Phone number validation in angular?

Hi, everyone,

I am using phone number validation using a pattern.

Validators.pattern("^((\\+91-?)|0)?[0-9]{10}$")  // this is what i found everywhere.

I want to restrict the first 2 digits of the number to 7 to 9 only. User should be able to add only 7,8,9 only.

for example 433333333333 to 973333333333

Please help