So I solved problem K using DP, but the tutorial video said that some contestants managed to solve it with regular expressions.

However, the K-quotation in general is a context free grammar not a regular expression. I guess if you fix K it will become a regular expression but the length will grow too fast as K grows and since matching it requires 2^(len of expression) preprocessing I don't see a realistic way of doing it.

How did they manage to use regular expressions to solve the problem??? problem link, solution video