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




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.   (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.

       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 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.

       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.

       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

       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

       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 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 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, then
       perluniintro.


ACKNOWLEDGEMENTS

       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.


AUTHOR

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


SEE ALSO

       perlunifaq(1), perlunicode(1), perluniintro(1), Encode(3)



perl v5.24.0                      2016-03-01                   PERLUNITUT(1pm)

perl 5.24 - Generated Sun Dec 4 20:16:30 CST 2016
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