| 1 | =head1 NAME |
| 2 | |
| 3 | perlunifaq - Perl Unicode FAQ |
| 4 | |
| 5 | =head1 DESCRIPTION |
| 6 | |
| 7 | This is a list of questions and answers about Unicode in Perl, intended to be |
| 8 | read after L<perlunitut>. |
| 9 | |
| 10 | =head2 perlunitut isn't really a Unicode tutorial, is it? |
| 11 | |
| 12 | No, and this isn't really a Unicode FAQ. |
| 13 | |
| 14 | Perl has an abstracted interface for all supported character encodings, so they |
| 15 | is actually a generic C<Encode> tutorial and C<Encode> FAQ. But many people |
| 16 | think that Unicode is special and magical, and I didn't want to disappoint |
| 17 | them, so I decided to call the document a Unicode tutorial. |
| 18 | |
| 19 | =head2 What about binary data, like images? |
| 20 | |
| 21 | Well, apart from a bare C<binmode $fh>, you shouldn't treat them specially. |
| 22 | (The binmode is needed because otherwise Perl may convert line endings on Win32 |
| 23 | systems.) |
| 24 | |
| 25 | Be careful, though, to never combine text strings with binary strings. If you |
| 26 | need text in a binary stream, encode your text strings first using the |
| 27 | appropriate encoding, then join them with binary strings. See also: "What if I |
| 28 | don't encode?". |
| 29 | |
| 30 | =head2 What about the UTF8 flag? |
| 31 | |
| 32 | Please, unless you're hacking the internals, or debugging weirdness, don't |
| 33 | think about the UTF8 flag at all. That means that you very probably shouldn't |
| 34 | use C<is_utf8>, C<_utf8_on> or C<_utf8_off> at all. |
| 35 | |
| 36 | Perl's internal format happens to be UTF-8. Unfortunately, Perl can't keep a |
| 37 | secret, so everyone knows about this. That is the source of much confusion. |
| 38 | It's better to pretend that the internal format is some unknown encoding, |
| 39 | and that you always have to encode and decode explicitly. |
| 40 | |
| 41 | =head2 When should I decode or encode? |
| 42 | |
| 43 | Whenever you're communicating with anything that is external to your perl |
| 44 | process, like a database, a text file, a socket, or another program. Even if |
| 45 | the thing you're communicating with is also written in Perl. |
| 46 | |
| 47 | =head2 What if I don't decode? |
| 48 | |
| 49 | Whenever your encoded, binary string is used together with a text string, Perl |
| 50 | will assume that your binary string was encoded with ISO-8859-1, also known as |
| 51 | latin-1. If it wasn't latin-1, then your data is unpleasantly converted. For |
| 52 | example, if it was UTF-8, the individual bytes of multibyte characters are seen |
| 53 | as separate characters, and then again converted to UTF-8. Such double encoding |
| 54 | can be compared to double HTML encoding (C<&gt;>), or double URI encoding |
| 55 | (C<%253E>). |
| 56 | |
| 57 | This silent implicit decoding is known as "upgrading". That may sound |
| 58 | positive, but it's best to avoid it. |
| 59 | |
| 60 | =head2 What if I don't encode? |
| 61 | |
| 62 | Your text string will be sent using the bytes in Perl's internal format. In |
| 63 | some cases, Perl will warn you that you're doing something wrong, with a |
| 64 | friendly warning: |
| 65 | |
| 66 | Wide character in print at example.pl line 2. |
| 67 | |
| 68 | Because the internal format is often UTF-8, these bugs are hard to spot, |
| 69 | because UTF-8 is usually the encoding you wanted! But don't be lazy, and don't |
| 70 | use the fact that Perl's internal format is UTF-8 to your advantage. Encode |
| 71 | explicitly to avoid weird bugs, and to show to maintenance programmers that you |
| 72 | thought this through. |
| 73 | |
| 74 | =head2 Is there a way to automatically decode or encode? |
| 75 | |
| 76 | If all data that comes from a certain handle is encoded in exactly the same |
| 77 | way, you can tell the PerlIO system to automatically decode everything, with |
| 78 | the C<encoding> layer. If you do this, you can't accidentally forget to decode |
| 79 | or encode anymore, on things that use the layered handle. |
| 80 | |
| 81 | You can provide this layer when C<open>ing the file: |
| 82 | |
| 83 | open my $fh, '>:encoding(UTF-8)', $filename; # auto encoding on write |
| 84 | open my $fh, '<:encoding(UTF-8)', $filename; # auto decoding on read |
| 85 | |
| 86 | Or if you already have an open filehandle: |
| 87 | |
| 88 | binmode $fh, ':encoding(UTF-8)'; |
| 89 | |
| 90 | Some database drivers for DBI can also automatically encode and decode, but |
| 91 | that is typically limited to the UTF-8 encoding, because they cheat. |
| 92 | |
| 93 | =head2 Cheat?! Tell me, how can I cheat? |
| 94 | |
| 95 | Well, because Perl's internal format is UTF-8, you can just skip the encoding |
| 96 | or decoding step, and manipulate the UTF8 flag directly. |
| 97 | |
| 98 | Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>. This is widely |
| 99 | accepted as good behavior when you're writing, but it can be dangerous when |
| 100 | reading, because it causes internal inconsistency when you have invalid byte |
| 101 | sequences. |
| 102 | |
| 103 | Instead of C<decode> and C<encode>, you could use C<_utf8_on> and C<_utf8_off>, |
| 104 | but this is considered bad style. Especially C<_utf8_on> can be dangerous, for |
| 105 | the same reason that C<:utf8> can. |
| 106 | |
| 107 | There are some shortcuts for oneliners; see C<-C> in L<perlrun>. |
| 108 | |
| 109 | =head2 What if I don't know which encoding was used? |
| 110 | |
| 111 | Do whatever you can to find out, and if you have to: guess. (Don't forget to |
| 112 | document your guess with a comment.) |
| 113 | |
| 114 | You could open the document in a web browser, and change the character set or |
| 115 | character encoding until you can visually confirm that all characters look the |
| 116 | way they should. |
| 117 | |
| 118 | There is no way to reliably detect the encoding automatically, so if people |
| 119 | keep sending you data without charset indication, you may have to educate them. |
| 120 | |
| 121 | =head2 Can I use Unicode in my Perl sources? |
| 122 | |
| 123 | Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the |
| 124 | C<use utf8> pragma. |
| 125 | |
| 126 | use utf8; |
| 127 | |
| 128 | This doesn't do anything to your input, or to your output. It only influences |
| 129 | the way your sources are read. You can use Unicode in string literals, in |
| 130 | identifiers (but they still have to be "word characters" according to C<\w>), |
| 131 | and even in custom delimiters. |
| 132 | |
| 133 | =head2 Data::Dumper doesn't restore the UTF8 flag; is it broken? |
| 134 | |
| 135 | No, Data::Dumper's Unicode abilities are as they should be. There have been |
| 136 | some complaints that it should restore the UTF8 flag when the data is read |
| 137 | again with C<eval>. However, you should really not look at the flag, and |
| 138 | nothing indicates that Data::Dumper should break this rule. |
| 139 | |
| 140 | Here's what happens: when Perl reads in a string literal, it sticks to 8 bit |
| 141 | encoding as long as it can. (But perhaps originally it was internally encoded |
| 142 | as UTF-8, when you dumped it.) When it has to give that up because other |
| 143 | characters are added to the text string, it silently upgrades the string to |
| 144 | UTF-8. |
| 145 | |
| 146 | If you properly encode your strings for output, none of this is of your |
| 147 | concern, and you can just C<eval> dumped data as always. |
| 148 | |
| 149 | =head2 How can I determine if a string is a text string or a binary string? |
| 150 | |
| 151 | You can't. Some use the UTF8 flag for this, but that's misuse, and makes well |
| 152 | behaved modules like Data::Dumper look bad. The flag is useless for this |
| 153 | purpose, because it's off when an 8 bit encoding (by default ISO-8859-1) is |
| 154 | used to store the string. |
| 155 | |
| 156 | This is something you, the programmer, has to keep track of; sorry. You could |
| 157 | consider adopting a kind of "Hungarian notation" to help with this. |
| 158 | |
| 159 | =head2 How do I convert from encoding FOO to encoding BAR? |
| 160 | |
| 161 | By first converting the FOO-encoded byte string to a text string, and then the |
| 162 | text string to a BAR-encoded byte string: |
| 163 | |
| 164 | my $text_string = decode('FOO', $foo_string); |
| 165 | my $bar_string = encode('BAR', $text_string); |
| 166 | |
| 167 | or by skipping the text string part, and going directly from one binary |
| 168 | encoding to the other: |
| 169 | |
| 170 | use Encode qw(from_to); |
| 171 | from_to($string, 'FOO', 'BAR'); # changes contents of $string |
| 172 | |
| 173 | or by letting automatic decoding and encoding do all the work: |
| 174 | |
| 175 | open my $foofh, '<:encoding(FOO)', 'example.foo.txt'; |
| 176 | open my $barfh, '>:encoding(BAR)', 'example.bar.txt'; |
| 177 | print { $barfh } $_ while <$foofh>; |
| 178 | |
| 179 | =head2 What about the C<use bytes> pragma? |
| 180 | |
| 181 | Don't use it. It makes no sense to deal with bytes in a text string, and it |
| 182 | makes no sense to deal with characters in a byte string. Do the proper |
| 183 | conversions (by decoding/encoding), and things will work out well: you get |
| 184 | character counts for decoded data, and byte counts for encoded data. |
| 185 | |
| 186 | C<use bytes> is usually a failed attempt to do something useful. Just forget |
| 187 | about it. |
| 188 | |
| 189 | =head2 What are C<decode_utf8> and C<encode_utf8>? |
| 190 | |
| 191 | These are alternate syntaxes for C<decode('utf8', ...)> and C<encode('utf8', |
| 192 | ...)>. |
| 193 | |
| 194 | =head2 What's the difference between C<UTF-8> and C<utf8>? |
| 195 | |
| 196 | C<UTF-8> is the official standard. C<utf8> is Perl's way of being liberal in |
| 197 | what it accepts. If you have to communicate with things that aren't so liberal, |
| 198 | you may want to consider using C<UTF-8>. If you have to communicate with things |
| 199 | that are too liberal, you may have to use C<utf8>. The full explanation is in |
| 200 | L<Encode>. |
| 201 | |
| 202 | C<UTF-8> is internally known as C<utf-8-strict>. The tutorial uses UTF-8 |
| 203 | consistently, even where utf8 is actually used internally, because the |
| 204 | distinction can be hard to make, and is mostly irrelevant. |
| 205 | |
| 206 | For example, utf8 can be used for code points that don't exist in Unicode, like |
| 207 | 9999999, but if you encode that to UTF-8, you get a substitution character (by |
| 208 | default; see L<Encode/"Handling Malformed Data"> for more ways of dealing with |
| 209 | this.) |
| 210 | |
| 211 | Okay, if you insist: the "internal format" is utf8, not UTF-8. (When it's not |
| 212 | some other encoding.) |
| 213 | |
| 214 | =head2 I lost track; what encoding is the internal format really? |
| 215 | |
| 216 | It's good that you lost track, because you shouldn't depend on the internal |
| 217 | format being any specific encoding. But since you asked: by default, the |
| 218 | internal format is either ISO-8859-1 (latin-1), or utf8, depending on the |
| 219 | history of the string. On EBCDIC platforms, this may be different even. |
| 220 | |
| 221 | Perl knows how it stored the string internally, and will use that knowledge |
| 222 | when you C<encode>. In other words: don't try to find out what the internal |
| 223 | encoding for a certain string is, but instead just encode it into the encoding |
| 224 | that you want. |
| 225 | |
| 226 | =head2 What character encodings does Perl support? |
| 227 | |
| 228 | To find out which character encodings your Perl supports, run: |
| 229 | |
| 230 | perl -MEncode -le "print for Encode->encodings(':all')" |
| 231 | |
| 232 | =head2 Which version of perl should I use? |
| 233 | |
| 234 | Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer. |
| 235 | The tutorial and FAQ are based on the status quo as of C<5.8.8>. |
| 236 | |
| 237 | You should also check your modules, and upgrade them if necessary. For example, |
| 238 | HTML::Entities requires version >= 1.32 to function correctly, even though the |
| 239 | changelog is silent about this. |
| 240 | |
| 241 | =head1 AUTHOR |
| 242 | |
| 243 | Juerd Waalboer <juerd@cpan.org> |
| 244 | |
| 245 | =head1 SEE ALSO |
| 246 | |
| 247 | L<perlunicode>, L<perluniintro>, L<Encode> |
| 248 | |