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

In reply to Re^4: Human-readable date format strings by Tux
in thread Human-readable date format strings by thewebsi

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.