Sed help?

sup /prog/, I need your help for a bash script I'm making.
The last thing I need is a 3-step line.
>1. find the first instance of one less than an existing variable ($X - 1)
>2. search for the first instance of a string above that
>3. take the line of step 2 and save it as a variable

How could I do this using sed/grep?
Thanks.

This is impossible using sed or grep alone.  You might be able to hack together some abomination with awk and sh, but you'd probably be better off just using Perl.