At work we have to split a potentially large ID into 2 10-digit long parts that will be passed to an outside system that will later return them with some more data to us.

A colleague had implemented it using regular expression, it passed code review, everything was ok, until he noticed a potential problem. For some cases, because the outside system stores them as int and therefore will remove any leading zeros, there will be no way to reconstruct the number.
So we brainstorm and I propose ether a modified regex, or to just use math like part1 = id % 10^10 and part2 = floor(id % 10^10) and then we can reconstruct it simply by: part2 * 10000000000 + part1

Colleague: - Well, the regex will be faster, there won't be any calculations
Me: - :| I disagree but ok..

We do some more brainstorming and testing and find a case where the proposed new regex fails as well
So I bring up my previous proposal, I explain what exactly it does.

Colleague: - I don't like the math, it has calculations, which won't be needed before we reach the 11th digit

Have I missed some major development in computer hardware? When did they become bad/slow at doing math? :|