code: public static int regexFlag(final char c) {
  
          int flag = FLAG_LOOKUP[c];
  
          if (flag == 0) {
              throw new IllegalArgumentException(String.format("Unrecognized flag [%c]", c));
          }
  
          return flag;
      }
docs: Converts a regular expression modifier from the database into Java regular expression flags.
  
  @param c regular expression modifier
  @return the Java flags
  @throws IllegalArgumentException If sequence contains invalid flags.