Yes, there are regional variations. In NW Wales 'iau' is used and also your suggestion (i)fengach. 'Ifancach' is the form used in the standard Welsh for Adults course.
There are many spoken variations of common words. The best thing is to look a word up in a dictionary to see if it is a generally known form or not.
For example, iau is commonly used for 'junior', as in ysgol iau (a junior school), as well as sometimes for 'younger'.