Regular Expression Exercise S1

The first in a series of exercises designed to teach you more about regular expressions, written by a guy who got partway through writing a regex book.

But first, a word about tools. It’s a lot easier to use a tool to do this sort of thing than it is to write code to do it. So, I suggest one of the following:

So, S stands for simple, 1 stands for 1, so this first one is going to be pretty simple.

S1 – Match a Social Security Number

Verify that a string is a social security number of the format ddd-dd-dddd.


Answer and explanation to follow.

Comments (11)

  1. Nat Luengnaruemitchai says:


  2. Dvae says:

    Surely you’d want the hypens to be optional or replaceable by spaces. Forcing a particlular format when enter a number like this, or a credit-card number can be a pain, unless you explictly tell the user in advance the format you want (as you have done).

    In any case, I’d use

    ^d{3}[-| ]?d{2}[-| ]?d{4}$

  3. David says:

    The best tool one can have when working with regular expressions is Regex Coach.

  4. James Curran says:


    Yes, but you’d want consistency in the separator use, so that it would accept "123-12-1234", "123 12 1234" or "123121234" but not "12312 1234" or "123-12 1234". Hence we’d need:

    ^d{3}(?<Sep>[- ]?)d{2}<Sep>d{4}$

    (as interpreted by Regex Workbench)

    ^ (anchor to start of string)

    Any digit

    Exactly 3 times

    Capture to <Sep>

    Any character in "- "

    ? (zero or one time)

    End Capture

    Any digit

    Exactly 2 times


    Any digit

    Exactly 4 times

    $ (anchor to end of string)

  5. Dave says:

    > Yes, but you’d want consistency in the separator use

    True, but if we wanted consistency, then we’d force them to use the properly structured format, and not allow for any variance 😉

    <strike>Dvae</strike> Dave

  6. Richard says:

    Why would I want to match one of those? Completely useless outside the US.

  7. James Curran says:

    Um… So, RIchard, what exactly is the format of a Social Security Number outside the US?

  8. Rob says:

    In the UK it is

    AA 00 00 00 A


    ^w{2} d{2} d{2} d{2} w$