Defining a type for email using template literal string and conditional types

email-type.ts

type Alfa = 'a' | 'b' | 'c' | 'd' 
    | 'e' | 'f' | 'g' | 'h' | 'i' 
    | 'j' | 'k' | 'l' | 'm' | 'n' 
    | 'o' | 'p' | 'q' | 'r' | 's' 
    | 't' | 'u' | 'v' | 'w' | 'x'
    | 'y' | 'z';

type Numeric = '1' | '2' | '3' | '4' 
    | '5' | '6' | '7' | '8' | '9';

type AllowedChars = '.' | Alfa | Numeric;

type Checker<T extends string, Check> = T extends `${infer CHAR}${infer REST}`
    ? CHAR extends Check ? Checker<REST, Check> : false
    : true;

type Domain<P extends string> = P extends `${infer PROVIDER}.${infer DOMAIN}`
    ? Checker<PROVIDER, Alfa> extends true
        ? Checker<DOMAIN, Alfa> extends true ? true : false
        : false
    : false;

type Email<S extends string> = S extends `${infer MAIL}@${infer DOMAIN}`
    ? Checker<MAIL, AllowedChars> extends true
        ? Domain<DOMAIN> extends true ? S : never
        : never
    : never;

type Mail = Email<'[email protected]'>; // '[email protected]'
type BadMail = Email<'bad*[email protected]'>; // never
type BadMail2 = Email<'badmailmail.com'>; // never
type BadMail3 = Email<'badmail@m=ail.com'>; // never
type BadMail4 = Email<'badmail@mailcom'>; // never
type BadMail5 = Email<'[email protected]'>; // never