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PERLUNIFAQ(1)	       Perl Programmers	Reference Guide		 PERLUNIFAQ(1)

NAME
       perlunifaq - Perl Unicode FAQ

Q and A
       This is a list of questions and answers about Unicode in	Perl, intended
       to be read after	perlunitut.

   perlunitut isn't really a Unicode tutorial, is it?
       No, and this isn't really a Unicode FAQ.

       Perl has	an abstracted interface	for all	supported character encodings,
       so this is actually a generic "Encode" tutorial and "Encode" FAQ. But
       many people think that Unicode is special and magical, and I didn't
       want to disappoint them,	so I decided to	call the document a Unicode
       tutorial.

   What	character encodings does Perl support?
       To find out which character encodings your Perl supports, run:

	   perl	-MEncode -le "print for	Encode->encodings(':all')"

   Which version of perl should	I use?
       Well, if	you can, upgrade to the	most recent, but certainly 5.8.1 or
       newer.  The tutorial and	FAQ assume the latest release.

       You should also check your modules, and upgrade them if necessary. For
       example,	HTML::Entities requires	version	>= 1.32	to function correctly,
       even though the changelog is silent about this.

   What	about binary data, like	images?
       Well, apart from	a bare "binmode	$fh", you shouldn't treat them
       specially.  (The	binmode	is needed because otherwise Perl may convert
       line endings on Win32 systems.)

       Be careful, though, to never combine text strings with binary strings.
       If you need text	in a binary stream, encode your	text strings first
       using the appropriate encoding, then join them with binary strings. See
       also: "What if I	don't encode?".

   When	should I decode	or encode?
       Whenever	you're communicating text with anything	that is	external to
       your perl process, like a database, a text file,	a socket, or another
       program.	Even if	the thing you're communicating with is also written in
       Perl.

   What	if I don't decode?
       Whenever	your encoded, binary string is used together with a text
       string, Perl will assume	that your binary string	was encoded with
       ISO-8859-1, also	known as latin-1. If it	wasn't latin-1,	then your data
       is unpleasantly converted. For example, if it was UTF-8,	the individual
       bytes of	multibyte characters are seen as separate characters, and then
       again converted to UTF-8. Such double encoding can be compared to
       double HTML encoding (">"), or double URI	encoding (%253E).

       This silent implicit decoding is	known as "upgrading". That may sound
       positive, but it's best to avoid	it.

   What	if I don't encode?
       Your text string	will be	sent using the bytes in	Perl's internal
       format. In some cases, Perl will	warn you that you're doing something
       wrong, with a friendly warning:

	   Wide	character in print at example.pl line 2.

       Because the internal format is often UTF-8, these bugs are hard to
       spot, because UTF-8 is usually the encoding you wanted! But don't be
       lazy, and don't use the fact that Perl's	internal format	is UTF-8 to
       your advantage. Encode explicitly to avoid weird	bugs, and to show to
       maintenance programmers that you	thought	this through.

   Is there a way to automatically decode or encode?
       If all data that	comes from a certain handle is encoded in exactly the
       same way, you can tell the PerlIO system	to automatically decode
       everything, with	the "encoding" layer. If you do	this, you can't
       accidentally forget to decode or	encode anymore,	on things that use the
       layered handle.

       You can provide this layer when "open"ing the file:

	 open my $fh, '>:encoding(UTF-8)', $filename;  # auto encoding on write
	 open my $fh, '<:encoding(UTF-8)', $filename;  # auto decoding on read

       Or if you already have an open filehandle:

	 binmode $fh, ':encoding(UTF-8)';

       Some database drivers for DBI can also automatically encode and decode,
       but that	is sometimes limited to	the UTF-8 encoding.

   What	if I don't know	which encoding was used?
       Do whatever you can to find out,	and if you have	to: guess. (Don't
       forget to document your guess with a comment.)

       You could open the document in a	web browser, and change	the character
       set or character	encoding until you can visually	confirm	that all
       characters look the way they should.

       There is	no way to reliably detect the encoding automatically, so if
       people keep sending you data without charset indication,	you may	have
       to educate them.

   Can I use Unicode in	my Perl	sources?
       Yes, you	can! If	your sources are UTF-8 encoded,	you can	indicate that
       with the	"use utf8" pragma.

	   use utf8;

       This doesn't do anything	to your	input, or to your output. It only
       influences the way your sources are read. You can use Unicode in	string
       literals, in identifiers	(but they still	have to	be "word characters"
       according to "\w"), and even in custom delimiters.

   Data::Dumper	doesn't	restore	the UTF8 flag; is it broken?
       No, Data::Dumper's Unicode abilities are	as they	should be. There have
       been some complaints that it should restore the UTF8 flag when the data
       is read again with "eval". However, you should really not look at the
       flag, and nothing indicates that	Data::Dumper should break this rule.

       Here's what happens: when Perl reads in a string	literal, it sticks to
       8 bit encoding as long as it can. (But perhaps originally it was
       internally encoded as UTF-8, when you dumped it.) When it has to	give
       that up because other characters	are added to the text string, it
       silently	upgrades the string to UTF-8.

       If you properly encode your strings for output, none of this is of your
       concern,	and you	can just "eval"	dumped data as always.

   Why do regex	character classes sometimes match only in the ASCII range?
       Starting	in Perl	5.14 (and partially in Perl 5.12), just	put a "use
       feature 'unicode_strings'" near the beginning of	your program.  Within
       its lexical scope you shouldn't have this problem.  It also is
       automatically enabled under "use	feature	':5.12'" or "use v5.12"	or
       using "-E" on the command line for Perl 5.12 or higher.

       The rationale for requiring this	is to not break	older programs that
       rely on the way things worked before Unicode came along.	 Those older
       programs	knew only about	the ASCII character set, and so	may not	work
       properly	for additional characters.  When a string is encoded in	UTF-8,
       Perl assumes that the program is	prepared to deal with Unicode, but
       when the	string isn't, Perl assumes that	only ASCII is wanted, and so
       those characters	that are not ASCII characters aren't recognized	as to
       what they would be in Unicode.  "use feature 'unicode_strings'" tells
       Perl to treat all characters as Unicode,	whether	the string is encoded
       in UTF-8	or not,	thus avoiding the problem.

       However,	on earlier Perls, or if	you pass strings to subroutines
       outside the feature's scope, you	can force Unicode rules	by changing
       the encoding to UTF-8 by	doing "utf8::upgrade($string)".	This can be
       used safely on any string, as it	checks and does	not change strings
       that have already been upgraded.

       For a more detailed discussion, see Unicode::Semantics on CPAN.

   Why do some characters not uppercase	or lowercase correctly?
       See the answer to the previous question.

   How can I determine if a string is a	text string or a binary	string?
       You can't. Some use the UTF8 flag for this, but that's misuse, and
       makes well behaved modules like Data::Dumper look bad. The flag is
       useless for this	purpose, because it's off when an 8 bit	encoding (by
       default ISO-8859-1) is used to store the	string.

       This is something you, the programmer, has to keep track	of; sorry. You
       could consider adopting a kind of "Hungarian notation" to help with
       this.

   How do I convert from encoding FOO to encoding BAR?
       By first	converting the FOO-encoded byte	string to a text string, and
       then the	text string to a BAR-encoded byte string:

	   my $text_string = decode('FOO', $foo_string);
	   my $bar_string  = encode('BAR', $text_string);

       or by skipping the text string part, and	going directly from one	binary
       encoding	to the other:

	   use Encode qw(from_to);
	   from_to($string, 'FOO', 'BAR');  # changes contents of $string

       or by letting automatic decoding	and encoding do	all the	work:

	   open	my $foofh, '<:encoding(FOO)', 'example.foo.txt';
	   open	my $barfh, '>:encoding(BAR)', 'example.bar.txt';
	   print { $barfh } $_ while <$foofh>;

   What	are "decode_utf8" and "encode_utf8"?
       These are alternate syntaxes for	"decode('utf8',	...)" and
       "encode('utf8', ...)". Do not use these functions for data exchange.
       Instead use "decode('UTF-8', ...)" and "encode('UTF-8', ...)"; see
       "What's the difference between UTF-8 and	utf8?" below.

   What	is a "wide character"?
       This is a term used for characters occupying more than one byte.

       The Perl	warning	"Wide character	in ..."	is caused by such a character.
       With no specified encoding layer, Perl tries to fit things into a
       single byte.  When it can't, it emits this warning (if warnings are
       enabled), and uses UTF-8	encoded	data instead.

       To avoid	this warning and to avoid having different output encodings in
       a single	stream,	always specify an encoding explicitly, for example
       with a PerlIO layer:

	   binmode STDOUT, ":encoding(UTF-8)";

INTERNALS
   What	is "the	UTF8 flag"?
       Please, unless you're hacking the internals, or debugging weirdness,
       don't think about the UTF8 flag at all. That means that you very
       probably	shouldn't use "is_utf8", "_utf8_on" or "_utf8_off" at all.

       The UTF8	flag, also called SvUTF8, is an	internal flag that indicates
       that the	current	internal representation	is UTF-8. Without the flag, it
       is assumed to be	ISO-8859-1. Perl converts between these	automatically.
       (Actually Perl usually assumes the representation is ASCII; see "Why do
       regex character classes sometimes match only in the ASCII range?"
       above.)

       One of Perl's internal formats happens to be UTF-8. Unfortunately, Perl
       can't keep a secret, so everyone	knows about this. That is the source
       of much confusion. It's better to pretend that the internal format is
       some unknown encoding, and that you always have to encode and decode
       explicitly.

   What	about the "use bytes" pragma?
       Don't use it. It	makes no sense to deal with bytes in a text string,
       and it makes no sense to	deal with characters in	a byte string. Do the
       proper conversions (by decoding/encoding), and things will work out
       well: you get character counts for decoded data,	and byte counts	for
       encoded data.

       "use bytes" is usually a	failed attempt to do something useful. Just
       forget about it.

   What	about the "use encoding" pragma?
       Don't use it. Unfortunately, it assumes that the	programmer's
       environment and that of the user	will use the same encoding. It will
       use the same encoding for the source code and for STDIN and STDOUT.
       When a program is copied	to another machine, the	source code does not
       change, but the STDIO environment might.

       If you need non-ASCII characters	in your	source code, make it a UTF-8
       encoded file and	"use utf8".

       If you need to set the encoding for STDIN, STDOUT, and STDERR, for
       example based on	the user's locale, "use	open".

   What	is the difference between ":encoding" and ":utf8"?
       Because UTF-8 is	one of Perl's internal formats,	you can	often just
       skip the	encoding or decoding step, and manipulate the UTF8 flag
       directly.

       Instead of ":encoding(UTF-8)", you can simply use ":utf8", which	skips
       the encoding step if the	data was already represented as	UTF8
       internally. This	is widely accepted as good behavior when you're
       writing,	but it can be dangerous	when reading, because it causes
       internal	inconsistency when you have invalid byte sequences. Using
       ":utf8" for input can sometimes result in security breaches, so please
       use ":encoding(UTF-8)" instead.

       Instead of "decode" and "encode", you could use "_utf8_on" and
       "_utf8_off", but	this is	considered bad style. Especially "_utf8_on"
       can be dangerous, for the same reason that ":utf8" can.

       There are some shortcuts	for oneliners; see -C in perlrun.

   What's the difference between "UTF-8" and "utf8"?
       "UTF-8" is the official standard. "utf8"	is Perl's way of being liberal
       in what it accepts. If you have to communicate with things that aren't
       so liberal, you may want	to consider using "UTF-8". If you have to
       communicate with	things that are	too liberal, you may have to use
       "utf8". The full	explanation is in "UTF-8 vs. utf8 vs. UTF8" in Encode.

       "UTF-8" is internally known as "utf-8-strict". The tutorial uses	UTF-8
       consistently, even where	utf8 is	actually used internally, because the
       distinction can be hard to make,	and is mostly irrelevant.

       For example, utf8 can be	used for code points that don't	exist in
       Unicode,	like 9999999, but if you encode	that to	UTF-8, you get a
       substitution character (by default; see "Handling Malformed Data" in
       Encode for more ways of dealing with this.)

       Okay, if	you insist: the	"internal format" is utf8, not UTF-8. (When
       it's not	some other encoding.)

   I lost track; what encoding is the internal format really?
       It's good that you lost track, because you shouldn't depend on the
       internal	format being any specific encoding. But	since you asked: by
       default,	the internal format is either ISO-8859-1 (latin-1), or utf8,
       depending on the	history	of the string. On EBCDIC platforms, this may
       be different even.

       Perl knows how it stored	the string internally, and will	use that
       knowledge when you "encode". In other words: don't try to find out what
       the internal encoding for a certain string is, but instead just encode
       it into the encoding that you want.

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

SEE ALSO
       perlunicode, perluniintro, Encode

perl v5.32.0			  2020-06-14			 PERLUNIFAQ(1)

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