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.
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));
      }
    }
  }
}