Regex pattern for HH:MM:SS time string

Question

I want to parse a hh:mm:ss string. A simple one is ([0-1]?\\d|2[0-3]):([0-5]?\\d):([0-5]?\\d) which expects 2:3:24 or 02:03:24<

Answer 1

^(?:(?:([01]?\d|2[0-3]):)?([0-5]?\d):)?([0-5]?\d)$

Explanation:

^                   # Start of string
(?:                 # Try to match...
 (?:                #  Try to match...
  ([01]?\d|2[0-3]): #   HH:
 )?                 #  (optionally).
 ([0-5]?\d):        #  MM: (required)
)?                  # (entire group optional, so either HH:MM:, MM: or nothing)
([0-5]?\d)          # SS (required)
$                   # End of string