How to write regex pattern from my function for validation custom phone numbers?

Question

I have my own function to check phone number:

function isPhoneNumber(phone) {
    var regexForPhoneWithCountryCode = /^[0-9+]*$/;
    var regexForPhoneWithOutCountryCode = /^[0-9]*$/;
    var tajikPhone = phone.substring(0,4);
    if(tajikPhone == "+161" && phone.length !== 13) {
        return false;
    } 
    if(phone.length == 9 && phone.match(regexForPhoneWithOutCountryCode)) {
        return true;
    } else if(phone.length > 12 && phone.length < 16 && phone.match(regexForPhoneWithCountryCode)) {
        return true;
    } else return false;
}

My function also work, but not completely correct.

Rules for validate phone number:

• Max length: 13
• Min length: 9

---

When max length == 13:

• Contain only: 0-9+
• First charackter match: +
• 3 charackters after "+" must be: 161

---

When max length == 9:

• Contain only: 0-9

---

Example valid numbers:

• +161674773312
• 674773312

Answer 1

A really simple method you could use is:

function isPhoneNumber(phone) {
    if (phone.match(/^(?:\+161)?\d{9}$/) {
        return true;
    } else {
        return false;
    }
}