Help me break this time regex!

regex.php

<?php

/*
  I'm trying to validate times in a PHP application. Some of the formats
  I want to allow include:
  
  - 12am (12 hour clock)
  - 1:45 pm (space optional)
  - 23:59 (24 hour clock)
  - 00:16
  - 0:00
  - 3 (hour by itself, 24-hour format)
  - 19
  
  Please help me test and break the regular expression below for this purpose.
  If you have any other suggestions for a type of time that could be included,
  I'm all ears!
  
  Thanks much. :)
  Eric "Aken" Roberts, @cryode
*/

preg_match('/^([01]?[0-9]|2[0123])(\:[0-5][0-9])?(?:\s?(?<!0|1[3-9])(am|pm))?$/i', $time);