An example of type system verified strings in TypeScript using template literal strings and tagged types.

safe_strings.ts

/**
 * Yet another demonstration of TypeScript awesomeness.
 * 
 * Demonstrates tagged types and template literal strings (v4.1) which allows 
 * * for the type system to infer the literal string type of value
 * * then deconstruct the literal string type into parts
 * * in order to parse and validate strings directly in the type system.
 * 
 * See the TRY comments for things to try. 
 * 
 * This example can be opened in the TS Playground (https://www.typescriptlang.org/play)
 * or locally. It does require v4.1 or later.
 */
module ReadMe {}

//--------------------------------------------------------------------------
// APIs
//--------------------------------------------------------------------------
type Tag<tag extends string> = { __tag: tag }

/** A string verified to be structured as an EMail. */
module EmailString {
    type EmailTag = Tag<"EMail">
    type EmailString = string & EmailTag
    /** 
     * An EmailString
     * - Assignable to string **BUT** string is not assignable to this. 
     * - Is a string with Something@SomethingElse. Essentially a string with an @ between two non-empty parts.
     **/
    export type Type = EmailString; 
    type FromLiteral<s extends string> =
        string extends s
        ? unknown // if s is general (unrefined string) then fail
        : s extends `${infer _prefix}@${infer _suffix}`
        ? _prefix extends ""
        ? never
        : _suffix extends ""
        ? never
        : s & EmailTag
        : unknown
    /** Refines the type of the given string to EmailString if it has two non-empty strings separated by @. */
    export const literal = <str extends string>(str: str): FromLiteral<str> => str as any;
    /** If you have a general string, test and refine whether it is an EMail string. */
    export const test = (str: string): str is Type => {
        const [prefix, suffix] = (str || "").split("@");
        return !!prefix && !!suffix;
    }
}

type EatPrefix<prefix extends string, s> = s extends `${prefix}${infer rest}` ? rest : false;
type EatDigit<s> = EatPrefix<"0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9", s>;

module USPhoneNumberString {
    type EatLeftParen<s> = EatPrefix<"(", s>;
    type EatRightParen<s> = EatPrefix<")", s>;

    type EatThreeDigits<s> = EatDigit<EatDigit<EatDigit<s>>>
    type EatFourDigits<s> = EatDigit<EatThreeDigits<s>>
    type EatAreaCode<s> = EatRightParen<EatThreeDigits<EatLeftParen<s>>>
    type EatPhoneNumber<s> = EatFourDigits<EatPrefix<"-", EatThreeDigits<s>>>
    type EatPhoneNumberWithAreaCode<s> = EatPhoneNumber<EatAreaCode<s>>

    type USPhoneNumberTag = Tag<"USPhoneNumber">
    type USPhoneNumberString = string & USPhoneNumberTag
    /** 
     * A USPhoneNumberString
     * - Assignable to string **BUT** string is not assignable to this. 
     * - Is a string that is either a seven digit or seven digit with area code.
     **/
    export type Type = USPhoneNumberString 
    
    type FromLiteral<s extends string> =
        EatPhoneNumber<s> extends string ? s & USPhoneNumberTag
        : EatPhoneNumberWithAreaCode<s> extends string ? s & USPhoneNumberTag
        : unknown;
    
    /** Refines the type of the given string to PhoneNumberString if it looks like a US phone number.. */
    export const literal = <str extends string>(str: str): FromLiteral<str> => str as any;

    /** If you have a general string, test and refine whether it is an EMail string. */
    export const test = (str: string): str is Type => {
        const [prefix, suffix] = (str || "").split("@");
        return !!prefix && !!suffix;
    }
}

type Person = {
    firstName: string;
    phoneNumber: USPhoneNumberString.Type;
    email: EmailString.Type;
}

const poorlyFormed: Person = {
    firstName: "Bob",
    // TRY: Remove the line below to see errors
    // @ts-expect-errors
    phoneNumber: "123",
    // TRY: Remove the line below to see errors
    // @ts-expect-errors
    email: "aol"
}

const validPerson: Person = {
    firstName: "Janice",
    // TRY: Remove a digit from the phone number
    phoneNumber: USPhoneNumberString.literal("(123)123-4567"),
    // TRY: Remove the @ symbol OR remove all the text to the left of the @.
    email: EmailString.literal("[email protected]")

}