that’s sounds more like a job for the filehandle buffer.
You should be able to match the first line with:
/^(.*)$/m
(as always, this is PCRE syntax)
the /m modifier makes ^ and $ match embedded newlines. Since there’s no /g modifier, it will just process the first occurrence, which is the first line, and then stop.
If you’re using a shell, use:
head -n1 file
or as a filter:
commandmakingoutput | head -n1
Please clarify your question, in case this is not wat you’re looking for.