public Checker tryCreate(String pattern) {
      Matcher matcher = MIDDLE_PATTERN.matcher(pattern);
      if (matcher.matches()) {
        return new MiddleChecker(matcher.group(1));
      }
      return null;
    }
Accepts simple REGEXP patterns like ".*abc.*" and creates corresponding checkers.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/FilterStringColRegExpStringScalar.java