pattern (value, schema, opts) {
    const pattern = schema.pattern
    if (utils.isString(value) && !value.match(pattern)) {
      return makeError(value, pattern, opts)
    }
  }
Validates the provided string matches a pattern defined in the Schema's `pattern` keyword.
Validation succeeds if the string is a match of the regex value of this keyword.

see http://json-schema.org/latest/json-schema-validation.html#anchor33
@name Schema.validationKeywords.pattern
@method
@param {*} value String to be validated.
@param {object} schema Schema containing the `pattern` keyword.
@param {object} [opts] Configuration options.
@returns {(array|undefined)} Array of errors or `undefined` if valid.