code: public static boolean isValidStringMatchingRegex(final GraphObject node, final PropertyKey<String> key, final String expression, final ErrorBuffer errorBuffer) {
  
  		final String value = node.getProperty(key);
  
  		if (isValidStringMatchingRegex(value, expression)) {
  			return true;
  		}
  
  		// no match
  		errorBuffer.add(new MatchToken(node.getType(), key, expression));
  		return false;
  	}
docs: Checks whether the value of the given property key of the given node
  if not null and matches the given regular expression.
  
  @param node
  @param key
  @param expression
  @param errorBuffer
  @return true if string matches expression