Optional Clause

     (command, preprocessor) A portion of a match pattern that is enclosed in
     square ([ ]) brackets.  An optional clause specifies part of a match
     pattern that need not be present for source text to match the pattern.
     An optional clause may contain any of the components legal within a
     match pattern, including other optional clauses.  When a match pattern
     contains a series of optional clauses that are immediately adjacent to
     each other, the matching portions of the source text are not required to
     appear in the same order as the clauses in the match pattern.  If an
     optional clause is matched by more than one part of the source text, the
     multiple matches may be handled using a repeating clause in the result
     pattern.