As the numbers of strings is infinite, brute forcing the two regexes over all the strings is not possible, so you are down to finding some theoretical solution (and I have no clue even how to start with it).

Only if the regexes determine a limited number of strings, you can perhaps work backward and calculate the set of acceptable strings for the "A" regex and the "B" regex. If both sets are the same, you have proven your point.