code: function pattern(rule, value, source, errors, options) {
    if (rule.pattern) {
      if (rule.pattern instanceof RegExp) {
        // if a RegExp instance is passed, reset `lastIndex` in case its `global`
        // flag is accidentally set to `true`, which in a validation scenario
        // is not necessary and the result might be misleading
        rule.pattern.lastIndex = 0;
        if (!rule.pattern.test(value)) {
          errors.push(util.format(options.messages.pattern.mismatch,
            rule.fullField, value, rule.pattern));
        }
      } else if (typeof rule.pattern === 'string') {
        const _pattern = new RegExp(rule.pattern);
        if (!_pattern.test(value)) {
          errors.push(util.format(options.messages.pattern.mismatch,
            rule.fullField, value, rule.pattern));
        }
      }
    }
  }
docs: Rule for validating a regular expression pattern.
  
  @param rule The validation rule.
  @param value The value of the field on the source object.
  @param source The source object being validated.
  @param errors An array of errors that this rule may add
  validation errors to.
  @param options The validation options.
  @param options.messages The validation messages.