One thing to consider is the cost to transform these strings into a format suitable for your regex to work with them. Presumably these strings live on your filesystem. They could all be in one huge file or each string can have its own file or anything in between these extremes.

A possible strategy could be to open the file(s) and upon reading each string immediately checking whether the string matches your query, discarding the string if not and keeping it if it does. This probably the fastest way, but will only work for one query. Each query would suffer again the cost of opening the file(s) and reading the strings.

If you need to run multiple queries, you must invent a data-structure to hold all the strings and timestamps (a hash of arrays or an array of arrays spring to mind). The set-up of these will also have a one-time cost and there is of course the memory issue to look into: too much memory needed and your machine will start swapping, which will degrade the performance of your queries. Fortunately memory if cheap, so adding another GigaByte of RAM will solve a lot of problems in that respect.

But you will still be re-inventing a lot of wheels and rather than playing at the "(re-)invent-a-database" game, why not dumping the data into a simple SQLite database and then running the usual SQL-queries against this database?