NAME
perlunifaq - Perl Unicode FAQ
Q and A
This is a list of questions and answers about Unicode in Perl, intended to be read after perlunitut.
perlunitut isn't really a Unicode tutorial, is it?
No, and this isnt really a Unicode FAQ.
Perl has an abstracted interface for all supported character encodings, so they is actually a generic
Encodetutorial and
EncodeFAQ. But many people think that Unicode is special and magical, and I didnt want to disappoint them, so I decided to call the document a Unicode tutorial.
What character encodings does Perl support?
To find out which character encodings your Perl supports, run:
perl -MEncode -le "print for Encode->encodings(:all)"
Which version of perl should I use?
Well, if you can, upgrade to the most recent, but certainly
5.8.1or newer. The tutorial and FAQ are based on the status quo as of
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.
What about binary data, like images?
Well, apart from a bare
binmode $fh, you shouldnt treat them specially. (The binmode is needed because otherwise Perl may convert line endings on Win32 systems.)
Be careful, though, to never combine text strings with binary strings. If you 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 dont encode?.
When should I decode or encode?
Whenever youre 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 youre communicating with is also written in Perl.
What if I don't decode?
Whenever your encoded, binary string is used together with a text string, Perl will assume that your binary string was encoded with ISO-8859-1, also known as latin-1. If it wasnt latin-1, then your data is unpleasantly converted. For example, if it was UTF-8, the individual bytes of multibyte characters are seen as separate characters, and then again converted to UTF-8. Such double encoding can be compared to double HTML encoding (
>), or double URI encoding (
%253E).
This silent implicit decoding is known as upgrading. That may sound positive, but its best to avoid it.
What if I don't encode?
Your text string will be sent using the bytes in Perls internal format. In some cases, Perl will warn you that youre doing something wrong, with a friendly warning:
Wide character in print at example.pl line 2.
Because the internal format is often UTF-8, these bugs are hard to spot, because UTF-8 is usually the encoding you wanted! But dont be lazy, and dont use the fact that Perls internal format is UTF-8 to your advantage. Encode explicitly to avoid weird bugs, and to show to maintenance programmers that you thought this through.
Is there a way to automatically decode or encode?
If all data that comes from a certain handle is encoded in exactly the same way, you can tell the PerlIO system to automatically decode everything, with the
encodinglayer. If you do this, you cant accidentally forget to decode or encode anymore, on things that use the layered handle.
You can provide this layer when
opening the file:
open my $fh, >:encoding(UTF-8), $filename; # auto encoding on write
open my $fh, <:encoding(UTF-8), $filename; # auto decoding on read
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 sometimes limited to the UTF-8 encoding.
What if I don't know which encoding was used?
Do whatever you can to find out, and if you have to: guess. (Dont forget to document your guess with a comment.)
You could open the document in a web browser, and change the character set or character encoding until you can visually confirm that all characters look the way they should.
There is no way to reliably detect the encoding automatically, so if people keep sending you data without charset indication, you may have to educate them.
Can I use Unicode in my Perl sources?
Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the
use utf8pragma.
use utf8;
This doesnt do anything to your input, or to your output. It only influences the way your sources are read. You can use Unicode in string literals, in identifiers (but they still have to be word characters according to
\w), and even in custom delimiters.
Data::Dumper doesn't restore the \s-1UTF8\s0 flag; is it broken?
No, Data::Dumpers Unicode abilities are as they should be. There have been some complaints that it should restore the UTF8 flag when the data is read again with
eval. However, you should really not look at the flag, and nothing indicates that Data::Dumper should break this rule.
Heres what happens: when Perl reads in a string literal, it sticks to 8 bit encoding as long as it can. (But perhaps originally it was internally encoded as UTF-8, when you dumped it.) When it has to give that up because other characters are added to the text string, it silently upgrades the string to UTF-8.
If you properly encode your strings for output, none of this is of your concern, and you can just
evaldumped data as always.
Why do regex character classes sometimes match only in the \s-1ASCII\s0 range?
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
uc,
lc,
ucfirst,
lcfirst,
\U,
\L,
\u,
\l,
\d,
\s,
\w,
\D,
\S,
\W,
/.../i,
(?i:...),
/[[:posix:]]/.
To force Unicode semantics, you can upgrade the internal representation to by doing
utf8::upgrade($string). This does not change strings that were already upgraded.
For a more detailed discussion, see Unicode::Semantics on CPAN.
How can I determine if a string is a text string or a binary string?
You cant. Some use the UTF8 flag for this, but thats misuse, and makes well behaved modules like Data::Dumper look bad. The flag is useless for this purpose, because its off when an 8 bit encoding (by default ISO-8859-1) is used to store the string.
This is something you, the programmer, has to keep track of; sorry. You could consider adopting a kind of Hungarian notation to help with this.
How do I convert from encoding \s-1FOO\s0 to encoding \s-1BAR\s0?
By first converting the FOO-encoded byte string to a text string, and then the text string to a BAR-encoded byte string:
my $text_string = decode(FOO, $foo_string);
my $bar_string = encode(BAR, $text_string);
or by skipping the text string part, and going directly from one binary encoding to the other:
use Encode qw(from_to);
from_to($string, FOO, BAR); # changes contents of $string
or by letting automatic decoding and encoding do all the work:
open my $foofh, <:encoding(FOO), example.foo.txt;
open my $barfh, >:encoding(BAR), example.bar.txt;
print { $barfh } $_ while <$foofh>;
What are \f(CWdecode_utf8\fP and \f(CWencode_utf8\fP?
These are alternate syntaxes for
decode(utf8, ...)and
encode(utf8, ...).
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 cant, 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)";
INTERNALS
What is ``the \s-1UTF8\s0 flag''?
Please, unless youre hacking the internals, or debugging weirdness, dont think about the UTF8 flag at all. That means that you very probably shouldnt use
is_utf8,
_utf8_onor
_utf8_offat 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 Perls internal formats happens to be UTF-8. Unfortunately, Perl cant keep a secret, so everyone knows about this. That is the source of much confusion. Its better to pretend that the internal format is some unknown encoding, and that you always have to encode and decode explicitly.
What about the \f(CWuse bytes\fP pragma?
Dont use it. It makes no sense to deal with bytes in a text string, and it makes no sense to deal with characters in a byte string. Do the proper conversions (by decoding/encoding), and things will work out well: you get character counts for decoded data, and byte counts for encoded data.
use bytesis usually a failed attempt to do something useful. Just forget about it.
What about the \f(CWuse encoding\fP pragma?
Dont use it. Unfortunately, it assumes that the programmers 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
use utf8.
If you need to set the encoding for STDIN, STDOUT, and STDERR, for example based on the users locale,
use open.
What is the difference between \f(CW:encoding\fP and \f(CW:utf8\fP?
Because UTF-8 is one of Perls internal formats, you can often just skip the encoding or decoding step, and manipulate the UTF8 flag directly.
Instead of
:encoding(UTF-8), you can simply use
:utf8, which skips the encoding step if the data was already represented as UTF8 internally. This is widely accepted as good behavior when youre writing, but it can be dangerous when reading, because it causes internal inconsistency when you have invalid byte sequences. Using
:utf8for input can sometimes result in security breaches, so please use
:encoding(UTF-8)instead.
Instead of
decodeand
encode, you could use
_utf8_onand
_utf8_off, but this is considered bad style. Especially
_utf8_oncan be dangerous, for the same reason that
:utf8can.
There are some shortcuts for oneliners; see
-Cin perlrun.
What's the difference between \f(CWUTF\-8\fP and \f(CWutf8\fP?
UTF-8is the official standard.
utf8is Perls way of being liberal in what it accepts. If you have to communicate with things that arent so liberal, you may want to consider using
UTF-8. If you have to communicate with things that are too liberal, you may have to use
utf8. The full explanation is in Encode.
UTF-8is internally known as
utf-8-strict. The tutorial uses UTF-8 consistently, even where utf8 is actually used internally, because the distinction can be hard to make, and is mostly irrelevant.
For example, utf8 can be used for code points that dont exist in Unicode, like 9999999, but if you encode that to UTF-8, you get a substitution character (by default; see Handling Malformed Data in Encode for more ways of dealing with this.)
Okay, if you insist: the internal format is utf8, not UTF-8. (When its not some other encoding.)
I lost track; what encoding is the internal format really?
Its good that you lost track, because you shouldnt depend on the internal format being any specific encoding. But since you asked: by default, the internal format is either ISO-8859-1 (latin-1), or utf8, depending on the history of the string. On EBCDIC platforms, this may be different even.
Perl knows how it stored the string internally, and will use that knowledge when you
encode. In other words: dont try to find out what the internal encoding for a certain string is, but instead just encode it into the encoding that you want.
AUTHOR
Juerd Waalboer <#####@juerd.nl>
SEE ALSO
perlunicode, perluniintro, Encode