code: public static String regexFlags(final int flags) {
          int processedFlags = flags;
          StringBuilder buf = new StringBuilder();
  
          for (int i = 0; i < FLAG_LOOKUP.length; i++) {
              if ((processedFlags & FLAG_LOOKUP[i]) > 0) {
                  buf.append((char) i);
                  processedFlags -= FLAG_LOOKUP[i];
              }
          }
  
          if (processedFlags > 0) {
              throw new IllegalArgumentException("Some flags could not be recognized.");
          }
  
          return buf.toString();
      }
docs: Converts Java regular expression flags into regular expression modifiers from the database.
  
  @param flags the Java flags
  @return the Java flags
  @throws IllegalArgumentException if some flags couldn't be recognized.