Testpat (regex)

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

   Notes:
      'regex' is a Perl regular expression like "orc", "^orc" or "^orc (.*)$".
         If it's a valid pattern, Testpat () returns 1; if it's an invalid
         pattern (or if 'regex' is an empty string), returns 0.
      See also the help for Testpat$ () which, for an invalid patterns, returns
         a string value containing the Perl error message.

   Compatibility:
      Since Axbasic v1.001
