Validating a Phone number field with Regular Expressions

Validating user input is a very common task when building a program with a user interface. While this task may appear daunting at first, it can actually be quite easy. Once you know of a good approach.

Regular expressions were born in the Unix world, but they can Be of great benefit anywhere. Regular expressions are complex and intimidating, but once you have mastered the basics they can be used in many tasks.

Let's look at a common usage of Regex: to validate user input. In the following code, we need to verify that the string phoneNumber contains a valid US phone number. First we need to verify that only numbers are imputed into the system.


private boolean IsUSPhoneNumber(string phoneNumber)
{
return Regex.IsMatch(phoneNumber, @"^(\d+)$");
}


Doing a quick test reveals that indeed a string of 123456789 is considered valid. However, so is 123 or 1234567890. Clearly we need to verify length of the string in addition to content. The following code will ensure this:


private boolean IsUSPhoneNumber(string phoneNumber)
{
return Regex.IsMatch(phoneNumber, @"^(\d{10})$");
}


While this is an improvement, there is still much more to consider. What if the user places the area code in parenthesis? This is a fairly common practice and should be accounted for.


private boolean IsUSPhoneNumber(string phoneNumber)
{
return Regex.IsMatch(phoneNumber, @"^([(]{0,1})(\d{3})([)]{0,1})(\d{7})$");
}


This code checks to see if there is zero or one parenthesis, 3 digits between the parenthesis and 7 digits after.

But, there are still other possibilities to consider; such as, what if the user uses white spaces to delineate the different sections of the phone number?


private boolean IsUSPhoneNumber(string phoneNumber)
{
return Regex.IsMatch(phoneNumber,
@"^([(]?)(\d{3})([)]?)([ ]?)(\d{3})([ ]?)(\d{4})$");
}


Or if periods are used?


private boolean IsUSPhoneNumber(string phoneNumber)
{
return Regex.IsMatch(phoneNumber,
@"^([(]?)(\d{3})([)-]?)([ ]?)(\d{3})([ -]?)(\d{4})$");
}


Or dashes?


private boolean IsUSPhoneNumber(string phoneNumber)
{
return Regex.IsMatch(phoneNumber,
@"^([(]?)(\d{3})([)-.]?)([ ]?)(\d{3})([ -.]?)(\d{4})$");
}



The final code snippet will validate most US phone number formats, in just 1 line of code. There are certainly other methods to validate a phone number, but can they be written in 1 line of code?