X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlunifaq.pod;h=b2913349cc4d27c6d58785151d91e26253147b9a;hb=03f3e794d79fbb3655a305de34b922dd4020d2ec;hp=4b2290abbf239cd33089008e89167950ad4fcc60;hpb=2575c402a8f9be55f848bdfb219afbf912c50ac1;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlunifaq.pod b/pod/perlunifaq.pod index 4b2290a..b291334 100644 --- a/pod/perlunifaq.pod +++ b/pod/perlunifaq.pod @@ -2,7 +2,7 @@ perlunifaq - Perl Unicode FAQ -=head1 DESCRIPTION +=head1 Q and A This is a list of questions and answers about Unicode in Perl, intended to be read after L. @@ -16,6 +16,21 @@ is actually a generic C tutorial and C FAQ. But many people think that Unicode is special and magical, and I didn't want to disappoint them, so I decided to call the document a Unicode tutorial. +=head2 What character encodings does Perl support? + +To find out which character encodings your Perl supports, run: + + perl -MEncode -le "print for Encode->encodings(':all')" + +=head2 Which version of perl should I use? + +Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer. +The tutorial and FAQ are based on the status quo as of C<5.8.8>. + +You should also check your modules, and upgrade them if necessary. For example, +HTML::Entities requires version >= 1.32 to function correctly, even though the +changelog is silent about this. + =head2 What about binary data, like images? Well, apart from a bare C, you shouldn't treat them specially. @@ -27,20 +42,9 @@ need text in a binary stream, encode your text strings first using the appropriate encoding, then join them with binary strings. See also: "What if I don't encode?". -=head2 What about the UTF8 flag? - -Please, unless you're hacking the internals, or debugging weirdness, don't -think about the UTF8 flag at all. That means that you very probably shouldn't -use C, C<_utf8_on> or C<_utf8_off> at all. - -Perl's internal format happens to be UTF-8. Unfortunately, Perl can't keep a -secret, so everyone knows about this. That is the source of much confusion. -It's better to pretend that the internal format is some unknown encoding, -and that you always have to encode and decode explicitly. - =head2 When should I decode or encode? -Whenever you're communicating with anything that is external to your perl +Whenever you're communicating text with anything that is external to your perl process, like a database, a text file, a socket, or another program. Even if the thing you're communicating with is also written in Perl. @@ -88,23 +92,7 @@ Or if you already have an open filehandle: binmode $fh, ':encoding(UTF-8)'; Some database drivers for DBI can also automatically encode and decode, but -that is typically limited to the UTF-8 encoding, because they cheat. - -=head2 Cheat?! Tell me, how can I cheat? - -Well, because Perl's internal format is UTF-8, you can just skip the encoding -or decoding step, and manipulate the UTF8 flag directly. - -Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>. This is widely -accepted as good behavior when you're writing, but it can be dangerous when -reading, because it causes internal inconsistency when you have invalid byte -sequences. - -Instead of C and C, you could use C<_utf8_on> and C<_utf8_off>, -but this is considered bad style. Especially C<_utf8_on> can be dangerous, for -the same reason that C<:utf8> can. - -There are some shortcuts for oneliners; see C<-C> in L. +that is sometimes limited to the UTF-8 encoding. =head2 What if I don't know which encoding was used? @@ -146,6 +134,25 @@ UTF-8. If you properly encode your strings for output, none of this is of your concern, and you can just C dumped data as always. +=head2 Why do regex character classes sometimes match only in the ASCII range? + +=head2 Why do some characters not uppercase or lowercase correctly? + +It seemed like a good idea at the time, to keep the semantics the same for +standard strings, when Perl got Unicode support. While it might be repaired +in the future, we now have to deal with the fact that Perl treats equal +strings differently, depending on the internal state. + +Affected are C, C, C, C, C<\U>, C<\L>, C<\u>, C<\l>, +C<\d>, C<\s>, C<\w>, C<\D>, C<\S>, C<\W>, C, C<(?i:...)>, +C. + +To force Unicode semantics, you can upgrade the internal representation to +by doing C. This does not change strings that were +already upgraded. + +For a more detailed discussion, see L on CPAN. + =head2 How can I determine if a string is a text string or a binary string? You can't. Some use the UTF8 flag for this, but that's misuse, and makes well @@ -176,6 +183,45 @@ or by letting automatic decoding and encoding do all the work: open my $barfh, '>:encoding(BAR)', 'example.bar.txt'; print { $barfh } $_ while <$foofh>; +=head2 What are C and C? + +These are alternate syntaxes for C and C. + +=head2 What is a "wide character"? + +This is a term used both for characters with an ordinal value greater than 127, +characters with an ordinal value greater than 255, or any character occupying +than one byte, depending on the context. + +The Perl warning "Wide character in ..." is caused by a character with an +ordinal value greater than 255. With no specified encoding layer, Perl tries to +fit things in ISO-8859-1 for backward compatibility reasons. When it can't, it +emits this warning (if warnings are enabled), and outputs UTF-8 encoded data +instead. + +To avoid this warning and to avoid having different output encodings in a single +stream, always specify an encoding explicitly, for example with a PerlIO layer: + + binmode STDOUT, ":encoding(UTF-8)"; + +=head1 INTERNALS + +=head2 What is "the UTF8 flag"? + +Please, unless you're hacking the internals, or debugging weirdness, don't +think about the UTF8 flag at all. That means that you very probably shouldn't +use C, C<_utf8_on> or C<_utf8_off> at all. + +The UTF8 flag, also called SvUTF8, is an internal flag that indicates that the +current internal representation is UTF-8. Without the flag, it is assumed to be +ISO-8859-1. Perl converts between these automatically. + +One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl can't +keep a secret, so everyone knows about this. That is the source of much +confusion. It's better to pretend that the internal format is some unknown +encoding, and that you always have to encode and decode explicitly. + =head2 What about the C pragma? Don't use it. It makes no sense to deal with bytes in a text string, and it @@ -186,10 +232,36 @@ character counts for decoded data, and byte counts for encoded data. C is usually a failed attempt to do something useful. Just forget about it. -=head2 What are C and C? +=head2 What about the C pragma? -These are alternate syntaxes for C and C. +Don't use it. Unfortunately, it assumes that the programmer's environment and +that of the user will use the same encoding. It will use the same encoding for +the source code and for STDIN and STDOUT. When a program is copied to another +machine, the source code does not change, but the STDIO environment might. + +If you need non-ASCII characters in your source code, make it a UTF-8 encoded +file and C. + +If you need to set the encoding for STDIN, STDOUT, and STDERR, for example +based on the user's locale, C. + +=head2 What is the difference between C<:encoding> and C<:utf8>? + +Because UTF-8 is one of Perl's internal formats, you can often just skip the +encoding or decoding step, and manipulate the UTF8 flag directly. + +Instead of C<:encoding(UTF-8)>, you can simply use C<:utf8>, which skips the +encoding step if the data was already represented as UTF8 internally. This is +widely accepted as good behavior when you're writing, but it can be dangerous +when reading, because it causes internal inconsistency when you have invalid +byte sequences. Using C<:utf8> for input can sometimes result in security +breaches, so please use C<:encoding(UTF-8)> instead. + +Instead of C and C, you could use C<_utf8_on> and C<_utf8_off>, +but this is considered bad style. Especially C<_utf8_on> can be dangerous, for +the same reason that C<:utf8> can. + +There are some shortcuts for oneliners; see C<-C> in L. =head2 What's the difference between C and C? @@ -223,24 +295,9 @@ when you C. In other words: don't try to find out what the internal encoding for a certain string is, but instead just encode it into the encoding that you want. -=head2 What character encodings does Perl support? - -To find out which character encodings your Perl supports, run: - - perl -MEncode -le "print for Encode->encodings(':all')" - -=head2 Which version of perl should I use? - -Well, if you can, upgrade to the most recent, but certainly C<5.8.1> or newer. -The tutorial and FAQ are based on the status quo as of C<5.8.8>. - -You should also check your modules, and upgrade them if necessary. For example, -HTML::Entities requires version >= 1.32 to function correctly, even though the -changelog is silent about this. - =head1 AUTHOR -Juerd Waalboer +Juerd Waalboer <#####@juerd.nl> =head1 SEE ALSO