docs: public FormFieldValidator createValidator(Element constraint, BpmnParse bpmnParse, ExpressionManager expressionManager) {
  
      String name = constraint.attribute("name");
      String config = constraint.attribute("config");
  
      if("validator".equals(name)) {
  
        // custom validators
  
        if(config == null || config.isEmpty()) {
          bpmnParse.addError("validator configuration needs to provide either a fully " +
          		"qualified classname or an expression resolving to a custom FormFieldValidator implementation.",
          		constraint);
  
        } else {
          if(StringUtil.isExpression(config)) {
            // expression
            Expression validatorExpression = expressionManager.createExpression(config);
            return new DelegateFormFieldValidator(validatorExpression);
          } else {
            // classname
            return new DelegateFormFieldValidator(config);
          }
        }
  
      } else {
  
        // built-in validators
  
        Class<? extends FormFieldValidator> validator = validators.get(name);
        if(validator != null) {
          FormFieldValidator validatorInstance = createValidatorInstance(validator);
          return validatorInstance;
  
        } else {
          bpmnParse.addError("Cannot find validator implementation for name '"+name+"'.", constraint);
  
        }
  
      }
  
      return null;
  
  
    }