src/validator/regexp.js
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);
}
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);
}
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.