Regular expression to validate specific input, but allow no input

Question

I'm trying to validate a textbox containing a variety of accepable inputs:

• "123456789"
• "123456789+1"
• ""

I'm using this regular expression to validate the first two conditions:

^[0-9]{9}\+[0-9]$

But now I can't figure out how to allow the user not to enter anything, I've tried things like encapsulating the expression in ()? , or changing the {9} to {0,9}, but this allows for a variable number of numbers in the first group (as it MUST be 9, or none)

Any help would be greatly appreciated!!

Answer 1

Try

^([0-9]{9}\+[0-9])?$

The ? means "0 or 1 copies of the preceding expression." In this case, the entire parenthesized expression is made optional.

Also, if you want the \+[0-9] part to be optional, you will likely want to do the same to that; wrap in in parentheses and make it optional with ?:

^([0-9]{9}(\+[0-9])?)?$

Answer 2

^(?:[0-9]{9}\+[0-9])?$