Why use an array and not a hash?

Because the order matters. If the regex checked for 'yy' before 'yyyy', then 'yyyy-mm-dd' would translate to '%y%y-%m-%d'.

You are missing...

I can only do what the POSIX standard supports. Thanks for the translation suggestions, they are easy enough to add.

What do you do with "yyy"? Leave it as is?

Like POSIX::strftime(), anything not recognized as a formatting element is left as-is, yes (although 'yyy' in specific would translate to '%yy'). Unlike strftime(), I didn't add an escape sequence in case you wanted a literal 'yy' in the string. Exercise for the user I guess. :-)

 Because the order matters. If the regex checked for 'yy' before 'yyyy', then 'yyyy-mm-dd' would translate to '%y%y-%m-%d'.

use warnings;

sub translate_date_format
{
    my %tr = (
        yyyy  => "%Y",
        yy    => "%y",
        mmmm  => "%B",
        mmm   => "%b",
        mm    => "%m",
        m     => "%e",
        ww    => "%V", # or %U
        w     => "%w", # or %u
        j     => "%j",
        dddd  => "%A",
        ddd   => "%a",
        dd    => "%d",
        d     => "%e",
        HH    => "%H",
        H     => "%k",
        hh    => "%I",
        h     => "%i",
        MM    => "%M",
        A     => "%p",
        AM    => "%p",
        PM    => "%p",
        a     => "%P",
        am    => "%P",
        pm    => "%P",
        SS    => "%S",
        ZZZ   => "%Z",
        );

    my $str = shift;
    $str =~ s/(?<!%)$_/$tr{$_}/g for reverse sort keys %tr;
    return ($str);
    }

### Test #############################################################
+#########

use POSIX;
use Time::Piece;

my @test = (
    "yyyy-mm-dd",  "d/mm/yy",
    "H:MM:SSAM",   "HH:MM:SSpm",
    "mmm ddd, yy", "dddd, mmmm dd, yyyy HH:MM:SS",
    );

foreach (@test) {
    my $tdf = translate_date_format ($_);
    print "$_ => '$tdf'\n";

    my $date = POSIX::strftime ($tdf, localtime ());
    print "  POSIX: $date\n";

    my $t = localtime ();
    $date = $t->strftime ($tdf);
    print "  Time::Piece: $date";
    $date = $t->strptime ($date, $tdf);
    print " => $date\n";
    }
[download]

I also see (using warnings) yet another reason never to use AM/PM :) :

HH:MM:SSpm => '%H:%M:%S%P'
  POSIX: 11:39:52am
garbage at end of string in strptime: am at /pro/lib/perl5/site_perl/5
+.20.0/x86_64-linux-thread-multi-ld/Time/Piece.pm line 469.
  Time::Piece: 11:39:52am => Thu Jan  1 11:39:52 1970
[download]


Enjoy, Have FUN! H.Merijn

reverse sort keys %tr

I'd probably sort them by negative length and join them to a single regex:

my $regex = '('
          . join('|', sort { length $b <=> length $a } keys %tr)
          . ')';
$str =~ s=$regex=$tr{$1} // $1=ge;
[download]

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