What is proper RegEx expression for SWIFT codes?

A swift code should be 8 or 11 letters or digits where the first six must be letters. But anyway it doesn't really matter what it is, what matters is that you understand how to create such an expression. Here is a regular expression with annotations to show you what the parts mean.

^[A-Z]{6}[A-Z0-9]{2}([A-Z0-9]{3})?$
       ^          ^           ^  ^
       |          |           |  |
       6 letters  2 letters   3 letters or digits
                  or digits      |
                                 last three are optional

All the examples on Wikipedia show only upper case letters A-Z. If you also want to allow lowercase letters then change A-Z to A-Za-z. I would check the ISO standard to see what that says, but unfortunately it's not free to obtain a copy.

This should do the trick

^([a-zA-Z]){4}([a-zA-Z]){2}([0-9a-zA-Z]){2}([0-9a-zA-Z]{3})?$

According to http://en.wikipedia.org/wiki/ISO_9362 ...

/[A-Z]{6}[A-Z0-9]{2}([A-Z0-9]{3})?/i