perlfunc: layout getpw*&c return values as a table

[perl5.git] / pod / perllocale.pod

=encoding utf8

=head1 NAME

perllocale - Perl locale handling (internationalization and localization)

=head1 DESCRIPTION

In the beginning there was ASCII, the "American Standard Code for
Information Interchange", which works quite well for Americans with
their English alphabet and dollar-denominated currency.  But it doesn't
work so well even for other English speakers, who may use different
currencies, such as the pound sterling (as the symbol for that currency
is not in ASCII); and it's hopelessly inadequate for many of the
thousands of the world's other languages.

To address these deficiencies, the concept of locales was invented
(formally the ISO C, XPG4, POSIX 1.c "locale system").  And applications
were and are being written that use the locale mechanism.  The process of
making such an application take account of its users' preferences in
these kinds of matters is called B<internationalization> (often
abbreviated as B<i18n>); telling such an application about a particular
set of preferences is known as B<localization> (B<l10n>).

Perl has been extended to support the locale system.  This
is controlled per application by using one pragma, one function call,
and several environment variables.

Unfortunately, there are quite a few deficiencies with the design (and
often, the implementations) of locales.  Unicode was invented (see
L<perlunitut> for an introduction to that) in part to address these
design deficiencies, and nowadays, there is a series of "UTF-8
locales", based on Unicode.  These are locales whose character set is
Unicode, encoded in UTF-8.  Starting in v5.20, Perl fully supports
UTF-8 locales, except for sorting and string comparisions.  (Use
L<Unicode::Collate> for these.)  Perl continues to support the old
non UTF-8 locales as well.

(Unicode is also creating C<CLDR>, the "Common Locale Data Repository",
L<http://cldr.unicode.org/> which includes more types of information than
are available in the POSIX locale system.  At the time of this writing,
there was no CPAN module that provides access to this XML-encoded data.
However, many of its locales have the POSIX-only data extracted, and are
available as UTF-8 locales at
L<http://unicode.org/Public/cldr/latest/>.)

=head1 WHAT IS A LOCALE

A locale is a set of data that describes various aspects of how various
communities in the world categorize their world.  These categories are
broken down into the following types (some of which include a brief
note here):

=over

=item Category LC_NUMERIC: Numeric formatting

This indicates how numbers should be formatted for human readability,
for example the character used as the decimal point.

=item Category LC_MONETARY: Formatting of monetary amounts

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=item Category LC_TIME: Date/Time formatting

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=item Category LC_MESSAGES: Error and other messages

This is used by Perl itself only for accessing operating system error
messages via L<$!|perlvar/$ERRNO> and L<$^E|perlvar/$EXTENDED_OS_ERROR>.

=item Category LC_COLLATE: Collation

This indicates the ordering of letters for comparison and sorting.
In Latin alphabets, for example, "b", generally follows "a".

=item Category LC_CTYPE: Character Types

This indicates, for example if a character is an uppercase letter.

=item Other categories

Some platforms have other categories, dealing with such things as
measurement units and paper sizes.  None of these are used directly by
Perl, but outside operations that Perl interacts with may use
these.  See L</Not within the scope of any "use locale" variant> below.

=back

More details on the categories used by Perl are given below in L</LOCALE
CATEGORIES>.

Together, these categories go a long way towards being able to customize
a single program to run in many different locations.  But there are
deficiencies, so keep reading.

=head1 PREPARING TO USE LOCALES

Perl itself will not use locales unless specifically requested to (but
again note that Perl may interact with code that does use them).  Even
if there is such a request, B<all> of the following must be true
for it to work properly:

=over 4

=item *

B<Your operating system must support the locale system>.  If it does,
you should find that the C<setlocale()> function is a documented part of
its C library.

=item *

B<Definitions for locales that you use must be installed>.  You, or
your system administrator, must make sure that this is the case. The
available locales, the location in which they are kept, and the manner
in which they are installed all vary from system to system.  Some systems
provide only a few, hard-wired locales and do not allow more to be
added.  Others allow you to add "canned" locales provided by the system
supplier.  Still others allow you or the system administrator to define
and add arbitrary locales.  (You may have to ask your supplier to
provide canned locales that are not delivered with your operating
system.)  Read your system documentation for further illumination.

=item *

B<Perl must believe that the locale system is supported>.  If it does,
C<perl -V:d_setlocale> will say that the value for C<d_setlocale> is
C<define>.

=back

If you want a Perl application to process and present your data
according to a particular locale, the application code should include
the S<C<use locale>> pragma (see L<The use locale pragma>) where
appropriate, and B<at least one> of the following must be true:

=over 4

=item 1

B<The locale-determining environment variables (see L</"ENVIRONMENT">)
must be correctly set up> at the time the application is started, either
by yourself or by whomever set up your system account; or

=item 2

B<The application must set its own locale> using the method described in
L<The setlocale function>.

=back

=head1 USING LOCALES

=head2 The use locale pragma

By default, Perl itself ignores the current locale.  The S<C<use locale>>
pragma tells Perl to use the current locale for some operations.
Starting in v5.16, there is an optional parameter to this pragma:

    use locale ':not_characters';

This parameter allows better mixing of locales and Unicode (less useful
in v5.20 and later), and is
described fully in L</Unicode and UTF-8>, but briefly, it tells Perl to
not use the character portions of the locale definition, that is
the C<LC_CTYPE> and C<LC_COLLATE> categories.  Instead it will use the
native character set (extended by Unicode).  When using this parameter,
you are responsible for getting the external character set translated
into the native/Unicode one (which it already will be if it is one of
the increasingly popular UTF-8 locales).  There are convenient ways of
doing this, as described in L</Unicode and UTF-8>.

The current locale is set at execution time by
L<setlocale()|/The setlocale function> described below.  If that function
hasn't yet been called in the course of the program's execution, the
current locale is that which was determined by the L</"ENVIRONMENT"> in
effect at the start of the program.
If there is no valid environment, the current locale is whatever the
system default has been set to.   On POSIX systems, it is likely, but
not necessarily, the "C" locale.  On Windows, the default is set via the
computer's S<C<Control Panel-E<gt>Regional and Language Options>> (or its
current equivalent).

The operations that are affected by locale are:

=over 4

=item B<Not within the scope of any C<"use locale"> variant>

Only operations originating outside Perl should be affected, as follows:

=over 4

=item *

The variable L<$!|perlvar/$ERRNO> (and its synonyms C<$ERRNO> and
C<$OS_ERROR>) when used as strings always are in terms of the current
locale.

=item *

The current locale is also used when going outside of Perl with
operations like L<system()|perlfunc/system LIST> or
L<qxE<sol>E<sol>|perlop/qxE<sol>STRINGE<sol>>, if those operations are
locale-sensitive.

=item *

Also Perl gives access to various C library functions through the
L<POSIX> module.  Some of those functions are always affected by the
current locale.  For example, C<POSIX::strftime()> uses C<LC_TIME>;
C<POSIX::strtod()> uses C<LC_NUMERIC>; C<POSIX::strcoll()> and
C<POSIX::strxfrm()> use C<LC_COLLATE>; and character classification
functions like C<POSIX::isalnum()> use C<LC_CTYPE>.  All such functions
will behave according to the current underlying locale, even if that
locale isn't exposed to Perl space.

=item *

Perl also provides lite wrappers for XS modules to use some C library
C<printf> functions.  These wrappers don't do anything with the locale,
and the underlying C library function is affected by the locale in
effect at the time of the wrapper call.
The affected functions are
L<perlapi/my_sprintf>,
L<perlapi/my_snprintf>,
and
L<perlapi/my_vsnprintf>.

=back

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=item B<Lingering effects of C<S<use locale>>>

Certain Perl operations that are set-up within the scope of a
C<use locale> variant retain that effect even outside the scope.
These include:

=over 4

=item *

The output format of a L<write()|perlfunc/write> is determined by an
earlier format declaration (L<perlfunc/format>), so whether or not the
output is affected by locale is determined by if the C<format()> is
within the scope of a C<use locale> variant, not whether the C<write()>
is.

=item *

Regular expression patterns can be compiled using
L<qrE<sol>E<sol>|perlop/qrE<sol>STRINGE<sol>msixpodual> with actual
matching deferred to later.  Again, it is whether or not the compilation
was done within the scope of C<use locale> that determines the match
behavior, not if the matches are done within such a scope or not.

=back

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=item B<Under C<"use locale ':not_characters';">>

=over 4

=item *

All the non-Perl operations.

=item *

B<Format declarations> (L<perlfunc/format>) and hence any subsequent
C<write()>s use C<LC_NUMERIC>.

=item *

B<stringification and output> use C<LC_NUMERIC>.
These include the results of
C<print()>,
C<printf()>,
C<say()>,
and
C<sprintf()>.

=back

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E<160>

=item B<Under just plain C<"use locale";>>

=over 4

=item *

All the above operations

=item *

B<The comparison operators> (C<lt>, C<le>, C<cmp>, C<ge>, and C<gt>) use
C<LC_COLLATE>.  C<sort()> is also affected if used without an
explicit comparison function, because it uses C<cmp> by default.

B<Note:> C<eq> and C<ne> are unaffected by locale: they always
perform a char-by-char comparison of their scalar operands.  What's
more, if C<cmp> finds that its operands are equal according to the
collation sequence specified by the current locale, it goes on to
perform a char-by-char comparison, and only returns I<0> (equal) if the
operands are char-for-char identical.  If you really want to know whether
two strings--which C<eq> and C<cmp> may consider different--are equal
as far as collation in the locale is concerned, see the discussion in
L<Category LC_COLLATE: Collation>.

=item *

B<Regular expressions and case-modification functions> (C<uc()>, C<lc()>,
C<ucfirst()>, and C<lcfirst()>) use C<LC_CTYPE>

=back

=back

The default behavior is restored with the S<C<no locale>> pragma, or
upon reaching the end of the block enclosing C<use locale>.
Note that C<use locale> and C<use locale ':not_characters'> may be
nested, and that what is in effect within an inner scope will revert to
the outer scope's rules at the end of the inner scope.

The string result of any operation that uses locale
information is tainted, as it is possible for a locale to be
untrustworthy.  See L<"SECURITY">.

=head2 The setlocale function

You can switch locales as often as you wish at run time with the
C<POSIX::setlocale()> function:

        # Import locale-handling tool set from POSIX module.
        # This example uses: setlocale -- the function call
        #                    LC_CTYPE -- explained below
        # (Showing the testing for success/failure of operations is
        # omitted in these examples to avoid distracting from the main
        # point)

        use POSIX qw(locale_h);
        use locale;
        my $old_locale;

        # query and save the old locale
        $old_locale = setlocale(LC_CTYPE);

        setlocale(LC_CTYPE, "fr_CA.ISO8859-1");
        # LC_CTYPE now in locale "French, Canada, codeset ISO 8859-1"

        setlocale(LC_CTYPE, "");
        # LC_CTYPE now reset to the default defined by the
        # LC_ALL/LC_CTYPE/LANG environment variables, or to the system
        # default.  See below for documentation.

        # restore the old locale
        setlocale(LC_CTYPE, $old_locale);

The first argument of C<setlocale()> gives the B<category>, the second the
B<locale>.  The category tells in what aspect of data processing you
want to apply locale-specific rules.  Category names are discussed in
L</LOCALE CATEGORIES> and L</"ENVIRONMENT">.  The locale is the name of a
collection of customization information corresponding to a particular
combination of language, country or territory, and codeset.  Read on for
hints on the naming of locales: not all systems name locales as in the
example.

If no second argument is provided and the category is something other
than LC_ALL, the function returns a string naming the current locale
for the category.  You can use this value as the second argument in a
subsequent call to C<setlocale()>, B<but> on some platforms the string
is opaque, not something that most people would be able to decipher as
to what locale it means.

If no second argument is provided and the category is LC_ALL, the
result is implementation-dependent.  It may be a string of
concatenated locale names (separator also implementation-dependent)
or a single locale name.  Please consult your L<setlocale(3)> man page for
details.

If a second argument is given and it corresponds to a valid locale,
the locale for the category is set to that value, and the function
returns the now-current locale value.  You can then use this in yet
another call to C<setlocale()>.  (In some implementations, the return
value may sometimes differ from the value you gave as the second
argument--think of it as an alias for the value you gave.)

As the example shows, if the second argument is an empty string, the
category's locale is returned to the default specified by the
corresponding environment variables.  Generally, this results in a
return to the default that was in force when Perl started up: changes
to the environment made by the application after startup may or may not
be noticed, depending on your system's C library.

Note that Perl ignores the current C<LC_CTYPE> and C<LC_COLLATE> locales
within the scope of a C<use locale ':not_characters'>.

If C<set_locale()> fails for some reason (for example, an attempt to set
to a locale unknown to the system), the locale for the category is not
changed, and the function returns C<undef>.


For further information about the categories, consult L<setlocale(3)>.

=head2 Finding locales

For locales available in your system, consult also L<setlocale(3)> to
see whether it leads to the list of available locales (search for the
I<SEE ALSO> section).  If that fails, try the following command lines:

        locale -a

        nlsinfo

        ls /usr/lib/nls/loc

        ls /usr/lib/locale

        ls /usr/lib/nls

	ls /usr/share/locale

and see whether they list something resembling these

        en_US.ISO8859-1     de_DE.ISO8859-1     ru_RU.ISO8859-5
        en_US.iso88591      de_DE.iso88591      ru_RU.iso88595
        en_US               de_DE               ru_RU
        en                  de                  ru
        english             german              russian
        english.iso88591    german.iso88591     russian.iso88595
        english.roman8                          russian.koi8r

Sadly, even though the calling interface for C<setlocale()> has been
standardized, names of locales and the directories where the
configuration resides have not been.  The basic form of the name is
I<language_territory>B<.>I<codeset>, but the latter parts after
I<language> are not always present.  The I<language> and I<country>
are usually from the standards B<ISO 3166> and B<ISO 639>, the
two-letter abbreviations for the countries and the languages of the
world, respectively.  The I<codeset> part often mentions some B<ISO
8859> character set, the Latin codesets.  For example, C<ISO 8859-1>
is the so-called "Western European codeset" that can be used to encode
most Western European languages adequately.  Again, there are several
ways to write even the name of that one standard.  Lamentably.

Two special locales are worth particular mention: "C" and "POSIX".
Currently these are effectively the same locale: the difference is
mainly that the first one is defined by the C standard, the second by
the POSIX standard.  They define the B<default locale> in which
every program starts in the absence of locale information in its
environment.  (The I<default> default locale, if you will.)  Its language
is (American) English and its character codeset ASCII or, rarely, a
superset thereof (such as the "DEC Multinational Character Set
(DEC-MCS)").  B<Warning>. The C locale delivered by some vendors
may not actually exactly match what the C standard calls for.  So
beware.

B<NOTE>: Not all systems have the "POSIX" locale (not all systems are
POSIX-conformant), so use "C" when you need explicitly to specify this
default locale.

=head2 LOCALE PROBLEMS

You may encounter the following warning message at Perl startup:

	perl: warning: Setting locale failed.
	perl: warning: Please check that your locale settings:
	        LC_ALL = "En_US",
	        LANG = (unset)
	    are supported and installed on your system.
	perl: warning: Falling back to the standard locale ("C").

This means that your locale settings had LC_ALL set to "En_US" and
LANG exists but has no value.  Perl tried to believe you but could not.
Instead, Perl gave up and fell back to the "C" locale, the default locale
that is supposed to work no matter what.  (On Windows, it first tries
falling back to the system default locale.)  This usually means your
locale settings were wrong, they mention locales your system has never
heard of, or the locale installation in your system has problems (for
example, some system files are broken or missing).  There are quick and
temporary fixes to these problems, as well as more thorough and lasting
fixes.

=head2 Testing for broken locales

If you are building Perl from source, the Perl test suite file
F<lib/locale.t> can be used to test the locales on your system.
Setting the environment variable C<PERL_DEBUG_FULL_TEST> to 1
will cause it to output detailed results.  For example, on Linux, you
could say

 PERL_DEBUG_FULL_TEST=1 ./perl -T -Ilib lib/locale.t > locale.log 2>&1

Besides many other tests, it will test every locale it finds on your
system to see if they conform to the POSIX standard.  If any have
errors, it will include a summary near the end of the output of which
locales passed all its tests, and which failed, and why.

=head2 Temporarily fixing locale problems

The two quickest fixes are either to render Perl silent about any
locale inconsistencies or to run Perl under the default locale "C".

Perl's moaning about locale problems can be silenced by setting the
environment variable PERL_BADLANG to a zero value, for example "0".
This method really just sweeps the problem under the carpet: you tell
Perl to shut up even when Perl sees that something is wrong.  Do not
be surprised if later something locale-dependent misbehaves.

Perl can be run under the "C" locale by setting the environment
variable LC_ALL to "C".  This method is perhaps a bit more civilized
than the PERL_BADLANG approach, but setting LC_ALL (or
other locale variables) may affect other programs as well, not just
Perl.  In particular, external programs run from within Perl will see
these changes.  If you make the new settings permanent (read on), all
programs you run see the changes.  See L<"ENVIRONMENT"> for
the full list of relevant environment variables and L<USING LOCALES>
for their effects in Perl.  Effects in other programs are
easily deducible.  For example, the variable LC_COLLATE may well affect
your B<sort> program (or whatever the program that arranges "records"
alphabetically in your system is called).

You can test out changing these variables temporarily, and if the
new settings seem to help, put those settings into your shell startup
files.  Consult your local documentation for the exact details.  For in
Bourne-like shells (B<sh>, B<ksh>, B<bash>, B<zsh>):

	LC_ALL=en_US.ISO8859-1
	export LC_ALL

This assumes that we saw the locale "en_US.ISO8859-1" using the commands
discussed above.  We decided to try that instead of the above faulty
locale "En_US"--and in Cshish shells (B<csh>, B<tcsh>)

	setenv LC_ALL en_US.ISO8859-1

or if you have the "env" application you can do in any shell

	env LC_ALL=en_US.ISO8859-1 perl ...

If you do not know what shell you have, consult your local
helpdesk or the equivalent.

=head2 Permanently fixing locale problems

The slower but superior fixes are when you may be able to yourself
fix the misconfiguration of your own environment variables.  The
mis(sing)configuration of the whole system's locales usually requires
the help of your friendly system administrator.

First, see earlier in this document about L<Finding locales>.  That tells
how to find which locales are really supported--and more importantly,
installed--on your system.  In our example error message, environment
variables affecting the locale are listed in the order of decreasing
importance (and unset variables do not matter).  Therefore, having
LC_ALL set to "En_US" must have been the bad choice, as shown by the
error message.  First try fixing locale settings listed first.

Second, if using the listed commands you see something B<exactly>
(prefix matches do not count and case usually counts) like "En_US"
without the quotes, then you should be okay because you are using a
locale name that should be installed and available in your system.
In this case, see L<Permanently fixing your system's locale configuration>.

=head2 Permanently fixing your system's locale configuration

This is when you see something like:

	perl: warning: Please check that your locale settings:
	        LC_ALL = "En_US",
	        LANG = (unset)
	    are supported and installed on your system.

but then cannot see that "En_US" listed by the above-mentioned
commands.  You may see things like "en_US.ISO8859-1", but that isn't
the same.  In this case, try running under a locale
that you can list and which somehow matches what you tried.  The
rules for matching locale names are a bit vague because
standardization is weak in this area.  See again the
L<Finding locales> about general rules.

=head2 Fixing system locale configuration

Contact a system administrator (preferably your own) and report the exact
error message you get, and ask them to read this same documentation you
are now reading.  They should be able to check whether there is something
wrong with the locale configuration of the system.  The L<Finding locales>
section is unfortunately a bit vague about the exact commands and places
because these things are not that standardized.

=head2 The localeconv function

The C<POSIX::localeconv()> function allows you to get particulars of the
locale-dependent numeric formatting information specified by the current
C<LC_NUMERIC> and C<LC_MONETARY> locales.  (If you just want the name of
the current locale for a particular category, use C<POSIX::setlocale()>
with a single parameter--see L<The setlocale function>.)

        use POSIX qw(locale_h);

        # Get a reference to a hash of locale-dependent info
        $locale_values = localeconv();

        # Output sorted list of the values
        for (sort keys %$locale_values) {
            printf "%-20s = %s\n", $_, $locale_values->{$_}
        }

C<localeconv()> takes no arguments, and returns B<a reference to> a hash.
The keys of this hash are variable names for formatting, such as
C<decimal_point> and C<thousands_sep>.  The values are the
corresponding, er, values.  See L<POSIX/localeconv> for a longer
example listing the categories an implementation might be expected to
provide; some provide more and others fewer.  You don't need an
explicit C<use locale>, because C<localeconv()> always observes the
current locale.

Here's a simple-minded example program that rewrites its command-line
parameters as integers correctly formatted in the current locale:

    use POSIX qw(locale_h);

    # Get some of locale's numeric formatting parameters
    my ($thousands_sep, $grouping) =
            @{localeconv()}{'thousands_sep', 'grouping'};

    # Apply defaults if values are missing
    $thousands_sep = ',' unless $thousands_sep;

    # grouping and mon_grouping are packed lists
    # of small integers (characters) telling the
    # grouping (thousand_seps and mon_thousand_seps
    # being the group dividers) of numbers and
    # monetary quantities.  The integers' meanings:
    # 255 means no more grouping, 0 means repeat
    # the previous grouping, 1-254 means use that
    # as the current grouping.  Grouping goes from
    # right to left (low to high digits).  In the
    # below we cheat slightly by never using anything
    # else than the first grouping (whatever that is).
    if ($grouping) {
        @grouping = unpack("C*", $grouping);
    } else {
        @grouping = (3);
    }

    # Format command line params for current locale
    for (@ARGV) {
        $_ = int;    # Chop non-integer part
        1 while
        s/(\d)(\d{$grouping[0]}($|$thousands_sep))/$1$thousands_sep$2/;
        print "$_";
    }
    print "\n";

=head2 I18N::Langinfo

Another interface for querying locale-dependent information is the
C<I18N::Langinfo::langinfo()> function, available at least in Unix-like
systems and VMS.

The following example will import the C<langinfo()> function itself and
three constants to be used as arguments to C<langinfo()>: a constant for
the abbreviated first day of the week (the numbering starts from
Sunday = 1) and two more constants for the affirmative and negative
answers for a yes/no question in the current locale.

    use I18N::Langinfo qw(langinfo ABDAY_1 YESSTR NOSTR);

    my ($abday_1, $yesstr, $nostr)
                = map { langinfo } qw(ABDAY_1 YESSTR NOSTR);

    print "$abday_1? [$yesstr/$nostr] ";

In other words, in the "C" (or English) locale the above will probably
print something like:

    Sun? [yes/no]

See L<I18N::Langinfo> for more information.

=head1 LOCALE CATEGORIES

The following subsections describe basic locale categories.  Beyond these,
some combination categories allow manipulation of more than one
basic category at a time.  See L<"ENVIRONMENT"> for a discussion of these.

=head2 Category LC_COLLATE: Collation

In the scope of S<C<use locale>> (but not a
C<use locale ':not_characters'>), Perl looks to the C<LC_COLLATE>
environment variable to determine the application's notions on collation
(ordering) of characters.  For example, "b" follows "a" in Latin
alphabets, but where do "E<aacute>" and "E<aring>" belong?  And while
"color" follows "chocolate" in English, what about in traditional Spanish?

The following collations all make sense and you may meet any of them
if you "use locale".

	A B C D E a b c d e
	A a B b C c D d E e
	a A b B c C d D e E
	a b c d e A B C D E

Here is a code snippet to tell what "word"
characters are in the current locale, in that locale's order:

        use locale;
        print +(sort grep /\w/, map { chr } 0..255), "\n";

Compare this with the characters that you see and their order if you
state explicitly that the locale should be ignored:

        no locale;
        print +(sort grep /\w/, map { chr } 0..255), "\n";

This machine-native collation (which is what you get unless S<C<use
locale>> has appeared earlier in the same block) must be used for
sorting raw binary data, whereas the locale-dependent collation of the
first example is useful for natural text.

As noted in L<USING LOCALES>, C<cmp> compares according to the current
collation locale when C<use locale> is in effect, but falls back to a
char-by-char comparison for strings that the locale says are equal. You
can use C<POSIX::strcoll()> if you don't want this fall-back:

        use POSIX qw(strcoll);
        $equal_in_locale =
            !strcoll("space and case ignored", "SpaceAndCaseIgnored");

C<$equal_in_locale> will be true if the collation locale specifies a
dictionary-like ordering that ignores space characters completely and
which folds case.

Perl only supports single-byte locales for C<LC_COLLATE>.  This means
that a UTF-8 locale likely will just give you machine-native ordering.
Use L<Unicode::Collate> for the full implementation of the Unicode
Collation Algorithm.

If you have a single string that you want to check for "equality in
locale" against several others, you might think you could gain a little
efficiency by using C<POSIX::strxfrm()> in conjunction with C<eq>:

        use POSIX qw(strxfrm);
        $xfrm_string = strxfrm("Mixed-case string");
        print "locale collation ignores spaces\n"
            if $xfrm_string eq strxfrm("Mixed-casestring");
        print "locale collation ignores hyphens\n"
            if $xfrm_string eq strxfrm("Mixedcase string");
        print "locale collation ignores case\n"
            if $xfrm_string eq strxfrm("mixed-case string");

C<strxfrm()> takes a string and maps it into a transformed string for use
in char-by-char comparisons against other transformed strings during
collation.  "Under the hood", locale-affected Perl comparison operators
call C<strxfrm()> for both operands, then do a char-by-char
comparison of the transformed strings.  By calling C<strxfrm()> explicitly
and using a non locale-affected comparison, the example attempts to save
a couple of transformations.  But in fact, it doesn't save anything: Perl
magic (see L<perlguts/Magic Variables>) creates the transformed version of a
string the first time it's needed in a comparison, then keeps this version around
in case it's needed again.  An example rewritten the easy way with
C<cmp> runs just about as fast.  It also copes with null characters
embedded in strings; if you call C<strxfrm()> directly, it treats the first
null it finds as a terminator.  don't expect the transformed strings
it produces to be portable across systems--or even from one revision
of your operating system to the next.  In short, don't call C<strxfrm()>
directly: let Perl do it for you.

Note: C<use locale> isn't shown in some of these examples because it isn't
needed: C<strcoll()> and C<strxfrm()> are POSIX functions
which use the standard system-supplied C<libc> functions that
always obey the current C<LC_COLLATE> locale.

=head2 Category LC_CTYPE: Character Types

In the scope of S<C<use locale>> (but not a
C<use locale ':not_characters'>), Perl obeys the C<LC_CTYPE> locale
setting.  This controls the application's notion of which characters are
alphabetic, numeric, punctuation, I<etc>.  This affects Perl's C<\w>
regular expression metanotation,
which stands for alphanumeric characters--that is, alphabetic,
numeric, and the platform's native underscore.
(Consult L<perlre> for more information about
regular expressions.)  Thanks to C<LC_CTYPE>, depending on your locale
setting, characters like "E<aelig>", "E<eth>", "E<szlig>", and
"E<oslash>" may be understood as C<\w> characters.
It also affects things like C<\s>, C<\D>, and the POSIX character
classes, like C<[[:graph:]]>.  (See L<perlrecharclass> for more
information on all these.)

The C<LC_CTYPE> locale also provides the map used in transliterating
characters between lower and uppercase.  This affects the case-mapping
functions--C<fc()>, C<lc()>, C<lcfirst()>, C<uc()>, and C<ucfirst()>; case-mapping
interpolation with C<\F>, C<\l>, C<\L>, C<\u>, or C<\U> in double-quoted
strings and C<s///> substitutions; and case-independent regular expression
pattern matching using the C<i> modifier.

Finally, C<LC_CTYPE> affects the (deprecated) POSIX character-class test
functions--C<POSIX::isalpha()>, C<POSIX::islower()>, and so on.  For
example, if you move from the "C" locale to a 7-bit Scandinavian one,
you may find--possibly to your surprise--that "|" moves from the
C<POSIX::ispunct()> class to C<POSIX::isalpha()>.
Unfortunately, this creates big problems for regular expressions. "|" still
means alternation even though it matches C<\w>.

Starting in v5.20, Perl supports UTF-8 locales for C<LC_CTYPE>, but
otherwise Perl only supports single-byte locales, such as the ISO 8859
series.  This means that wide character locales, for example for Asian
languages, are not supported.  The UTF-8 locale support is actually a
superset of POSIX locales, because it is really full Unicode behavior
as if no locale were in effect at all (except for tainting; see
L</SECURITY>).  POSIX locales, even UTF-8 ones,
are lacking certain concepts in Unicode, such as the idea that changing
the case of a character could expand to be more than one character.
Perl in a UTF-8 locale, will give you that expansion.  Prior to v5.20,
Perl treated a UTF-8 locale on some platforms like an ISO 8859-1 one,
with some restrictions, and on other platforms more like the "C" locale.
For releases v5.16 and v5.18, C<S<use locale 'not_characters>> could be
used as a workaround for this (see L</Unicode and UTF-8>).

Note that there are quite a few things that are unaffected by the
current locale.  All the escape sequences for particular characters,
C<\n> for example, always mean the platform's native one.  This means,
for example, that C<\N> in regular expressions (every character
but new-line) works on the platform character set.

B<Note:> A broken or malicious C<LC_CTYPE> locale definition may result
in clearly ineligible characters being considered to be alphanumeric by
your application.  For strict matching of (mundane) ASCII letters and
digits--for example, in command strings--locale-aware applications
should use C<\w> with the C</a> regular expression modifier.  See L<"SECURITY">.

=head2 Category LC_NUMERIC: Numeric Formatting

After a proper C<POSIX::setlocale()> call, and within the scope of one
of the C<use locale> variants, Perl obeys the C<LC_NUMERIC>
locale information, which controls an application's idea of how numbers
should be formatted for human readability.
In most implementations the only effect is to
change the character used for the decimal point--perhaps from "."  to ",".
The functions aren't aware of such niceties as thousands separation and
so on. (See L<The localeconv function> if you care about these things.)

 use POSIX qw(strtod setlocale LC_NUMERIC);
 use locale;

 setlocale LC_NUMERIC, "";

 $n = 5/2;   # Assign numeric 2.5 to $n

 $a = " $n"; # Locale-dependent conversion to string

 print "half five is $n\n";       # Locale-dependent output

 printf "half five is %g\n", $n;  # Locale-dependent output

 print "DECIMAL POINT IS COMMA\n"
          if $n == (strtod("2,5"))[0]; # Locale-dependent conversion

See also L<I18N::Langinfo> and C<RADIXCHAR>.

=head2 Category LC_MONETARY: Formatting of monetary amounts

The C standard defines the C<LC_MONETARY> category, but not a function
that is affected by its contents.  (Those with experience of standards
committees will recognize that the working group decided to punt on the
issue.)  Consequently, Perl essentially takes no notice of it.  If you
really want to use C<LC_MONETARY>, you can query its contents--see
L<The localeconv function>--and use the information that it returns in your
application's own formatting of currency amounts.  However, you may well
find that the information, voluminous and complex though it may be, still
does not quite meet your requirements: currency formatting is a hard nut
to crack.

See also L<I18N::Langinfo> and C<CRNCYSTR>.

=head2 LC_TIME

Output produced by C<POSIX::strftime()>, which builds a formatted
human-readable date/time string, is affected by the current C<LC_TIME>
locale.  Thus, in a French locale, the output produced by the C<%B>
format element (full month name) for the first month of the year would
be "janvier".  Here's how to get a list of long month names in the
current locale:

        use POSIX qw(strftime);
        for (0..11) {
            $long_month_name[$_] =
                strftime("%B", 0, 0, 0, 1, $_, 96);
        }

Note: C<use locale> isn't needed in this example: C<strftime()> is a POSIX
function which uses the standard system-supplied C<libc> function that
always obeys the current C<LC_TIME> locale.

See also L<I18N::Langinfo> and C<ABDAY_1>..C<ABDAY_7>, C<DAY_1>..C<DAY_7>,
C<ABMON_1>..C<ABMON_12>, and C<ABMON_1>..C<ABMON_12>.

=head2 Other categories

The remaining locale categories are not currently used by Perl itself.
But again note that things Perl interacts with may use these, including
extensions outside the standard Perl distribution, and by the
operating system and its utilities.  Note especially that the string
value of C<$!> and the error messages given by external utilities may
be changed by C<LC_MESSAGES>.  If you want to have portable error
codes, use C<%!>.  See L<Errno>.

=head1 SECURITY

Although the main discussion of Perl security issues can be found in
L<perlsec>, a discussion of Perl's locale handling would be incomplete
if it did not draw your attention to locale-dependent security issues.
Locales--particularly on systems that allow unprivileged users to
build their own locales--are untrustworthy.  A malicious (or just plain
broken) locale can make a locale-aware application give unexpected
results.  Here are a few possibilities:

=over 4

=item *

Regular expression checks for safe file names or mail addresses using
C<\w> may be spoofed by an C<LC_CTYPE> locale that claims that
characters such as "E<gt>" and "|" are alphanumeric.

=item *

String interpolation with case-mapping, as in, say, C<$dest =
"C:\U$name.$ext">, may produce dangerous results if a bogus LC_CTYPE
case-mapping table is in effect.

=item *

A sneaky C<LC_COLLATE> locale could result in the names of students with
"D" grades appearing ahead of those with "A"s.

=item *

An application that takes the trouble to use information in
C<LC_MONETARY> may format debits as if they were credits and vice versa
if that locale has been subverted.  Or it might make payments in US
dollars instead of Hong Kong dollars.

=item *

The date and day names in dates formatted by C<strftime()> could be
manipulated to advantage by a malicious user able to subvert the
C<LC_DATE> locale.  ("Look--it says I wasn't in the building on
Sunday.")

=back

Such dangers are not peculiar to the locale system: any aspect of an
application's environment which may be modified maliciously presents
similar challenges.  Similarly, they are not specific to Perl: any
programming language that allows you to write programs that take
account of their environment exposes you to these issues.

Perl cannot protect you from all possibilities shown in the
examples--there is no substitute for your own vigilance--but, when
C<use locale> is in effect, Perl uses the tainting mechanism (see
L<perlsec>) to mark string results that become locale-dependent, and
which may be untrustworthy in consequence.  Here is a summary of the
tainting behavior of operators and functions that may be affected by
the locale:

=over 4

=item  *

B<Comparison operators> (C<lt>, C<le>, C<ge>, C<gt> and C<cmp>):

Scalar true/false (or less/equal/greater) result is never tainted.

=item  *

B<Case-mapping interpolation> (with C<\l>, C<\L>, C<\u>, C<\U>, or C<\F>)

Result string containing interpolated material is tainted if
C<use locale> (but not S<C<use locale ':not_characters'>>) is in effect.

=item  *

B<Matching operator> (C<m//>):

Scalar true/false result never tainted.

All subpatterns, either delivered as a list-context result or as C<$1>
I<etc>., are tainted if C<use locale> (but not
S<C<use locale ':not_characters'>>) is in effect, and the subpattern
regular expression contains a locale-dependent construct.  These
constructs include C<\w> (to match an alphanumeric character), C<\W>
(non-alphanumeric character), C<\b> and C<\B> (word-boundary and
non-boundardy, which depend on what C<\w> and C<\W> match), C<\s>
(whitespace character), C<\S> (non whitespace character), C<\d> and
C<\D> (digits and non-digits), and the POSIX character classes, such as
C<[:alpha:]> (see L<perlrecharclass/POSIX Character Classes>).

Tainting is also likely if the pattern is to be matched
case-insensitively (via C</i>).  The exception is if all the code points
to be matched this way are above 255 and do not have folds under Unicode
rules to below 256.  Tainting is not done for these because Perl
only uses Unicode rules for such code points, and those rules are the
same no matter what the current locale.

The matched-pattern variables, C<$&>, C<$`> (pre-match), C<$'>
(post-match), and C<$+> (last match) also are tainted.

=item  *

B<Substitution operator> (C<s///>):

Has the same behavior as the match operator.  Also, the left
operand of C<=~> becomes tainted when C<use locale>
(but not S<C<use locale ':not_characters'>>) is in effect if modified as
a result of a substitution based on a regular
expression match involving any of the things mentioned in the previous
item, or of case-mapping, such as C<\l>, C<\L>,C<\u>, C<\U>, or C<\F>.

=item *

B<Output formatting functions> (C<printf()> and C<write()>):

Results are never tainted because otherwise even output from print,
for example C<print(1/7)>, should be tainted if C<use locale> is in
effect.

=item *

B<Case-mapping functions> (C<lc()>, C<lcfirst()>, C<uc()>, C<ucfirst()>):

Results are tainted if C<use locale> (but not
S<C<use locale ':not_characters'>>) is in effect.

=item *

B<POSIX locale-dependent functions> (C<localeconv()>, C<strcoll()>,
C<strftime()>, C<strxfrm()>):

Results are never tainted.

=item *

B<POSIX character class tests> (C<POSIX::isalnum()>,
C<POSIX::isalpha()>, C<POSIX::isdigit()>, C<POSIX::isgraph()>,
C<POSIX::islower()>, C<POSIX::isprint()>, C<POSIX::ispunct()>,
C<POSIX::isspace()>, C<POSIX::isupper()>, C<POSIX::isxdigit()>):

True/false results are never tainted.

=back

Three examples illustrate locale-dependent tainting.
The first program, which ignores its locale, won't run: a value taken
directly from the command line may not be used to name an output file
when taint checks are enabled.

        #/usr/local/bin/perl -T
        # Run with taint checking

        # Command line sanity check omitted...
        $tainted_output_file = shift;

        open(F, ">$tainted_output_file")
            or warn "Open of $tainted_output_file failed: $!\n";

The program can be made to run by "laundering" the tainted value through
a regular expression: the second example--which still ignores locale
information--runs, creating the file named on its command line
if it can.

        #/usr/local/bin/perl -T

        $tainted_output_file = shift;
        $tainted_output_file =~ m%[\w/]+%;
        $untainted_output_file = $&;

        open(F, ">$untainted_output_file")
            or warn "Open of $untainted_output_file failed: $!\n";

Compare this with a similar but locale-aware program:

        #/usr/local/bin/perl -T

        $tainted_output_file = shift;
        use locale;
        $tainted_output_file =~ m%[\w/]+%;
        $localized_output_file = $&;

        open(F, ">$localized_output_file")
            or warn "Open of $localized_output_file failed: $!\n";

This third program fails to run because C<$&> is tainted: it is the result
of a match involving C<\w> while C<use locale> is in effect.

=head1 ENVIRONMENT

=over 12

=item PERL_SKIP_LOCALE_INIT

This environment variable, available starting in Perl v5.20, and if it
evaluates to a TRUE value, tells Perl to not use the rest of the
environment variables to initialize with.  Instead, Perl uses whatever
the current locale settings are.  This is particularly useful in
embedded environments, see
L<perlembed/Using embedded Perl with POSIX locales>.

=item PERL_BADLANG

A string that can suppress Perl's warning about failed locale settings
at startup.  Failure can occur if the locale support in the operating
system is lacking (broken) in some way--or if you mistyped the name of
a locale when you set up your environment.  If this environment
variable is absent, or has a value that does not evaluate to integer
zero--that is, "0" or ""-- Perl will complain about locale setting
failures.

B<NOTE>: PERL_BADLANG only gives you a way to hide the warning message.
The message tells about some problem in your system's locale support,
and you should investigate what the problem is.

=back

The following environment variables are not specific to Perl: They are
part of the standardized (ISO C, XPG4, POSIX 1.c) C<setlocale()> method
for controlling an application's opinion on data.  Windows is non-POSIX,
but Perl arranges for the following to work as described anyway.
If the locale given by an environment variable is not valid, Perl tries
the next lower one in priority.  If none are valid, on Windows, the
system default locale is then tried.  If all else fails, the C<"C">
locale is used.  If even that doesn't work, something is badly broken,
but Perl tries to forge ahead with whatever the locale settinga might
be.

=over 12

=item LC_ALL

C<LC_ALL> is the "override-all" locale environment variable. If
set, it overrides all the rest of the locale environment variables.

=item LANGUAGE

B<NOTE>: C<LANGUAGE> is a GNU extension, it affects you only if you
are using the GNU libc.  This is the case if you are using e.g. Linux.
If you are using "commercial" Unixes you are most probably I<not>
using GNU libc and you can ignore C<LANGUAGE>.

However, in the case you are using C<LANGUAGE>: it affects the
language of informational, warning, and error messages output by
commands (in other words, it's like C<LC_MESSAGES>) but it has higher
priority than C<LC_ALL>.  Moreover, it's not a single value but
instead a "path" (":"-separated list) of I<languages> (not locales).
See the GNU C<gettext> library documentation for more information.

=item LC_CTYPE

In the absence of C<LC_ALL>, C<LC_CTYPE> chooses the character type
locale.  In the absence of both C<LC_ALL> and C<LC_CTYPE>, C<LANG>
chooses the character type locale.

=item LC_COLLATE

In the absence of C<LC_ALL>, C<LC_COLLATE> chooses the collation
(sorting) locale.  In the absence of both C<LC_ALL> and C<LC_COLLATE>,
C<LANG> chooses the collation locale.

=item LC_MONETARY

In the absence of C<LC_ALL>, C<LC_MONETARY> chooses the monetary
formatting locale.  In the absence of both C<LC_ALL> and C<LC_MONETARY>,
C<LANG> chooses the monetary formatting locale.

=item LC_NUMERIC

In the absence of C<LC_ALL>, C<LC_NUMERIC> chooses the numeric format
locale.  In the absence of both C<LC_ALL> and C<LC_NUMERIC>, C<LANG>
chooses the numeric format.

=item LC_TIME

In the absence of C<LC_ALL>, C<LC_TIME> chooses the date and time
formatting locale.  In the absence of both C<LC_ALL> and C<LC_TIME>,
C<LANG> chooses the date and time formatting locale.

=item LANG

C<LANG> is the "catch-all" locale environment variable. If it is set, it
is used as the last resort after the overall C<LC_ALL> and the
category-specific C<LC_...>.

=back

=head2 Examples

The LC_NUMERIC controls the numeric output:

   use locale;
   use POSIX qw(locale_h); # Imports setlocale() and the LC_ constants.
   setlocale(LC_NUMERIC, "fr_FR") or die "Pardon";
   printf "%g\n", 1.23; # If the "fr_FR" succeeded, probably shows 1,23.

and also how strings are parsed by C<POSIX::strtod()> as numbers:

   use locale;
   use POSIX qw(locale_h strtod);
   setlocale(LC_NUMERIC, "de_DE") or die "Entschuldigung";
   my $x = strtod("2,34") + 5;
   print $x, "\n"; # Probably shows 7,34.

=head1 NOTES

=head2 String C<eval> and C<LC_NUMERIC>

A string L<eval|perlfunc/eval EXPR> parses its expression as standard
Perl.  It is therefore expecting the decimal point to be a dot.  If
C<LC_NUMERIC> is set to have this be a comma instead, the parsing will
be confused, perhaps silently.

 use locale;
 use POSIX qw(locale_h);
 setlocale(LC_NUMERIC, "fr_FR") or die "Pardon";
 my $a = 1.2;
 print eval "$a + 1.5";
 print "\n";

prints C<13,5>.  This is because in that locale, the comma is the
decimal point character.  The C<eval> thus expands to:

 eval "1,2 + 1.5"

and the result is not what you likely expected.  No warnings are
generated.  If you do string C<eval>'s within the scope of
S<C<use locale>>, you should instead change the C<eval> line to do
something like:

 print eval "no locale; $a + 1.5";

This prints C<2.7>.

=head2 Backward compatibility

Versions of Perl prior to 5.004 B<mostly> ignored locale information,
generally behaving as if something similar to the C<"C"> locale were
always in force, even if the program environment suggested otherwise
(see L<The setlocale function>).  By default, Perl still behaves this
way for backward compatibility.  If you want a Perl application to pay
attention to locale information, you B<must> use the S<C<use locale>>
pragma (see L<The use locale pragma>) or, in the unlikely event
that you want to do so for just pattern matching, the
C</l> regular expression modifier (see L<perlre/Character set
modifiers>) to instruct it to do so.

Versions of Perl from 5.002 to 5.003 did use the C<LC_CTYPE>
information if available; that is, C<\w> did understand what
were the letters according to the locale environment variables.
The problem was that the user had no control over the feature:
if the C library supported locales, Perl used them.

=head2 I18N:Collate obsolete

In versions of Perl prior to 5.004, per-locale collation was possible
using the C<I18N::Collate> library module.  This module is now mildly
obsolete and should be avoided in new applications.  The C<LC_COLLATE>
functionality is now integrated into the Perl core language: One can
use locale-specific scalar data completely normally with C<use locale>,
so there is no longer any need to juggle with the scalar references of
C<I18N::Collate>.

=head2 Sort speed and memory use impacts

Comparing and sorting by locale is usually slower than the default
sorting; slow-downs of two to four times have been observed.  It will
also consume more memory: once a Perl scalar variable has participated
in any string comparison or sorting operation obeying the locale
collation rules, it will take 3-15 times more memory than before.  (The
exact multiplier depends on the string's contents, the operating system
and the locale.) These downsides are dictated more by the operating
system's implementation of the locale system than by Perl.

=head2 Freely available locale definitions

The Unicode CLDR project extracts the POSIX portion of many of its
locales, available at

  http://unicode.org/Public/cldr/latest/

There is a large collection of locale definitions at:

  http://std.dkuug.dk/i18n/WG15-collection/locales/

You should be aware that it is
unsupported, and is not claimed to be fit for any purpose.  If your
system allows installation of arbitrary locales, you may find the
definitions useful as they are, or as a basis for the development of
your own locales.

=head2 I18n and l10n

"Internationalization" is often abbreviated as B<i18n> because its first
and last letters are separated by eighteen others.  (You may guess why
the internalin ... internaliti ... i18n tends to get abbreviated.)  In
the same way, "localization" is often abbreviated to B<l10n>.

=head2 An imperfect standard

Internationalization, as defined in the C and POSIX standards, can be
criticized as incomplete, ungainly, and having too large a granularity.
(Locales apply to a whole process, when it would arguably be more useful
to have them apply to a single thread, window group, or whatever.)  They
also have a tendency, like standards groups, to divide the world into
nations, when we all know that the world can equally well be divided
into bankers, bikers, gamers, and so on.

=head1 Unicode and UTF-8

The support of Unicode is new starting from Perl version v5.6, and more fully
implemented in versions v5.8 and later.  See L<perluniintro>.

Starting in Perl v5.20, UTF-8 locales are supported in Perl, except for
C<LC_COLLATE> (use L<Unicode::Collate> instead).  If you have Perl v5.16
or v5.18 and can't upgrade, you can use

    use locale ':not_characters';

When this form of the pragma is used, only the non-character portions of
locales are used by Perl, for example C<LC_NUMERIC>.  Perl assumes that
you have translated all the characters it is to operate on into Unicode
(actually the platform's native character set (ASCII or EBCDIC) plus
Unicode).  For data in files, this can conveniently be done by also
specifying

    use open ':locale';

This pragma arranges for all inputs from files to be translated into
Unicode from the current locale as specified in the environment (see
L</ENVIRONMENT>), and all outputs to files to be translated back
into the locale.  (See L<open>).  On a per-filehandle basis, you can
instead use the L<PerlIO::locale> module, or the L<Encode::Locale>
module, both available from CPAN.  The latter module also has methods to
ease the handling of C<ARGV> and environment variables, and can be used
on individual strings.  If you know that all your locales will be
UTF-8, as many are these days, you can use the L<B<-C>|perlrun/-C>
command line switch.

This form of the pragma allows essentially seamless handling of locales
with Unicode.  The collation order will be by Unicode code point order.
It is strongly
recommended that when you need to order and sort strings that you use
the standard module L<Unicode::Collate> which gives much better results
in many instances than you can get with the old-style locale handling.

All the modules and switches just described can be used in v5.20 with
just plain C<use locale>, and, should the input locales not be UTF-8,
you'll get the less than ideal behavior, described below, that you get
with pre-v5.16 Perls, or when you use the locale pragma without the
C<:not_characters> parameter in v5.16 and v5.18.  If you are using
exclusively UTF-8 locales in v5.20 and higher, the rest of this section
does not apply to you.

There are two cases, multi-byte and single-byte locales.  First
multi-byte:

The only multi-byte (or wide character) locale that Perl is ever likely
to support is UTF-8.  This is due to the difficulty of implementation,
the fact that high quality UTF-8 locales are now published for every
area of the world (L<http://unicode.org/Public/cldr/latest/>), and that
failing all that you can use the L<Encode> module to translate to/from
your locale.  So, you'll have to do one of those things if you're using
one of these locales, such as Big5 or Shift JIS.  For UTF-8 locales, in
Perls (pre v5.20) that don't have full UTF-8 locale support, they may
work reasonably well (depending on your C library implementation)
simply because both
they and Perl store characters that take up multiple bytes the same way.
However, some, if not most, C library implementations may not process
the characters in the upper half of the Latin-1 range (128 - 255)
properly under LC_CTYPE.  To see if a character is a particular type
under a locale, Perl uses the functions like C<isalnum()>.  Your C
library may not work for UTF-8 locales with those functions, instead
only working under the newer wide library functions like C<iswalnum()>.
However, they are treated like single-byte locales, and will have the
restrictions described below.

For single-byte locales,
Perl generally takes the tack to use locale rules on code points that can fit
in a single byte, and Unicode rules for those that can't (though this
isn't uniformly applied, see the note at the end of this section).  This
prevents many problems in locales that aren't UTF-8.  Suppose the locale
is ISO8859-7, Greek.  The character at 0xD7 there is a capital Chi. But
in the ISO8859-1 locale, Latin1, it is a multiplication sign.  The POSIX
regular expression character class C<[[:alpha:]]> will magically match
0xD7 in the Greek locale but not in the Latin one.

However, there are places where this breaks down.  Certain Perl constructs are
for Unicode only, such as C<\p{Alpha}>.  They assume that 0xD7 always has its
Unicode meaning (or the equivalent on EBCDIC platforms).  Since Latin1 is a
subset of Unicode and 0xD7 is the multiplication sign in both Latin1 and
Unicode, C<\p{Alpha}> will never match it, regardless of locale.  A similar
issue occurs with C<\N{...}>.  Prior to v5.20, It is therefore a bad
idea to use C<\p{}> or
C<\N{}> under plain C<use locale>--I<unless> you can guarantee that the
locale will be a ISO8859-1.  Use POSIX character classes instead.

Another problem with this approach is that operations that cross the
single byte/multiple byte boundary are not well-defined, and so are
disallowed.  (This boundary is between the codepoints at 255/256.).
For example, lower casing LATIN CAPITAL LETTER Y WITH DIAERESIS (U+0178)
should return LATIN SMALL LETTER Y WITH DIAERESIS (U+00FF).  But in the
Greek locale, for example, there is no character at 0xFF, and Perl
has no way of knowing what the character at 0xFF is really supposed to
represent.  Thus it disallows the operation.  In this mode, the
lowercase of U+0178 is itself.

The same problems ensue if you enable automatic UTF-8-ification of your
standard file handles, default C<open()> layer, and C<@ARGV> on non-ISO8859-1,
non-UTF-8 locales (by using either the B<-C> command line switch or the
C<PERL_UNICODE> environment variable; see L<perlrun>).
Things are read in as UTF-8, which would normally imply a Unicode
interpretation, but the presence of a locale causes them to be interpreted
in that locale instead.  For example, a 0xD7 code point in the Unicode
input, which should mean the multiplication sign, won't be interpreted by
Perl that way under the Greek locale.  This is not a problem
I<provided> you make certain that all locales will always and only be either
an ISO8859-1, or, if you don't have a deficient C library, a UTF-8 locale.

Still another problem is that this approach can lead to two code
points meaning the same character.  Thus in a Greek locale, both U+03A7
and U+00D7 are GREEK CAPITAL LETTER CHI.

Vendor locales are notoriously buggy, and it is difficult for Perl to test
its locale-handling code because this interacts with code that Perl has no
control over; therefore the locale-handling code in Perl may be buggy as
well.  (However, the Unicode-supplied locales should be better, and
there is a feed back mechanism to correct any problems.  See
L</Freely available locale definitions>.)

If you have Perl v5.16, the problems mentioned above go away if you use
the C<:not_characters> parameter to the locale pragma (except for vendor
bugs in the non-character portions).  If you don't have v5.16, and you
I<do> have locales that work, using them may be worthwhile for certain
specific purposes, as long as you keep in mind the gotchas already
mentioned.  For example, if the collation for your locales works, it
runs faster under locales than under L<Unicode::Collate>; and you gain
access to such things as the local currency symbol and the names of the
months and days of the week.  (But to hammer home the point, in v5.16,
you get this access without the downsides of locales by using the
C<:not_characters> form of the pragma.)

Note: The policy of using locale rules for code points that can fit in a
byte, and Unicode rules for those that can't is not uniformly applied.
Pre-v5.12, it was somewhat haphazard; in v5.12 it was applied fairly
consistently to regular expression matching except for bracketed
character classes; in v5.14 it was extended to all regex matches; and in
v5.16 to the casing operations such as C<"\L"> and C<uc()>.  For
collation, in all releases, the system's C<strxfrm()> function is called,
and whatever it does is what you get.

=head1 BUGS

=head2 Broken systems

In certain systems, the operating system's locale support
is broken and cannot be fixed or used by Perl.  Such deficiencies can
and will result in mysterious hangs and/or Perl core dumps when
C<use locale> is in effect.  When confronted with such a system,
please report in excruciating detail to <F<perlbug@perl.org>>, and
also contact your vendor: bug fixes may exist for these problems
in your operating system.  Sometimes such bug fixes are called an
operating system upgrade.  If you have the source for Perl, include in
the perlbug email the output of the test described above in L</Testing
for broken locales>.

=head1 SEE ALSO

L<I18N::Langinfo>, L<perluniintro>, L<perlunicode>, L<open>,
L<POSIX/isalnum>, L<POSIX/isalpha>,
L<POSIX/isdigit>, L<POSIX/isgraph>, L<POSIX/islower>,
L<POSIX/isprint>, L<POSIX/ispunct>, L<POSIX/isspace>,
L<POSIX/isupper>, L<POSIX/isxdigit>, L<POSIX/localeconv>,
L<POSIX/setlocale>, L<POSIX/strcoll>, L<POSIX/strftime>,
L<POSIX/strtod>, L<POSIX/strxfrm>.

For special considerations when Perl is embedded in a C program,
see L<perlembed/Using embedded Perl with POSIX locales>.

=head1 HISTORY

Jarkko Hietaniemi's original F<perli18n.pod> heavily hacked by Dominic
Dunlop, assisted by the perl5-porters.  Prose worked over a bit by
Tom Christiansen, and updated by Perl 5 porters.