code: public static Constraint pattern(final Object regex) {
  		return new Constraint("pattern", patternPayload(regex)) {
  			public boolean isValid(Object actualValue) {
  				if (actualValue != null) {
  					if (regex != null && regex instanceof String) {
  						if (!(actualValue instanceof String) || !((String) actualValue).matches((String) regex)) {
  							return false;
  						}
  					}
  				}
  				return true;
  			}
  		};
  	}
docs: The 'pattern' constraint - field must contain a value matching a regular expression.
  @param regex a regular expression
  @return constraint