I believe you're missing the OP's point. The regexes are not part of the given data; in fact they're the desired answer.

I.e. given the data

Error 123 on SystemA file not found error
Error 123 on SystemB file not found error
Error 123 on SystemC file not found error
[download]

the task is to come up with a regex that matches these, using as little variables as possible.

That's how I read it anyway