whole_func_string: function regexp(rule, value, callback, source, options) {
    const errors = [];
    const validate = rule.required || (!rule.required && source.hasOwnProperty(rule.field));
    if (validate) {
      if (isEmptyValue(value) && !rule.required) {
        return callback();
      }
      rules.required(rule, value, source, errors, options);
      if (!isEmptyValue(value)) {
        rules.type(rule, value, source, errors, options);
      }
    }
    callback(errors);
  }
func_code_string: function regexp(rule, value, callback, source, options) {
    const errors = [];
    const validate = rule.required || (!rule.required && source.hasOwnProperty(rule.field));
    if (validate) {
      if (isEmptyValue(value) && !rule.required) {
        return callback();
      }
      rules.required(rule, value, source, errors, options);
      if (!isEmptyValue(value)) {
        rules.type(rule, value, source, errors, options);
      }
    }
    callback(errors);
  }
func_documentation_string: Validates the regular expression type.
  
  @param rule The validation rule.
  @param value The value of the field on the source object.
  @param callback The callback function.
  @param source The source object being validated.
  @param options The validation options.
  @param options.messages The validation messages.