Sure! Here's an example of how you could define a type for a valid email that follows the format of [alphanumeric characters]@[alphanumeric characters].[top domain]:
``` 
type Email = string & { __email: never };

const emailRegExp = /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/;

function isEmail(email: string): email is Email {
  return emailRegExp.test(email);
}
``` 
In this code, we define a type called Email, which is essentially a string with an additional property of __email of type never. The purpose of this additional property is to prevent other string types from being assigned to the Email type.

We also define a regular expression emailRegExp that matches the format of a valid email address.

The isEmail function takes a string and returns a boolean indicating whether or not the string is a valid email address. This function uses the emailRegExp regular expression to test whether the input string matches the format of a valid email address, and returns true if it does.

You can use this Email type to ensure that a string conforms to the format of a valid email address. For example:
``` 
const validEmail: Email = 'example123@example.com'; // Valid email
const invalidEmail: Email = 'invalid_email'; // Error: Type '"invalid_email"' is not assignable to type 'Email'.
```
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