TESTPAT$ (regex)

   Synopsis:
      TESTPAT$ checks the validity of a pattern.
         
   Notes:
      'regex' is a Perl regular expression like "orc", "^orc" or "^orc (.*)$".
         If it's a valid pattern, TESTPAT$ returns an empty string. If 'regex'
         is itself an empty string, returns an empty string. Otherwise, TESTPAT$
         returns the Perl error message generated by the invalid pattern.
      See also the help for TESTPAT, which returns a numerical value.

   Compatibility:
      Since Axbasic v1.001
