Performs validation for string types.

@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.
function string(rule, value, callback, source, options) {
  const errors = [];
  const validate = rule.required || (!rule.required && source.hasOwnProperty(rule.field));
  if (validate) {
    if (isEmptyValue(value, 'string') && !rule.required) {
      return callback();
    }
    rules.required(rule, value, source, errors, options, 'string');
    if (!isEmptyValue(value, 'string')) {
      rules.type(rule, value, source, errors, options);
      rules.range(rule, value, source, errors, options);
      rules.pattern(rule, value, source, errors, options);
      if (rule.whitespace === true) {
        rules.whitespace(rule, value, source, errors, options);
      }
    }
  }
  callback(errors);
}