It does not really matter. Either characters to use would be included in regex itself, such as [j-g] or you could use any.

For example "\D{2}\S{3}" means match 2 non-digit characters followed by 3 nonspace characters. Some things that match "aaaaa", "bbbbb", "ALUSD", "AA123"...

Site I linked really explains things quite well on mouseover. Once you complete first challenge (make sure you open this in chrome, I've had exciting quest of delete elements in firefox until I could save my solution) you can register and come back anytime later.