regular expression for a text box that is not required, but needs to validate if entered

On my form I have a text box for a phone number. I have a regular expression that is fairly universal & can accept almost every variation for开发者_Python百科 a phone number, local or international. the expression is as follows:

^((+)?[1-9]{1,2})?([-\s.])?(((\d{1,4}))|\d{1,4})(([-\s.])?[0-9]{1,12}){1,2}(x?[0-9]{1,})?$

The issue is, the field is not required, but needs to be validated if they decide to enter a number. is there any possible way of doing this?

Surround the entire regex, except the ^ and $ with an extra ( )?

^(((+)?[1-9]{1,2})?([-\s.])?(((\d{1,4}))|\d{1,4})(([-\s.])?[0-9]{1,12}){1,2}(x?[0-9]{1,})?)?$

I don't do ASP programming, but couldn't you do some sort of condition that says something like:

if( textbox.value.length > 0 ) then validate

So it only validates if the user entered something