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X11R7.4 - man page for perlunitut (x11r4 section 1)

PERLUNITUT(1)			 Perl Programmers Reference Guide		    PERLUNITUT(1)

       perlunitut - Perl Unicode Tutorial

       The days of just flinging strings around are over. It's well established that modern pro-
       grams 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. Recom-
       mended reading is "The Absolute Minimum Every Software Developer Absolutely, Positively
       Must Know About Unicode and Character Sets (No Excuses!)" by Joel Spolsky, at

       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.


       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...


       Unicode is a character set with room for lots of characters. The ordinal value of a char-
       acter is called a code point.

       There are many, many code points, but computers work with bytes, and a byte can have only
       256 values. Unicode has many more characters, so you need a method to make these accessi-

       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.


       UTF-8 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 or more (up to six) bytes using a
       complex scheme. Fortunately, Perl handles this for us, so we don't have to worry about

       Text strings (character strings)

       Text strings, or character strings are made of characters. Bytes are irrelevant here, and
       so are encodings. Each character is just that: the character.

       Text strings are also called Unicode strings, because in Perl, every text string is a Uni-
       code string.

       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.

       Binary strings (byte strings)

       Binary strings, or byte strings 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;


       Encoding (as a verb) is the conversion from text to binary. 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.


       Decoding is the conversion from binary to text. 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.

       Internal format

       Perl has an internal format, 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.

       Your new toolkit

       Add to your standard heading the following line:

	   use Encode qw(encode decode);

       Or, if you're lazy, just:

	   use Encode;

       I/O flow (the actual 5 minute tutorial)

       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 char-
       acters 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 char-
       acter 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 bytes, which is always a good idea to
       supply if the number is known.

	   "Content-Type: text/plain; charset=UTF-8",
	   "Content-Length: $byte_count"

       Decode everything you receive, encode everything you send out. (If it's text data.)

Q and A (or FAQ)
       After reading this document, you ought to read perlunifaq too.

       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 Work-
       shop 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.

       Juerd Waalboer <#####@juerd.nl>

       perlunifaq, perlunicode, perluniintro, Encode

perl v5.8.9				    2007-11-17				    PERLUNITUT(1)

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