Testpat$ (regex)

   Synopsis:
      Checks the validity of a pattern (regular expression)
         
   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 numeric value.

   Compatibility:
      Since Axbasic v1.001
