Includes

Validates that a value contains a given substring anywhere within it.

Configuration

Config flag	Type	Description
`includes`	`string`	The required substring.
`includes`	`[string, string]`	Tuple form — the substring plus a custom message.

const form = useForm({
  initialValues: { referral: '' },
  validationSchema: {
    referral: { includes: '@' },
  },
});

With a custom message:

referral: { includes: ['@', 'The value must contain an @'] }

import { V } from 'formify-js';

V.includes('@');

Scenario	Message
Substring not found	`Must include "@"` (the message names your substring)

Value	Rule	Result
`'user@site.com'`	`includes: '@'`	✅ Valid
`'usersite.com'`	`includes: '@'`	❌ Invalid

The check is case-sensitive: 'HELLO' fails includes: 'hello'.
Position is not restricted — the substring may appear at the start, middle, or end.