qr/^       # Anchors to the beginning of the line --
             # I think this may be unnecessary, but
             # it's not a biggie.

     foobar  # Must have 'foobar'

     (       # First group ..
      a?     # zero or one 'a'
      b?     # zero or one 'b'
      c?     # zero or one 'c'
     )       # .. end of first group (is this missing
             # a '?' to show it's optional?)

     (?:     # Start of a non-capturing group ..
      X      # Must have an 'X'
      (?=.)  # Must have a character (lookahead assertion)

      (      # Second group ..
       a?    # zero or one 'a'
       b?    # zero or one 'b'
       c?    # zero or one 'c'
      )      # .. end of second group
     )?      # .. end of non-capturing group, shows group
             # is optional.

     $       # Anchors end of line. Same comment as for
             # beginning anchor.