Up until now, in over 20 years of punting, a regular would be one I go back to a second time
As it happens I am now actively interviewing potential regulars, thanks to a succession of lackluster to average punts and some recent threads on here extolling the benefits of having one. Those touring in an old Audi with Romanian plates, working in short rental flats for a bloke called Sergei who calls McDonald's his office don't get to the interview stage - unless they have a nice arse of course. Equal opportunities and all that shit.
To me a regular would be a prossie I see for a fair proportion of my punts, 50 % or so, and would also be a reliable choice if nothing else appeals. I'm thinking one visit a month, maybe twice when horn demands and funds allow.
Having a reliable experience is a big draw, as are the potential benefits of experiences getting better. A prossie might view it differently though and only consider you to be a regular if you offer a monthly income stream, so if you only go along once every three months you're unlikely to reap the benefits them putting you in the regular bracket, even though she might represent 50% of your punts. Big guess work in this though and I'm sure others with experience of having a regular will correct me - please do if I've got it wrong.