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

NAME

       perlunitut - Perl Unicode Tutorial

DESCRIPTION

       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.

       Definitions

       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

       Unicode is a character set with room for lots of characters. The ordinal value of a character 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 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.

       UTF-8

       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 encod-
       ings, 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 this.

       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 Unicode 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;
	   $binary =~ s/x00x0F/xFFxF0/;  # for the brave :)
	   print {$fh} $binary;
	   my $byte_count = length $binary;

       Encoding

       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 repre-
       sented are lost in the conversion.

       Decoding

       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 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 mem-
       ory, 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 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"

SUMMARY

       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.

ACKNOWLEDGEMENTS

       Thanks to Johan Vromans from Squirrel Consultancy. His UTF-8 rants during the Amsterdam Perl Mongers meetings got me interested and deter-
       mined 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 expla-
       nation 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.

AUTHOR

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

SEE ALSO

       perlunifaq, perlunicode, perluniintro, Encode

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