Regular expression for 2 or 5 digits

Use 3 optional digits:

^\d{2}\d{3}?$

Note that some regex engines will interpret the ? after any repetition modifier (even a fixed one) as an ungreedy modifier, which seems to cause problems for the case of two digits. If you experience this, use:

^\d{2}(?:\d{3})?$

You can read up on some regex basics in this great tutorial.

By the way, the above is effectively equivalent (but marginally more efficient) to this one using alternation:

^(?:\d{2}|\d{5})$

(Just for the sake of showing you another regex concept.)