PERLUNITUT(1) Perl Programmers Reference Guide PERLUNITUT(1) #
PERLUNITUT(1) Perl Programmers Reference Guide PERLUNITUT(1)
NNAAMMEE #
perlunitut - Perl Unicode Tutorial
DDEESSCCRRIIPPTTIIOONN #
The days of just flinging strings around are over. It's well established
that modern programs need to be capable of communicating funny accented
letters, and things like euro symbols. This means that programmers need
new habits. It's easy to program Unicode capable software, but it does
require discipline to do it right.
There's a lot to know about character sets, and text encodings. It's
probably best to spend a full day learning all this, but the basics can
be learned in minutes.
These are not the very basics, though. It is assumed that you already
know the difference between bytes and characters, and realise (and
accept!) that there are many different character sets and encodings, and
that your program has to be explicit about them. Recommended reading is
"The Absolute Minimum Every Software Developer Absolutely, Positively
Must Know About Unicode and Character Sets (No Excuses!)" by Joel
Spolsky, at <http://joelonsoftware.com/articles/Unicode.html>.
This tutorial speaks in rather absolute terms, and provides only a
limited view of the wealth of character string related features that Perl
has to offer. For most projects, this information will probably suffice.
DDeeffiinniittiioonnss It’s important to set a few things straight first. This is the most important part of this tutorial. This view may conflict with other information that you may have found on the web, but that’s mostly because many sources are wrong.
You may have to re-read this entire section a few times...
_U_n_i_c_o_d_e
UUnniiccooddee is a character set with room for lots of characters. The ordinal
value of a character is called a ccooddee ppooiinntt. (But in practice, the
distinction between code point and character is blurred, so the terms
often are used interchangeably.)
There are many, many code points, but computers work with bytes, and a
byte has room for only 256 values. Unicode has many more characters than
that, so you need a method to make these accessible.
Unicode is encoded using several competing encodings, of which UTF-8 is
the most used. In a Unicode encoding, multiple subsequent bytes can be
used to store a single code point, or simply: character.
_U_T_F_-_8 #
UUTTFF--88 is a Unicode encoding. Many people think that Unicode and UTF-8 are
the same thing, but they're not. There are more Unicode encodings, but
much of the world has standardized on UTF-8.
UTF-8 treats the first 128 codepoints, 0..127, the same as ASCII. They
take only one byte per character. All other characters are encoded as two
to four bytes using a complex scheme. Fortunately, Perl handles this for
us, so we don't have to worry about this.
_T_e_x_t _s_t_r_i_n_g_s _(_c_h_a_r_a_c_t_e_r _s_t_r_i_n_g_s_)
TTeexxtt ssttrriinnggss, or cchhaarraacctteerr ssttrriinnggss are made of characters. Bytes are
irrelevant here, and so are encodings. Each character is just that: the
character.
On a text string, you would do things like:
$text =~ s/foo/bar/;
if ($string =~ /^\d+$/) { ... }
$text = ucfirst $text;
my $character_count = length $text;
The value of a character ("ord", "chr") is the corresponding Unicode code
point.
_B_i_n_a_r_y _s_t_r_i_n_g_s _(_b_y_t_e _s_t_r_i_n_g_s_)
BBiinnaarryy ssttrriinnggss, or bbyyttee ssttrriinnggss are made of bytes. Here, you don't have
characters, just bytes. All communication with the outside world
(anything outside of your current Perl process) is done in binary.
On a binary string, you would do things like:
my (@length_content) = unpack "(V/a)*", $binary;
$binary =~ s/\x00\x0F/\xFF\xF0/; # for the brave :)
print {$fh} $binary;
my $byte_count = length $binary;
_E_n_c_o_d_i_n_g
EEnnccooddiinngg (as a verb) is the conversion from _t_e_x_t to _b_i_n_a_r_y. To encode,
you have to supply the target encoding, for example "iso-8859-1" or
"UTF-8". Some encodings, like the "iso-8859" ("latin") range, do not
support the full Unicode standard; characters that can't be represented
are lost in the conversion.
_D_e_c_o_d_i_n_g
DDeeccooddiinngg is the conversion from _b_i_n_a_r_y to _t_e_x_t. To decode, you have to
know what encoding was used during the encoding phase. And most of all,
it must be something decodable. It doesn't make much sense to decode a
PNG image into a text string.
_I_n_t_e_r_n_a_l _f_o_r_m_a_t
Perl has an iinntteerrnnaall ffoorrmmaatt, an encoding that it uses to encode text
strings so it can store them in memory. All text strings are in this
internal format. In fact, text strings are never in any other format!
You shouldn't worry about what this format is, because conversion is
automatically done when you decode or encode.
YYoouurr nneeww ttoooollkkiitt Add to your standard heading the following line:
use Encode qw(encode decode);
Or, if you're lazy, just:
use Encode;
II//OO ffllooww ((tthhee aaccttuuaall 55 mmiinnuuttee ttuuttoorriiaall)) The typical input/output flow of a program is:
1. Receive and decode
2. Process
3. Encode and output
If your input is binary, and is supposed to remain binary, you shouldn't
decode it to a text string, of course. But in all other cases, you should
decode it.
Decoding can't happen reliably if you don't know how the data was
encoded. If you get to choose, it's a good idea to standardize on UTF-8.
my $foo = decode('UTF-8', get 'http://example.com/');
my $bar = decode('ISO-8859-1', readline STDIN);
my $xyzzy = decode('Windows-1251', $cgi->param('foo'));
Processing happens as you knew before. The only difference is that you're
now using characters instead of bytes. That's very useful if you use
things like "substr", or "length".
It's important to realize that there are no bytes in a text string. Of
course, Perl has its internal encoding to store the string in memory, but
ignore that. If you have to do anything with the number of bytes, it's
probably best to move that part to step 3, just after you've encoded the
string. Then you know exactly how many bytes it will be in the
destination string.
The syntax for encoding text strings to binary strings is as simple as
decoding:
$body = encode('UTF-8', $body);
If you needed to know the length of the string in bytes, now's the
perfect time for that. Because $body is now a byte string, "length" will
report the number of bytes, instead of the number of characters. The
number of characters is no longer known, because characters only exist in
text strings.
my $byte_count = length $body;
And if the protocol you're using supports a way of letting the recipient
know which character encoding you used, please help the receiving end by
using that feature! For example, E-mail and HTTP support MIME headers, so
you can use the "Content-Type" header. They can also have
"Content-Length" to indicate the number of _b_y_t_e_s, which is always a good
idea to supply if the number is known.
"Content-Type: text/plain; charset=UTF-8",
"Content-Length: $byte_count"
SSUUMMMMAARRYY #
Decode everything you receive, encode everything you send out. (If it's
text data.)
QQ aanndd AA ((oorr FFAAQQ)) After reading this document, you ought to read perlunifaq too, then perluniintro.
AACCKKNNOOWWLLEEDDGGEEMMEENNTTSS #
Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8 rants during
the Amsterdam Perl Mongers meetings got me interested and determined to
find out how to use character encodings in Perl in ways that don't break
easily.
Thanks to Gerard Goossen from TTY. His presentation "UTF-8 in the wild"
(Dutch Perl Workshop 2006) inspired me to publish my thoughts and write
this tutorial.
Thanks to the people who asked about this kind of stuff in several Perl
IRC channels, and have constantly reminded me that a simpler explanation
was needed.
Thanks to the people who reviewed this document for me, before it went
public. They are: Benjamin Smith, Jan-Pieter Cornet, Johan Vromans,
Lukas Mai, Nathan Gray.
AAUUTTHHOORR #
Juerd Waalboer <#####@juerd.nl>
SSEEEE AALLSSOO #
perlunifaq, perlunicode, perluniintro, Encode
perl v5.36.3 2017-02-05 PERLUNITUT(1)