code: public FessMessages addConstraintsPatternMessage(String property, String regexp) {
          assertPropertyNotNull(property);
          add(property, new UserMessage(CONSTRAINTS_Pattern_MESSAGE, regexp));
          return this;
      }
docs: Add the created action message for the key 'constraints.Pattern.message' with parameters.
  <pre>
  message: {item} must match "{regexp}".
  </pre>
  @param property The property name for the message. (NotNull)
  @param regexp The parameter regexp for message. (NotNull)
  @return this. (NotNull)