17
To rephrase:
Given a string, determine whether the string contains at least one uppercase letter (A-Z), at least one lowercase letter (a-z), at least one digit (0-9) and at least one space (ASCII 0x20).
If that's correct, do you really want spaces in passwords? I would think it would be better to have special characters, such as $ and %.
I'll see what I can come up with later, unless you or someone else solves it first. I'm not sure that a regex is the best way of handling this.