applied patch, with indentation tweaks

[p5sagit/p5-mst-13.2.git] / pod / perllocale.pod

=head1 NAME

perllocale - Perl locale handling (internationalization and localization)

=head1 DESCRIPTION

Perl supports language-specific notions of data such as "is this a
letter", "what is the uppercase equivalent of this letter", and "which
of these letters comes first".  These are important issues, especially
for languages other than English - but also for English: it would be
very naE<iuml>ve to think that C<A-Za-z> defines all the "letters". Perl
is also aware that some character other than '.' may be preferred as a
decimal point, and that output date representations may be
language-specific.  The process of making an application take account of
its users' preferences in such 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 can understand language-specific data via the standardized (ISO C,
XPG4, POSIX 1.c) method called "the locale system". The locale system is
controlled per application using one pragma, one function call, and
several environment variables.

B<NOTE>: This feature is new in Perl 5.004, and does not apply unless an
application specifically requests it - see L<Backward compatibility>.
The one exception is that write() now B<always> uses the current locale
- see L<"NOTES">.

=head1 PREPARING TO USE LOCALES

If Perl applications are to be able to understand and present your data
correctly according a locale of your choice, B<all> of the following
must be true:

=over 4

=item *

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

=item *

B<Definitions for the locales which 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, 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 which 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 *

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

=item *

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 ignores the current locale.  The S<C<use locale>>
pragma tells Perl to use the current locale for some operations:

=over 4

=item *

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

B<Note:> C<eq> and C<ne> are unaffected by the locale: they always
perform a byte-by-byte 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 byte-by-byte comparison, and only returns I<0> (equal) if the
operands are bit-for-bit 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> (uc(), lc(),
ucfirst(), and lcfirst()) use C<LC_CTYPE>

=item *

B<The formatting functions> (printf(), sprintf() and write()) use
C<LC_NUMERIC>

=item *

B<The POSIX date formatting function> (strftime()) uses C<LC_TIME>.

=back

C<LC_COLLATE>, C<LC_CTYPE>, and so on, are discussed further in L<LOCALE
CATEGORIES>.

The default behavior returns with S<C<no locale>> or on reaching the
end of the enclosing block.

Note that 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
POSIX::setlocale() function:

        # This functionality not usable prior to Perl 5.004
        require 5.004;

        # Import locale-handling tool set from POSIX module.
        # This example uses: setlocale -- the function call
        #                    LC_CTYPE -- explained below
        use POSIX qw(locale_h);

        # 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 default defined by LC_ALL/LC_CTYPE/LANG
        # environment variables.  See below for documentation.

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

The first argument of 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, 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 setlocale().  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 use this in a subsequent call to
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 that 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 which 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 the implementation of your system's C library.

If the second argument does not correspond to a valid locale, the locale
for the category is not changed, and the function returns I<undef>.

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

=head2 Finding locales

For the locales available in your system, also consult L<setlocale(3)>
and see whether it leads you to the list of the 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

and see whether they list something resembling these

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

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

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 and the second by
the POSIX standard.  What they define is the B<default locale> in which
every program starts in the absence of locale information in its
environment.  (The default default locale, if you will.)  Its language
is (American) English and its character codeset ASCII.

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 meet 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 were that LC_ALL equals "En_US"
and LANG exists but has no value.  Perl tried to believe you but it
could not.  Instead Perl gave up and fell back to the "C" locale, the
default locale that is supposed to work no matter what.  This usually
means either or both of the two problems: either your locale settings
were wrong, they talk of locales your system has never heard of, or
that the locale installation in your system has problems, for example
some system files are broken or missing.  For the problems there are
quick and temporary fixes and more thorough and lasting fixes.

=head2 Temporarily fixing locale problems

The two quickest fixes are either to make Perl be 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 non-zero value, for example
"1".  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 works funny.

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

You can first try out changing these variables temporarily and if the
new settings seem to help then put the settings into the startup files
of your environment.  Please consult your local documentation for the
exact details but very shortly for UNIXish systems: in Bourneish
shells (sh, ksh, bash, zsh) for example

        LC_ALL=en_US.ISO8859-1
        export LC_ALL

We assume here that we saw with the above discussed commands the
locale "en_US.ISO8859-1" and decided to try that instead of the above
faulty locale "En_US" -- and in Cshish shells (csh, tcsh)

        setenv LC_ALL en_US.ISO8859-1
        
If you do not know what shell you have, please consult your local
helpdesk or the equivalent.

=head2 Permanently fixing locale problems

Then the slower but better fixes: the misconfiguration of your own
environment variables you may be able to fix yourself; 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 you can find which locales really are supported and more
importantly, installed, in your system.  In our example error message
the environment variables affecting the locale are listed in the order
of decreasing importance and unset variables do not matter, therefore
in the above error message the LC_ALL being "En_US" must have been the
bad choice.  Always try fixing first the locale settings listed first.

Second, if you see with the listed commands something B<exactly> (for
example prefix matches do not count and case usually matters) 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 skip forward to L<Fixing the system locale
configuration>.

=head2 Permantently fixing your locale configuration

This is the case when for example you see

        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 is not
the same thing.  In this case you might try running under a locale
that you could list and somehow matches with what you tried.  The
rules for matching locale names are a bit vague because
standardisation is weak in this area.  See again the L<Finding
locales> about the general rules.

=head2 Permanently fixing the system locale configuration

Please contact your system administrator and tell her the exact error
message you get and ask her to read this same documentation you are
now reading.  She 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 standardised.

=head2 The localeconv function

The 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 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", $_, $locale_values->{$_}
        }

localeconv() takes no arguments, and returns B<a reference to> a hash.
The keys of this hash are formatting variable names such as
C<decimal_point> and C<thousands_sep>; the values are the corresponding
values.  See L<POSIX (3)/localeconv> for a longer example, which lists
all the categories an implementation might be expected to provide; some
provide more and others fewer, however.  Note that you don't need C<use
locale>: as a function with the job of querying the locale, localeconv()
always observes the current locale.

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

        # See comments in previous example
        require 5.004;
        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 = 3 unless $grouping;

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

=head1 LOCALE CATEGORIES

The subsections which follow describe basic locale categories.  As well
as these, there are some combination categories which allow the
manipulation of more than one basic category at a time.  See
L<"ENVIRONMENT"> for a discussion of these.

=head2 Category LC_COLLATE: Collation

When in the scope of S<C<use locale>>, Perl looks to the C<LC_COLLATE>
environment variable to determine the application's notions on the
collation (ordering) of characters.  ('b' follows 'a' in Latin
alphabets, but where do 'E<aacute>' and 'E<aring>' belong?)

Here is a code snippet that will tell you what are the alphanumeric
characters in the current locale, in the locale 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
byte-by-byte comparison for strings which the locale says are equal. You
can use POSIX::strcoll() if you don't want this fall-back:

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

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

If you have a single string which you want to check for "equality in
locale" against several others, you might think you could gain a little
efficiency by using 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");

strxfrm() takes a string and maps it into a transformed string for use
in byte-by-byte comparisons against other transformed strings during
collation.  "Under the hood", locale-affected Perl comparison operators
call strxfrm() for both their operands, then do a byte-by-byte
comparison of the transformed strings.  By calling strxfrm() explicitly,
and using a non locale-affected comparison, the example attempts to save
a couple of transformations.  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 it 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 strxfrm() directly, it treats the first
null it finds as a terminator.  And 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 strxfrm()
directly: let Perl do it for you.

Note: C<use locale> isn't shown in some of these examples, as it isn't
needed: strcoll() and strxfrm() exist only to generate locale-dependent
results, and so always obey the current C<LC_COLLATE> locale.

=head2 Category LC_CTYPE: Character Types

When in the scope of S<C<use locale>>, Perl obeys the C<LC_CTYPE> locale
setting.  This controls the application's notion of which characters are
alphabetic.  This affects Perl's C<\w> regular expression metanotation,
which stands for alphanumeric characters - that is, alphabetic and
numeric characters.  (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.

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

Finally, C<LC_CTYPE> affects the POSIX character-class test functions -
isalpha(), 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 ispunct() class to isalpha().

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 (unaccented) letters and
digits - for example, in command strings - locale-aware applications
should use C<\w> inside a C<no locale> block.  See L<"SECURITY">.

=head2 Category LC_NUMERIC: Numeric Formatting

When in the scope of S<C<use locale>>, Perl obeys the C<LC_NUMERIC>
locale information, which controls application's idea of how numbers
should be formatted for human readability by the printf(), sprintf(),
and write() functions.  String to numeric conversion by the
POSIX::strtod() function is also affected.  In most implementations the
only effect is to change the character used for the decimal point -
perhaps from '.'  to ',': these functions aren't aware of such niceties
as thousands separation and so on.  (See L<The localeconv function> if
you care about these things.)

Note that output produced by print() is B<never> affected by the
current locale: it is independent of whether C<use locale> or C<no
locale> is in effect, and corresponds to what you'd get from printf()
in the "C" locale.  The same is true for Perl's internal conversions
between numeric and string formats:

        use POSIX qw(strtod);
        use locale;

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

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

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

=head2 Category LC_MONETARY: Formatting of monetary amounts

The C standard defines the C<LC_MONETARY> category, but no 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 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, though voluminous and complex, does not quite
meet your requirements: currency formatting is a hard nut to crack.

=head2 LC_TIME

The output produced by 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 the 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: as a function which
exists only to generate locale-dependent results, strftime() always
obeys the current C<LC_TIME> locale.

=head2 Other categories

The remaining locale category, C<LC_MESSAGES> (possibly supplemented by
others in particular implementations) is not currently used by Perl -
except possibly to affect the behavior of library functions called by
extensions which are not part of the standard Perl distribution.

=head1 SECURITY

While 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 which 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 which 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 *

If the decimal point character in the C<LC_NUMERIC> locale is
surreptitiously changed from a dot to a comma, C<sprintf("%g",
0.123456e3)> produces a string result of "123,456". Many people would
interpret this as one hundred and twenty-three thousand, four hundred
and fifty-six.

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

=item *

The date and day names in dates formatted by 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 maliciously be modified presents
similar challenges.  Similarly, they are not specific to Perl: any
programming language which allows you to write programs which take
account of their environment exposes you to these issues.

Perl cannot protect you from all of the 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 which become locale-dependent, and
which may be untrustworthy in consequence.  Here is a summary of the
tainting behavior of operators and functions which 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> or C<\U>)

Result string containing interpolated material is tainted if
C<use locale> is in effect.

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

Scalar true/false result never tainted.

Subpatterns, either delivered as an array-context result, or as $1 etc.
are tainted if C<use locale> is in effect, and the subpattern regular
expression contains C<\w> (to match an alphanumeric character), C<\W>
(non-alphanumeric character), C<\s> (white-space character), or C<\S>
(non white-space character).  The matched pattern variable, $&, $`
(pre-match), $' (post-match), and $+ (last match) are also tainted if
C<use locale> is in effect and the regular expression contains C<\w>,
C<\W>, C<\s>, or C<\S>.

=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> in effect,
if it is modified as a result of a substitution based on a regular
expression match involving C<\w>, C<\W>, C<\s>, or C<\S>; or of
case-mapping with C<\l>, C<\L>,C<\u> or C<\U>.

=item B<In-memory formatting function> (sprintf()):

Result is tainted if "use locale" is in effect.

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

Success/failure result is never tainted.

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

Results are tainted if C<use locale> is in effect.

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

Results are never tainted.

=item B<POSIX character class tests> (isalnum(), isalpha(), isdigit(),
isgraph(), islower(), isprint(), ispunct(), isspace(), isupper(),
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 $untainted_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 very similar program which is locale-aware:

        #/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 $& is tainted: it is the result
of a match involving C<\w> when C<use locale> is in effect.

=head1 ENVIRONMENT

=over 12

=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 which 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) setlocale() method
for controlling an application's opinion on data.

=over 12

=item LC_ALL

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

=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

=head1 NOTES

=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 (see
L<The setlocale function>) was always in force, even if the program
environment suggested otherwise.  By default, Perl still behaves this
way so as to maintain 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>) to
instruct it to do so.

Versions of Perl from 5.002 to 5.003 did use the C<LC_CTYPE>
information if that was available, that is, C<\w> did understand what
are 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 write() and LC_NUMERIC

Formats are the only part of Perl which unconditionally use information
from a program's locale; if a program's environment specifies an
LC_NUMERIC locale, it is always used to specify the decimal point
character in formatted output.  Formatted output cannot be controlled by
C<use locale> because the pragma is tied to the block structure of the
program, and, for historical reasons, formats exist outside that block
structure.

=head2 Freely available locale definitions

There is a large collection of locale definitions at
C<ftp://dkuug.dk/i18n/WG15-collection>.  You should be aware that it is
unsupported, and is not claimed to be fit for any purpose.  If your
system allows the 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.  But, for now, it's the only
standard we've got.  This may be construed as a bug.

=head1 BUGS

=head2 Broken systems

In certain system environments 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 the
C<use locale> is in effect.  When confronted with such a system,
please report in excruciating detail to <F<perlbug@perl.com>>, and
complain to your vendor: maybe some bug fixes exist for these problems
in your operating system.  Sometimes such bug fixes are called an
operating system upgrade.

=head1 SEE ALSO

L<POSIX (3)/isalnum>

L<POSIX (3)/isalpha>

L<POSIX (3)/isdigit>

L<POSIX (3)/isgraph>

L<POSIX (3)/islower>

L<POSIX (3)/isprint>,

L<POSIX (3)/ispunct>

L<POSIX (3)/isspace>

L<POSIX (3)/isupper>,

L<POSIX (3)/isxdigit>

L<POSIX (3)/localeconv>

L<POSIX (3)/setlocale>,

L<POSIX (3)/strcoll>

L<POSIX (3)/strftime>

L<POSIX (3)/strtod>,

L<POSIX (3)/strxfrm>

=head1 HISTORY

Jarkko Hietaniemi's original F<perli18n.pod> heavily hacked by Dominic
Dunlop, assisted by the perl5-porters.

Last update: Mon Nov 17 22:48:48 EET 1997