code: private static Pattern importStringToPattern(String s, Processor p, Log log) {
          if (isValidImportString(s)) {
              return validImportStringToPattern(s);
          } else {
              log.warning("proc.malformed.supported.string", s, p.getClass().getName());
              return noMatches; // won't match any valid identifier
          }
      }
docs: Convert import-style string for supported annotations into a
  regex matching that string.  If the string is a valid
  import-style string, return a regex that won't match anything.