How to use regex search

I can't find any documentation about how to use regex search, I found there is the search-regexp option, but when I try inputting any regex it just repeats the options.

How would I go about finding, for example, any message starting with a number in the #general channel of team FOOBAR?

Start with the existing search-regexp option and reproduce the reported behavior for a message beginning with a number in the #general channel of team FOOBAR. Trace the relevant help or documentation entry, then document the supported syntax and a working example so users can tell how to perform the search.

Written by the indexing model from the issue text.