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 internationalization
       (often abbreviated as i18n); telling such an application about a
       particular set of preferences is known as localization (l10n).

       Perl was 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, and their use for character
       sets has mostly been supplanted by Unicode (see perlunitut for an
       introduction to that, and keep on reading here for how Unicode
       interacts with locales in Perl).

       Perl continues to support the old locale system, and starting in v5.16,
       provides a hybrid way to use the Unicode character set, along with the
       other portions of locales that may not be so problematic.  (Unicode is
       also creating "CLDR", the "Common Locale Data Repository",
       <> 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 at

       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):

       Category LC_NUMERIC: Numeric formatting
           This indicates how numbers should be formatted for human
           readability, for example the character used as the decimal point.

       Category LC_MONETARY: Formatting of monetary amounts

       Category LC_TIME: Date/Time formatting
       More details on the categories are given below in "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.

       Perl will not use locales unless specifically requested to (see "NOTES"
       below for the partial exception of "write()").  But even if there is
       such a request, all of the following must be true for it to work

       o   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.

       o   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.

       o   Perl must believe that the locale system is supported.  If it does,
           "perl -V:d_setlocale" will say that the value for "d_setlocale" is

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

       1.  The locale-determining environment variables (see "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

       2.  The application must set its own locale using the method described
           in "The setlocale function".

   The use locale pragma
       By default, Perl ignores the current locale.  The "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, and is
       described fully in "Unicode and UTF-8", but briefly, it tells Perl to
       "LC_NUMERIC" is always initialized to the C locale (mentioned under
       "Finding locales").  If there is no valid environment, the current
       locale is undefined.  It is likely, but not necessarily, the "C"

       The operations that are affected by locale are:

       Under "use locale ':not_characters';"
           o   Format declarations (format()) use "LC_NUMERIC"

           o   The POSIX date formatting function (strftime()) uses "LC_TIME".

       Under just plain "use locale;"
           The above operations are affected, as well as the following:

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

               Note: "eq" and "ne" are unaffected by locale: they always
               perform a char-by-char comparison of their scalar operands.
               What's more, if "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 0 (equal) if the operands are char-for-char
               identical.  If you really want to know whether two
               strings--which "eq" and "cmp" may consider different--are equal
               as far as collation in the locale is concerned, see the
               discussion in "Category LC_COLLATE: Collation".

           o   Regular expressions and case-modification functions (uc(),
               lc(), ucfirst(), and lcfirst()) use "LC_CTYPE"

       The default behavior is restored with the "no locale" pragma, or upon
       reaching the end of the block enclosing "use locale".  Note that "use
       locale" and "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

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

               # Import locale-handling tool set from POSIX module.
               # This example uses: setlocale -- the function call
               #                    LC_CTYPE -- explained below
               # restore the old locale
               setlocale(LC_CTYPE, $old_locale);

       The first argument of setlocale() gives the category, the second the
       locale.  The category tells in what aspect of data processing you want
       to apply locale-specific rules.  Category names are discussed in
       "LOCALE CATEGORIES" and "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 else
       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 setlocale().

       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 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 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.

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

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

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

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

               locale -a


               ls /usr/lib/nls/loc
               en                  de                  ru
               english             german              russian
               english.iso88591    german.iso88591     russian.iso88595
               english.roman8                          russian.koi8r

       Sadly, even though the calling interface for setlocale() has been
       standardized, names of locales and the directories where the
       configuration resides have not been.  The basic form of the name is
       language_territory.codeset, but the latter parts after language are not
       always present.  The language and country are usually from the
       standards ISO 3166 and ISO 639, the two-letter abbreviations for the
       countries and the languages of the world, respectively.  The codeset
       part often mentions some ISO 8859 character set, the Latin codesets.
       For example, "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 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.  Warning. The C locale
       delivered by some vendors may not actually exactly match what the C
       standard calls for.  So beware.

       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.

       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.  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.

   Temporarily fixing locale problems
       The two quickest fixes are either to render Perl silent about any
       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 "ENVIRONMENT" for the full list
       of relevant environment variables and "USING LOCALES" for their effects
       in Perl.  Effects in other programs are 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

       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 (sh, ksh, bash, zsh):

               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 (csh, 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.

   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 "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 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 "Permanently fixing your system's locale configuration".

   Permanently fixing your system's locale configuration
       This is when you see something like:

               perl: warning: Please check that your locale settings:

       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 "Finding locales" section is unfortunately a bit vague
       about the exact commands and places because these things are not that

   The localeconv function
       The POSIX::localeconv() function allows you to get particulars of the
       locale-dependent numeric formatting information specified by the
       current "LC_NUMERIC" and "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 "The setlocale

               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 a reference to a hash.
       The keys of this hash are variable names for formatting, such as
       "decimal_point" and "thousands_sep".  The values are the corresponding,
       er, values.  See "localeconv" in POSIX 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 "use locale",
       because 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
               1 while
               print "$_";
           print "\n";

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

       The following example will import the langinfo() function itself and
       three constants to be used as arguments to 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 I18N::Langinfo for more information.

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

   Category LC_COLLATE: Collation
       In the scope of "use locale" (but not a "use locale
       ':not_characters'"), Perl looks to the "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 "a" and "aa" 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:
       sorting raw binary data, whereas the locale-dependent collation of the
       first example is useful for natural text.

       As noted in "USING LOCALES", "cmp" compares according to the current
       collation locale when "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 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 that ignores space characters completely and
       which folds case.

       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 POSIX::strxfrm() in conjunction with "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 char-by-char comparisons against other transformed strings during
       collation.  "Under the hood", locale-affected Perl comparison operators
       call strxfrm() for both operands, then do a char-by-char 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.  But in fact, it doesn't save anything: Perl magic
       (see "Magic Variables" in perlguts) 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 "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.  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: "use locale" isn't shown in some of these examples because it
       isn't needed: strcoll() and strxfrm() exist only to generate locale-
       dependent results, and so always obey the current "LC_COLLATE" locale.

   Category LC_CTYPE: Character Types
       In the scope of "use locale" (but not a "use locale
       ':not_characters'"), Perl obeys the "LC_CTYPE" locale setting.  This
       controls the application's notion of which characters are alphabetic.
       pattern matching using the "i" modifier.

       Finally, "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().
       Unfortunately, this creates big problems for regular expressions. "|"
       still means alternation even though it matches "\w".

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

       Note: A broken or malicious "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 "\w" with the "/a" regular expression modifier.  See

   Category LC_NUMERIC: Numeric Formatting
       After a proper POSIX::setlocale() call, Perl obeys the "LC_NUMERIC"
       locale information, which controls an 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 "The localeconv
       function" if you care about these things.)

       Output produced by print() is also affected by the current locale: it
       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

               use POSIX qw(strtod setlocale LC_NUMERIC);

               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 I18N::Langinfo and "RADIXCHAR".

       See also I18N::Langinfo and "CRNCYSTR".

       Output produced by POSIX::strftime(), which builds a formatted human-
       readable date/time string, is affected by the current "LC_TIME" locale.
       Thus, in a French locale, the output produced by the %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: "use locale" isn't needed in this example: as a function that
       exists only to generate locale-dependent results, strftime() always
       obeys the current "LC_TIME" locale.

       See also I18N::Langinfo and "ABDAY_1".."ABDAY_7", "DAY_1".."DAY_7",
       "ABMON_1".."ABMON_12", and "ABMON_1".."ABMON_12".

   Other categories
       The remaining locale category, "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 outside the standard Perl distribution and by the
       operating system and its utilities.  Note especially that the string
       value of $! and the error messages given by external utilities may be
       changed by "LC_MESSAGES".  If you want to have portable error codes,
       use "%!".  See Errno.

       Although the main discussion of Perl security issues can be found in
       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:

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

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

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

       o   An application that takes the trouble to use information in
       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 "use
       locale" is in effect, Perl uses the tainting mechanism (see 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:

       o   Comparison operators ("lt", "le", "ge", "gt" and "cmp"):

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

       o   Case-mapping interpolation (with "\l", "\L", "\u" or "\U")

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

       o   Matching operator ("m//"):

           Scalar true/false result never tainted.

           Subpatterns, either delivered as a list-context result or as $1
           etc.  are tainted if "use locale" (but not
           "use locale ':not_characters'") is in effect, and the subpattern
           regular expression contains "\w" (to match an alphanumeric
           character), "\W" (non-alphanumeric character), "\s" (whitespace
           character), or "\S" (non whitespace character).  The matched-
           pattern variable, $&, $` (pre-match), $' (post-match), and $+ (last
           match) are also tainted if "use locale" is in effect and the
           regular expression contains "\w", "\W", "\s", or "\S".

       o   Substitution operator ("s///"):

           Has the same behavior as the match operator.  Also, the left
           operand of "=~" becomes tainted when "use locale" (but not
           "use locale ':not_characters'") is in effect if modified as a
           result of a substitution based on a regular expression match
           involving "\w", "\W", "\s", or "\S"; or of case-mapping with "\l",
           "\L","\u" or "\U".

       o   Output formatting functions (printf() and write()):

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

       o   Case-mapping functions (lc(), lcfirst(), uc(), ucfirst()):

           Results are tainted if "use locale" (but not
           "use locale ':not_characters'") is in effect.

       o   POSIX locale-dependent functions (localeconv(), strcoll(),
       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 $& is tainted: it is the result
       of a match involving "\w" while "use locale" is in effect.

                   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.

                   NOTE: PERL_BADLANG only gives you a way to hide the warning
                   message.  The message tells about some problem in your

       part of the standardized (ISO C, XPG4, POSIX 1.c) setlocale() method
       for controlling an application's opinion on data.

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

       LANGUAGE    NOTE: "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 not using GNU libc and you can ignore

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

       LC_CTYPE    In the absence of "LC_ALL", "LC_CTYPE" chooses the
                   character type locale.  In the absence of both "LC_ALL" and
                   "LC_CTYPE", "LANG" chooses the character type locale.

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

       LC_MONETARY In the absence of "LC_ALL", "LC_MONETARY" chooses the
                   monetary formatting locale.  In the absence of both
                   "LC_ALL" and "LC_MONETARY", "LANG" chooses the monetary
                   formatting locale.

       LC_NUMERIC  In the absence of "LC_ALL", "LC_NUMERIC" chooses the
                   numeric format locale.  In the absence of both "LC_ALL" and
                   "LC_NUMERIC", "LANG" chooses the numeric format.

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

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

       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";

       Versions of Perl prior to 5.004 mostly ignored locale information,
       generally behaving as if something similar to the "C" locale were
       always in force, even if the program environment suggested otherwise
       (see "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 must use the "use locale" pragma
       (see "The use locale pragma") or, in the unlikely event that you want
       to do so for just pattern matching, the "/l" regular expression
       modifier (see "Character set modifiers" in perlre) to instruct it to do

       Versions of Perl from 5.002 to 5.003 did use the "LC_CTYPE" information
       if available; that is, "\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.

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

   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.

   write() and LC_NUMERIC
       If a program's environment specifies an LC_NUMERIC locale and "use
       locale" is in effect when the format is declared, the locale is used to
       specify the decimal point character in formatted output.  Formatted
       output cannot be controlled by "use locale" at the time when write() is

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

       There is a large collection of locale definitions at:

       You should be aware that it is unsupported, and is not claimed to 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.

Unicode and UTF-8
       The support of Unicode is new starting from Perl version v5.6, and more
       fully implemented in version v5.8 and later.  See perluniintro.  It is
       strongly recommended that when combining Unicode and locale (starting
       in v5.16), you 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 "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
       "ENVIRONMENT"), and all outputs to files to be translated back into the
       locale.  (See open).  On a per-filehandle basis, you can instead use
       the PerlIO::locale module, or the Encode::Locale module, both available
       from CPAN.  The latter module also has methods to ease the handling of
       "ARGV" and environment variables, and can be used on individual
       strings.  Also, if you know that all your locales will be UTF-8, as
       many are these days, you can use the -C command line switch.

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

       For pre-v5.16 Perls, or if you use the locale pragma without the
       ":not_characters" parameter, Perl tries to work with both Unicode and
       locales--but there are problems.

       Perl does not handle multi-byte locales in this case, such as have been
       used for various Asian languages, such as Big5 or Shift JIS.  However,
       the increasingly common multi-byte UTF-8 locales, if properly
       implemented, may work reasonably well (depending on your C library
       implementation) in this form of the locale pragma, 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 "isalnum()".  Your C

       However, there are places where this breaks down.  Certain constructs
       are for Unicode only, such as "\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, "\p{Alpha}" will never match it, regardless
       of locale.  A similar issue occurs with "\N{...}".  It is therefore a
       bad idea to use "\p{}" or "\N{}" under plain "use locale"--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 "open()" layer, and @ARGV on
       non-ISO8859-1, non-UTF-8 locales (by using either the -C command line
       switch or the "PERL_UNICODE" environment variable; see 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 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.

       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 "Freely available locale definitions".)

       If you have Perl v5.16, the problems mentioned above go away if you use
       the ":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
       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 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
       ":not_characters" form of the pragma.)

       Note: The policy of using locale rules for code points that can fit in
       in mysterious hangs and/or Perl core dumps when "use locale" is in
       effect.  When confronted with such a system, please report in
       excruciating detail to <>, 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

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

       Jarkko Hietaniemi's original 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.

perl v5.18.2                      2018-11-20                     PERLLOCALE(1)
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