How about approaching from a slightly different angle:
grab the bits of info you do want from the string, then
use them to construct a new string.
Do a regex match on your initial string, using parens and
array context to get back the pieces you're interested in,
like so:
($month, $day, $hour, $min, $ampm) = ($stamp =~ m|(\d+)/(\d+)/\d+\s+(\
+d+):(\d+)(\w)|);
Then use those pieces to build your new string:
$newstamp = sprintf("%02d_%02d_%02d%02d%s", $month, $day, $hour, $min,
+ $ampm);
(Using %02d in sprintf will give you leading zeros in front
of single digit numbers, which will make your resulting
string more consistent -- change them to just %d to leave
off the leading zeros)
This has the added benefit of making the code's function
much more obvious 6 months from now than a cryptic series
of substitutions, and being easier to update if and when
your formats change.
