For example, I would like to find ty in:
erytypotym5ty    
etytyty    
koetymitywty
or il in:
keililmil    
ilwrilltyil5ile
^.*(\w{2}).*\1.*\1.*$.
The two letters (also digits and _; you could replace \w with [a-zA-Z], if you don't want them) will be in group 1.
