrably incorrect for locales that use | |
224 | characters above that range (when mapped into Unicode). It will also | |
225 | tend to run slower. Avoidance of locales is strongly encouraged. | |
226 | ||
227 | =head1 SEE ALSO | |
228 | ||
229 | L<byte>, L<utf8>, L<perlvar/"$^U"> | |
230 | ||
231 | =cut |