The days of just flinging strings around are over. It's well established thatmodern programs need to be capable of communicating funny accented letters, andthings like euro symbols. This means that programmers need new habits. It'seasy to program Unicode capable software, but it does require discipline to doit right.

There's a lot to know about character sets, and text encodings. It's probablybest to spend a full day learning all this, but the basics can be learned inminutes.

These are not the very basics, though. It is assumed that you alreadyknow the difference between bytes and characters, and realise (and accept!)that there are many different character sets and encodings, and that yourprogram has to be explicit about them. Recommended reading is "The AbsoluteMinimum Every Software Developer Absolutely, Positively Must Know About Unicodeand Character Sets (No Excuses!)" by Joel Spolsky, athttp://joelonsoftware.com/articles/Unicode.html.

This tutorial speaks in rather absolute terms, and provides only a limited viewof the wealth of character string related features that Perl has to offer. Formost projects, this information will probably suffice.

Definitions

It's important to set a few things straight first. This is the most importantpart of this tutorial. This view may conflict with other information that youmay 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 ordinalvalue of a character is called a code point. (But in practice, thedistinction between code point and character is blurred, so the terms oftenare used interchangeably.)

There are many, many code points, but computers work with bytes, and a byte hasroom 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 themost used. In a Unicode encoding, multiple subsequent bytes can be used tostore a single code point, or simply: character.

UTF-8

UTF-8 is a Unicode encoding. Many people think that Unicode and UTF-8 arethe same thing, but they're not. There are more Unicode encodings, but much ofthe world has standardized on UTF-8.

UTF-8 treats the first 128 codepoints, 0..127, the same as ASCII. They takeonly 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 forus, so we don't have to worry about this.

Text strings (character strings)

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

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 codepoint.

Binary strings (byte strings)

Binary strings, or byte strings are made of bytes. Here, you don't havecharacters, just bytes. All communication with the outside world (anythingoutside 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

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 fullUnicode standard; characters that can't be represented are lost in theconversion.

Decoding

Decoding is the conversion from binary to text. To decode, you have toknow what encoding was used during the encoding phase. And most of all, it mustbe something decodable. It doesn't make much sense to decode a PNG image into atext string.

Internal format

Perl has an internal format, an encoding that it uses to encode text stringsso 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 isautomatically 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 decodeit 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. Ifyou 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 nowusing characters instead of bytes. That's very useful if you use things likesubstr, 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 movethat part to step 3, just after you've encoded the string. Then you knowexactly 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 timefor that. Because $body is now a byte string, length will report thenumber of bytes, instead of the number of characters. The number ofcharacters 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 knowwhich character encoding you used, please help the receiving end by using thatfeature! For example, E-mail and HTTP support MIME headers, so you can use theContent-Type header. They can also have Content-Length to indicate thenumber of bytes, which is always a good idea to supply if the number isknown.

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

SUMMARY

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

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 theAmsterdam Perl Mongers meetings got me interested and determined to find outhow 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" (DutchPerl 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 IRCchannels, and have constantly reminded me that a simpler explanation wasneeded.

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, NathanGray.

AUTHOR

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

Perl Unicode Tutorial

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