perlunicode.1

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each of two worlds: the old world of bytes and the new world of
characters, upgrading from bytes to characters when necessary.
If your legacy code does not explicitly use Unicode, no automatic
switch-over to characters should happen.  Characters shouldn't get
downgraded to bytes, either.  It is possible to accidentally mix bytes
and characters, however (see perluniintro), in which case \f(CW\*(C`\ew\*(C'\fR in
regular expressions might start behaving differently.  Review your
code.  Use warnings and the \f(CW\*(C`strict\*(C'\fR pragma.
.Sh "Unicode in Perl on \s-1EBCDIC\s0"
.IX Subsection "Unicode in Perl on EBCDIC"
The way Unicode is handled on \s-1EBCDIC\s0 platforms is still
experimental.  On such platforms, references to \s-1UTF\-8\s0 encoding in this
document and elsewhere should be read as meaning the UTF-EBCDIC
specified in Unicode Technical Report 16, unless \s-1ASCII\s0 vs. \s-1EBCDIC\s0 issues
are specifically discussed. There is no \f(CW\*(C`utfebcdic\*(C'\fR pragma or
\&\*(L":utfebcdic\*(R" layer; rather, \*(L"utf8\*(R" and \*(L":utf8\*(R" are reused to mean
the platform's \*(L"natural\*(R" 8\-bit encoding of Unicode. See perlebcdic
for more discussion of the issues.
.Sh "Locales"
.IX Subsection "Locales"
Usually locale settings and Unicode do not affect each other, but
there are a couple of exceptions:
.IP "\(bu" 4
You can enable automatic UTF\-8\-ification of your standard file
handles, default \f(CW\*(C`open()\*(C'\fR layer, and \f(CW@ARGV\fR by using either
the \f(CW\*(C`\-C\*(C'\fR command line switch or the \f(CW\*(C`PERL_UNICODE\*(C'\fR environment
variable, see perlrun for the documentation of the \f(CW\*(C`\-C\*(C'\fR switch.
.IP "\(bu" 4
Perl tries really hard to work both with Unicode and the old
byte-oriented world. Most often this is nice, but sometimes Perl's
straddling of the proverbial fence causes problems.
.Sh "When Unicode Does Not Happen"
.IX Subsection "When Unicode Does Not Happen"
While Perl does have extensive ways to input and output in Unicode,
and few other 'entry points' like the \f(CW@ARGV\fR which can be interpreted
as Unicode (\s-1UTF\-8\s0), there still are many places where Unicode (in some
encoding or another) could be given as arguments or received as
results, or both, but it is not.
.PP
The following are such interfaces.  For all of these interfaces Perl
currently (as of 5.8.3) simply assumes byte strings both as arguments
and results, or \s-1UTF\-8\s0 strings if the \f(CW\*(C`encoding\*(C'\fR pragma has been used.
.PP
One reason why Perl does not attempt to resolve the role of Unicode in
this cases is that the answers are highly dependent on the operating
system and the file system(s).  For example, whether filenames can be
in Unicode, and in exactly what kind of encoding, is not exactly a
portable concept.  Similarly for the qx and system: how well will the
\&'command line interface' (and which of them?) handle Unicode?
.IP "\(bu" 4
chdir, chmod, chown, chroot, exec, link, lstat, mkdir, 
rename, rmdir, stat, symlink, truncate, unlink, utime, \-X
.IP "\(bu" 4
\&\f(CW%ENV\fR
.IP "\(bu" 4
glob (aka the <*>)
.IP "\(bu" 4
open, opendir, sysopen
.IP "\(bu" 4
qx (aka the backtick operator), system
.IP "\(bu" 4
readdir, readlink
.Sh "Forcing Unicode in Perl (Or Unforcing Unicode in Perl)"
.IX Subsection "Forcing Unicode in Perl (Or Unforcing Unicode in Perl)"
Sometimes (see \*(L"When Unicode Does Not Happen\*(R") there are
situations where you simply need to force Perl to believe that a byte
string is \s-1UTF\-8\s0, or vice versa.  The low-level calls
utf8::upgrade($bytestring) and utf8::downgrade($utf8string) are
the answers.
.PP
Do not use them without careful thought, though: Perl may easily get
very confused, angry, or even crash, if you suddenly change the 'nature'
of scalar like that.  Especially careful you have to be if you use the
\&\fIutf8::upgrade()\fR: any random byte string is not valid \s-1UTF\-8\s0.
.Sh "Using Unicode in \s-1XS\s0"
.IX Subsection "Using Unicode in XS"
If you want to handle Perl Unicode in \s-1XS\s0 extensions, you may find the
following C APIs useful.  See also \*(L"Unicode Support\*(R" in perlguts for an
explanation about Unicode at the \s-1XS\s0 level, and perlapi for the \s-1API\s0
details.
.IP "\(bu" 4
\&\f(CW\*(C`DO_UTF8(sv)\*(C'\fR returns true if the \f(CW\*(C`UTF8\*(C'\fR flag is on and the bytes
pragma is not in effect.  \f(CW\*(C`SvUTF8(sv)\*(C'\fR returns true is the \f(CW\*(C`UTF8\*(C'\fR
flag is on; the bytes pragma is ignored.  The \f(CW\*(C`UTF8\*(C'\fR flag being on
does \fBnot\fR mean that there are any characters of code points greater
than 255 (or 127) in the scalar or that there are even any characters
in the scalar.  What the \f(CW\*(C`UTF8\*(C'\fR flag means is that the sequence of
octets in the representation of the scalar is the sequence of \s-1UTF\-8\s0
encoded code points of the characters of a string.  The \f(CW\*(C`UTF8\*(C'\fR flag
being off means that each octet in this representation encodes a
single character with code point 0..255 within the string.  Perl's
Unicode model is not to use \s-1UTF\-8\s0 until it is absolutely necessary.
.IP "\(bu" 4
\&\f(CW\*(C`uvuni_to_utf8(buf, chr)\*(C'\fR writes a Unicode character code point into
a buffer encoding the code point as \s-1UTF\-8\s0, and returns a pointer
pointing after the \s-1UTF\-8\s0 bytes.
.IP "\(bu" 4
\&\f(CW\*(C`utf8_to_uvuni(buf, lenp)\*(C'\fR reads \s-1UTF\-8\s0 encoded bytes from a buffer and
returns the Unicode character code point and, optionally, the length of
the \s-1UTF\-8\s0 byte sequence.
.IP "\(bu" 4
\&\f(CW\*(C`utf8_length(start, end)\*(C'\fR returns the length of the \s-1UTF\-8\s0 encoded buffer
in characters.  \f(CW\*(C`sv_len_utf8(sv)\*(C'\fR returns the length of the \s-1UTF\-8\s0 encoded
scalar.
.IP "\(bu" 4
\&\f(CW\*(C`sv_utf8_upgrade(sv)\*(C'\fR converts the string of the scalar to its \s-1UTF\-8\s0
encoded form.  \f(CW\*(C`sv_utf8_downgrade(sv)\*(C'\fR does the opposite, if
possible.  \f(CW\*(C`sv_utf8_encode(sv)\*(C'\fR is like sv_utf8_upgrade except that
it does not set the \f(CW\*(C`UTF8\*(C'\fR flag.  \f(CW\*(C`sv_utf8_decode()\*(C'\fR does the
opposite of \f(CW\*(C`sv_utf8_encode()\*(C'\fR.  Note that none of these are to be
used as general-purpose encoding or decoding interfaces: \f(CW\*(C`use Encode\*(C'\fR
for that.  \f(CW\*(C`sv_utf8_upgrade()\*(C'\fR is affected by the encoding pragma
but \f(CW\*(C`sv_utf8_downgrade()\*(C'\fR is not (since the encoding pragma is
designed to be a one-way street).
.IP "\(bu" 4
\&\f(CWis_utf8_char(s)\fR returns true if the pointer points to a valid \s-1UTF\-8\s0
character.
.IP "\(bu" 4
\&\f(CW\*(C`is_utf8_string(buf, len)\*(C'\fR returns true if \f(CW\*(C`len\*(C'\fR bytes of the buffer
are valid \s-1UTF\-8\s0.
.IP "\(bu" 4
\&\f(CW\*(C`UTF8SKIP(buf)\*(C'\fR will return the number of bytes in the \s-1UTF\-8\s0 encoded
character in the buffer.  \f(CW\*(C`UNISKIP(chr)\*(C'\fR will return the number of bytes
required to UTF\-8\-encode the Unicode character code point.  \f(CW\*(C`UTF8SKIP()\*(C'\fR
is useful for example for iterating over the characters of a \s-1UTF\-8\s0
encoded buffer; \f(CW\*(C`UNISKIP()\*(C'\fR is useful, for example, in computing
the size required for a \s-1UTF\-8\s0 encoded buffer.
.IP "\(bu" 4
\&\f(CW\*(C`utf8_distance(a, b)\*(C'\fR will tell the distance in characters between the
two pointers pointing to the same \s-1UTF\-8\s0 encoded buffer.
.IP "\(bu" 4
\&\f(CW\*(C`utf8_hop(s, off)\*(C'\fR will return a pointer to an \s-1UTF\-8\s0 encoded buffer
that is \f(CW\*(C`off\*(C'\fR (positive or negative) Unicode characters displaced
from the \s-1UTF\-8\s0 buffer \f(CW\*(C`s\*(C'\fR.  Be careful not to overstep the buffer:
\&\f(CW\*(C`utf8_hop()\*(C'\fR will merrily run off the end or the beginning of the
buffer if told to do so.
.IP "\(bu" 4
\&\f(CW\*(C`pv_uni_display(dsv, spv, len, pvlim, flags)\*(C'\fR and
\&\f(CW\*(C`sv_uni_display(dsv, ssv, pvlim, flags)\*(C'\fR are useful for debugging the
output of Unicode strings and scalars.  By default they are useful
only for debugging\*(--they display \fBall\fR characters as hexadecimal code
points\*(--but with the flags \f(CW\*(C`UNI_DISPLAY_ISPRINT\*(C'\fR,
\&\f(CW\*(C`UNI_DISPLAY_BACKSLASH\*(C'\fR, and \f(CW\*(C`UNI_DISPLAY_QQ\*(C'\fR you can make the
output more readable.
.IP "\(bu" 4
\&\f(CW\*(C`ibcmp_utf8(s1, pe1, u1, l1, u1, s2, pe2, l2, u2)\*(C'\fR can be used to
compare two strings case-insensitively in Unicode.  For case-sensitive
comparisons you can just use \f(CW\*(C`memEQ()\*(C'\fR and \f(CW\*(C`memNE()\*(C'\fR as usual.
.PP
For more information, see perlapi, and \fIutf8.c\fR and \fIutf8.h\fR
in the Perl source code distribution.
.SH "BUGS"
.IX Header "BUGS"
.Sh "Interaction with Locales"
.IX Subsection "Interaction with Locales"
Use of locales with Unicode data may lead to odd results.  Currently,
Perl attempts to attach 8\-bit locale info to characters in the range
0..255, but this technique is demonstrably incorrect for locales that
use characters above that range when mapped into Unicode.  Perl's
Unicode support will also tend to run slower.  Use of locales with
Unicode is discouraged.
.Sh "Interaction with Extensions"
.IX Subsection "Interaction with Extensions"
When Perl exchanges data with an extension, the extension should be
able to understand the \s-1UTF8\s0 flag and act accordingly. If the
extension doesn't know about the flag, it's likely that the extension
will return incorrectly-flagged data.
.PP
So if you're working with Unicode data, consult the documentation of
every module you're using if there are any issues with Unicode data
exchange. If the documentation does not talk about Unicode at all,
suspect the worst and probably look at the source to learn how the
module is implemented. Modules written completely in Perl shouldn't
cause problems. Modules that directly or indirectly access code written
in other programming languages are at risk.
.PP
For affected functions, the simple strategy to avoid data corruption is
to always make the encoding of the exchanged data explicit. Choose an
encoding that you know the extension can handle. Convert arguments passed
to the extensions to that encoding and convert results back from that
encoding. Write wrapper functions that do the conversions for you, so
you can later change the functions when the extension catches up.
.PP
To provide an example, let's say the popular Foo::Bar::escape_html
function doesn't deal with Unicode data yet. The wrapper function
would convert the argument to raw \s-1UTF\-8\s0 and convert the result back to
Perl's internal representation like so:
.PP
.Vb 5
\&    sub my_escape_html ($) {
\&      my($what) = shift;
\&      return unless defined $what;
\&      Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
\&    }
.Ve
.PP
Sometimes, when the extension does not convert data but just stores
and retrieves them, you will be in a position to use the otherwise
dangerous \fIEncode::_utf8_on()\fR function. Let's say the popular
\&\f(CW\*(C`Foo::Bar\*(C'\fR extension, written in C, provides a \f(CW\*(C`param\*(C'\fR method that
lets you store and retrieve data according to these prototypes:
.PP
.Vb 2
\&    $self\->param($name, $value);            # set a scalar
\&    $value = $self\->param($name);           # retrieve a scalar
.Ve
.PP
If it does not yet provide support for any encoding, one could write a
derived class with such a \f(CW\*(C`param\*(C'\fR method:
.PP
.Vb 12
\&    sub param {
\&      my($self,$name,$value) = @_;
\&      utf8::upgrade($name);     # make sure it is UTF\-8 encoded
\&      if (defined $value) {
\&        utf8::upgrade($value);  # make sure it is UTF\-8 encoded
\&        return $self\->SUPER::param($name,$value);
\&      } else {
\&        my $ret = $self\->SUPER::param($name);
\&        Encode::_utf8_on($ret); # we know, it is UTF\-8 encoded
\&        return $ret;
\&      }
\&    }
.Ve
.PP
Some extensions provide filters on data entry/exit points, such as
DB_File::filter_store_key and family. Look out for such filters in
the documentation of your extensions, they can make the transition to
Unicode data much easier.
.Sh "Speed"
.IX Subsection "Speed"
Some functions are slower when working on \s-1UTF\-8\s0 encoded strings than
on byte encoded strings.  All functions that need to hop over
characters such as \fIlength()\fR, \fIsubstr()\fR or \fIindex()\fR, or matching regular
expressions can work \fBmuch\fR faster when the underlying data are
byte-encoded.
.PP
In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1
a caching scheme was introduced which will hopefully make the slowness
somewhat less spectacular, at least for some operations.  In general,
operations with \s-1UTF\-8\s0 encoded strings are still slower. As an example,
the Unicode properties (character classes) like \f(CW\*(C`\ep{Nd}\*(C'\fR are known to
be quite a bit slower (5\-20 times) than their simpler counterparts
like \f(CW\*(C`\ed\*(C'\fR (then again, there 268 Unicode characters matching \f(CW\*(C`Nd\*(C'\fR
compared with the 10 \s-1ASCII\s0 characters matching \f(CW\*(C`d\*(C'\fR).
.Sh "Porting code from perl\-5.6.X"
.IX Subsection "Porting code from perl-5.6.X"
Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer
was required to use the \f(CW\*(C`utf8\*(C'\fR pragma to declare that a given scope
expected to deal with Unicode data and had to make sure that only
Unicode data were reaching that scope. If you have code that is
working with 5.6, you will need some of the following adjustments to
your code. The examples are written such that the code will continue
to work under 5.6, so you should be safe to try them out.
.IP "\(bu" 4
A filehandle that should read or write \s-1UTF\-8\s0
.Sp
.Vb 3
\&  if ($] > 5.007) {
\&    binmode $fh, ":encoding(utf8)";
\&  }
.Ve
.IP "\(bu" 4
A scalar that is going to be passed to some extension
.Sp
Be it Compress::Zlib, Apache::Request or any extension that has no
mention of Unicode in the manpage, you need to make sure that the
\&\s-1UTF8\s0 flag is stripped off. Note that at the time of this writing
(October 2002) the mentioned modules are not UTF\-8\-aware. Please
check the documentation to verify if this is still true.
.Sp
.Vb 4
\&  if ($] > 5.007) {
\&    require Encode;
\&    $val = Encode::encode_utf8($val); # make octets
\&  }
.Ve
.IP "\(bu" 4
A scalar we got back from an extension
.Sp
If you believe the scalar comes back as \s-1UTF\-8\s0, you will most likely
want the \s-1UTF8\s0 flag restored:
.Sp
.Vb 4
\&  if ($] > 5.007) {
\&    require Encode;
\&    $val = Encode::decode_utf8($val);
\&  }
.Ve
.IP "\(bu" 4
Same thing, if you are really sure it is \s-1UTF\-8\s0
.Sp
.Vb 4
\&  if ($] > 5.007) {
\&    require Encode;
\&    Encode::_utf8_on($val);
\&  }
.Ve
.IP "\(bu" 4
A wrapper for fetchrow_array and fetchrow_hashref
.Sp
When the database contains only \s-1UTF\-8\s0, a wrapper function or method is
a convenient way to replace all your fetchrow_array and
fetchrow_hashref calls. A wrapper function will also make it easier to
adapt to future enhancements in your database driver. Note that at the
time of this writing (October 2002), the \s-1DBI\s0 has no standardized way
to deal with \s-1UTF\-8\s0 data. Please check the documentation to verify if
that is still true.
.Sp
.Vb 10
\&  sub fetchrow {
\&    my($self, $sth, $what) = @_; # $what is one of fetchrow_{array,hashref}
\&    if ($] < 5.007) {
\&      return $sth\->$what;
\&    } else {
\&      require Encode;
\&      if (wantarray) {
\&        my @arr = $sth\->$what;
\&        for (@arr) {
\&          defined && /[^\e000\-\e177]/ && Encode::_utf8_on(