Regular expression for a interval of non-negative integers

Let $m,n\in \mathbb{N}$ and $m\le n$. I wrote a program in Haskell that generate a regular expression that matches all decimal representation of some number in between $a$ and $b$ inclusive. The regular expression would have length $O(\log n\log \log n)$. I also included a simple version, which shows how the algorithm is done.

The code was made for the decimal system, but of course it can be generalized to any base.

Example:

matchIntRange 123 4321

1(2[3-9]|[3-9]\d)|[2-9]\d\d|[123]\d{3}|4([012]\d\d|3([01]\d|2[01]))

The main idea is to consider a set of tries. Each one contain strings with the same length. The regular expression has a structure closely related to this trie. It be nice if there is a polytime algorithm to generate the shortest regular expression for this problem if all we can use is (), |.

It is important to also handle cases where we will have repeated elements

matchIntRange 12121212 12121212

(12){4}

This is done by recursively try to compress a free monoid element with the FreeMonoidCompress module.