PERLUNIFAQ(1) Perl Programmers Reference Guide PERLUNIFAQ(1) #
PERLUNIFAQ(1) Perl Programmers Reference Guide PERLUNIFAQ(1)
NNAAMMEE #
perlunifaq - Perl Unicode FAQ
QQ aanndd AA This is a list of questions and answers about Unicode in Perl, intended to be read after perlunitut.
ppeerrlluunniittuutt iissnn’’tt rreeaallllyy aa UUnniiccooddee ttuuttoorriiaall,, iiss iitt?? No, and this isn’t really a Unicode FAQ.
Perl has an abstracted interface for all supported character encodings,
so this is actually a generic "Encode" tutorial and "Encode" 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.
WWhhaatt cchhaarraacctteerr eennccooddiinnggss ddooeess PPeerrll ssuuppppoorrtt?? To find out which character encodings your Perl supports, run:
perl -MEncode -le "print for Encode->encodings(':all')"
WWhhiicchh vveerrssiioonn ooff ppeerrll sshhoouulldd II uussee?? Well, if you can, upgrade to the most recent, but certainly 5.8.1 or newer. The tutorial and FAQ assume the latest release.
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.
WWhhaatt aabboouutt bbiinnaarryy ddaattaa,, lliikkee iimmaaggeess?? Well, apart from a bare “binmode $fh”, you shouldn’t 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 don't encode?".
WWhheenn sshhoouulldd II ddeeccooddee oorr eennccooddee?? 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.
WWhhaatt iiff II ddoonn’’tt ddeeccooddee?? 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 wasn’t 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 it's best to avoid it.
WWhhaatt iiff II ddoonn’’tt eennccooddee?? It depends on what you output and how you output it.
_O_u_t_p_u_t _v_i_a _a _f_i_l_e_h_a_n_d_l_e
• If the string's characters are all code point 255 or lower, Perl
outputs bytes that match those code points. This is what happens with
encoded strings. It can also, though, happen with unencoded strings
that happen to be all code point 255 or lower.
• Otherwise, Perl outputs the string encoded as UTF-8. This only
happens with strings you neglected to encode. Since that should not
happen, Perl also throws a "wide character" warning in this case.
_O_t_h_e_r _o_u_t_p_u_t _m_e_c_h_a_n_i_s_m_s _(_e_._g_._, _"_e_x_e_c_"_, _"_c_h_d_i_r_"_, _._._)
Your text string will be sent using the bytes in Perl's internal format.
Because the internal format is often UTF-8, these bugs are hard to spot,
because UTF-8 is usually the encoding you wanted! But don't be lazy, and
don't use the fact that Perl's 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.
IIss tthheerree aa wwaayy ttoo aauuttoommaattiiccaallllyy ddeeccooddee oorr eennccooddee?? 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 “encoding” layer. If you do this, you can’t accidentally forget to decode or encode anymore, on things that use the layered handle.
You can provide this layer when "open"ing 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.
WWhhaatt iiff II ddoonn’’tt kknnooww wwhhiicchh eennccooddiinngg wwaass uusseedd?? Do whatever you can to find out, and if you have to: guess. (Don’t 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.
CCaann II uussee UUnniiccooddee iinn mmyy PPeerrll ssoouurrcceess?? Yes, you can! If your sources are UTF-8 encoded, you can indicate that with the “use utf8” pragma.
use utf8;
This doesn't 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.
DDaattaa::::DDuummppeerr ddooeessnn’’tt rreessttoorree tthhee UUTTFF88 ffllaagg;; iiss iitt bbrrookkeenn?? No, Data::Dumper’s 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.
Here's 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 "eval" dumped data as always.
WWhhyy ddoo rreeggeexx cchhaarraacctteerr ccllaasssseess ssoommeettiimmeess mmaattcchh oonnllyy iinn tthhee AASSCCIIII rraannggee?? Starting in Perl 5.14 (and partially in Perl 5.12), just put a “use feature ‘unicode_strings’” near the beginning of your program. Within its lexical scope you shouldn’t have this problem. It also is automatically enabled under “use feature ‘:5.12’” or “use v5.12” or using “-E” on the command line for Perl 5.12 or higher.
The rationale for requiring this is to not break older programs that rely
on the way things worked before Unicode came along. Those older programs
knew only about the ASCII character set, and so may not work properly for
additional characters. When a string is encoded in UTF-8, Perl assumes
that the program is prepared to deal with Unicode, but when the string
isn't, Perl assumes that only ASCII is wanted, and so those characters
that are not ASCII characters aren't recognized as to what they would be
in Unicode. "use feature 'unicode_strings'" tells Perl to treat all
characters as Unicode, whether the string is encoded in UTF-8 or not,
thus avoiding the problem.
However, on earlier Perls, or if you pass strings to subroutines outside
the feature's scope, you can force Unicode rules by changing the encoding
to UTF-8 by doing "utf8::upgrade($string)". This can be used safely on
any string, as it checks and does not change strings that have already
been upgraded.
For a more detailed discussion, see Unicode::Semantics on CPAN.
WWhhyy ddoo ssoommee cchhaarraacctteerrss nnoott uuppppeerrccaassee oorr lloowweerrccaassee ccoorrrreeccttllyy?? See the answer to the previous question.
HHooww ccaann II ddeetteerrmmiinnee iiff aa ssttrriinngg iiss aa tteexxtt ssttrriinngg oorr aa bbiinnaarryy ssttrriinngg?? You can’t. Some use the UTF8 flag for this, but that’s misuse, and makes well behaved modules like Data::Dumper look bad. The flag is useless for this purpose, because it’s 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.
HHooww ddoo II ccoonnvveerrtt ffrroomm eennccooddiinngg FFOOOO ttoo eennccooddiinngg BBAARR?? 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>;
WWhhaatt aarree “"ddeeccooddee__uuttff88"” aanndd “"eennccooddee__uuttff88"”?? These are alternate syntaxes for “decode(‘utf8’, …)” and “encode(‘utf8’, …)”. Do not use these functions for data exchange. Instead use “decode(‘UTF-8’, …)” and “encode(‘UTF-8’, …)”; see “What’s the difference between UTF-8 and utf8?” below.
WWhhaatt iiss aa “"wwiiddee cchhaarraacctteerr"”?? This is a term used for characters occupying more than one byte.
The Perl warning "Wide character in ..." is caused by such a character.
With no specified encoding layer, Perl tries to fit things into a single
byte. When it can't, it emits this warning (if warnings are enabled),
and uses 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)";
IINNTTEERRNNAALLSS #
WWhhaatt iiss “"tthhee UUTTFF88 ffllaagg"”?? 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 “is_utf8”, “_utf8_on” or “_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.
(Actually Perl usually assumes the representation is ASCII; see "Why do
regex character classes sometimes match only in the ASCII range?" above.)
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.
WWhhaatt aabboouutt tthhee “"uussee bbyytteess"” pprraaggmmaa?? Don’t 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 bytes" is usually a failed attempt to do something useful. Just
forget about it.
WWhhaatt aabboouutt tthhee “"uussee eennccooddiinngg"” pprraaggmmaa?? 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 "use utf8".
If you need to set the encoding for STDIN, STDOUT, and STDERR, for
example based on the user's locale, "use open".
WWhhaatt iiss tthhee ddiiffffeerreennccee bbeettwweeeenn “”::eennccooddiinngg"" aanndd “”::uuttff88""?? 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 ":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 you're writing, but it can
be dangerous when reading, because it causes internal inconsistency when
you have invalid byte sequences. Using ":utf8" for input can sometimes
result in security breaches, so please use ":encoding(UTF-8)" instead.
Instead of "decode" and "encode", you could use "_utf8_on" and
"_utf8_off", but this is considered bad style. Especially "_utf8_on" can
be dangerous, for the same reason that ":utf8" can.
There are some shortcuts for oneliners; see -C in perlrun.
WWhhaatt’’ss tthhee ddiiffffeerreennccee bbeettwweeeenn “"UUTTFF--88"” aanndd “"uuttff88"”?? “UTF-8” is the official standard. “utf8” is Perl’s way of being liberal in what it accepts. If you have to communicate with things that aren’t 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 “UTF-8 vs. utf8 vs. UTF8” in Encode.
"UTF-8" is 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 don't 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 it's
not some other encoding.)
II lloosstt ttrraacckk;; wwhhaatt eennccooddiinngg iiss tthhee iinntteerrnnaall ffoorrmmaatt rreeaallllyy?? It’s good that you lost track, because you shouldn’t 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: 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.
AAUUTTHHOORR #
Juerd Waalboer <#####@juerd.nl>
SSEEEE AALLSSOO #
perlunicode, perluniintro, Encode
perl v5.36.3 2023-02-15 PERLUNIFAQ(1)