Basically I want to take as input text from a file, remove a line from that file, and send the output back to the same file. Something along these lines if that makes it any
Use sponge for this kind of tasks. Its part of moreutils.
Try this command:
grep -v 'seg[0-9]\{1,\}\.[0-9]\{1\}' file_name | sponge file_name