How do I define a custom validator in zod?

Question

RepoPilot · Accepted Answer

I can see from the source that Zod supports custom validation through its `refinement` method. Based on the code in [packages/zod/src/v3/types.ts:L1035-L1041], there's a `_regex` method that demonstrates the pattern:

```typescript
protected _regex(regex: RegExp, validation: StringValidation, message?: errorUtil.ErrMessage) {
  return this.refinement((data) => regex.test(data), {
    validation,
    code: ZodIssueCode.invalid_string,
    ...errorUtil.errToObj(message),
  });
}
```

This shows that you can use `.refinement()` to define custom validators. The refinement accepts:
1. A validation function that returns a boolean
2. An options object with error details (code, message, etc.)

However, the repository data I have doesn't include the full API documentation or examples of custom validator usage. To get more complete information about how to define custom validators with different patterns and options, you would want to check the main Zod documentation or look at the full type definitions for the `refinement` method.

The code also shows various built-in string validators (email, uuid, url, regex, etc.) defined as checks in [packages/zod/src/v3/types.ts:L545-L587], which might serve as useful patterns for understanding how validators are structured.

FOLLOW_UPS:
- What's the difference between refinement and superRefine?
- How do custom validators interact with error handling?
- Can I create reusable custom validator functions?