RegEx for matching forward reference exercise

Question

I am working on an exercise on regex forward reference like the one asked here

The task is:

• string s consists of tic or tac
• tic should not be immediate neighbour of itself
• The first tic must occur only when tac has appeared at least twice before

I've used this regex to solve it. But it doesn't use forward reference so I really want to know how this can be solved using it.

^tac(tac)+(tic(tac)+){0,}(tic)?$

Valid:

tactactic
tactactictactic

Invalid:

tactactictactictictac
tactictac

How do I solve this problem?

Answer 1

This uses a forward reference:

^(\2tic|(tac))+$

Demo

It's actually very similar to the example given on this page.