java regular expression optional characters

Question

I'm having problems getting my regular expression working.

The regular expression is:

([0-9]m)* ([0-9]f)*

A digit must come before "m" or "f" but "m" or "f" are optional. Example are:

1m 2f
1m
6f

What have I done wrong?

Should 2f 1m be allowed? What about 2m 1m? What about 11m? — Mark Byers
– Mark Byers, Commented Apr 29, 2012 at 20:16

Mark Byers · Accepted Answer · 2012-04-29 20:20:50Z

9

The * means match the previous token 0 or more times, which doesn't look like what you want.

These should help you to build the regular expression you need:

Knowing that, I imagine that you can probably find a solution by yourself, but for the sake of completion, I'll show one possible solution.

Your question isn't very clear so I'm just going to assume that you want the following to match:

1m 2f
1m
6f 
0m

and that you want the following to fail to match:

1
m    
11m
1m 1m
2f 3m
1m  2f
"1m 2f"

If those assumptions are incorrect, then please make your question more clear.

With those assumptions, try this:

^[0-9]m(?: [0-9]f)?$|^[0-9]f$

If you also want 2f 3m to match then use this:

^[0-9]m(?: [0-9]f)?$|^[0-9]f(?: [0-9]m)?$

answered Apr 29, 2012 at 20:13

Mark Byers

844k202 gold badges1.6k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

1 Answer 1