Ken Thompson created 'grep' to find instances of a regular expression in files, inspired by the 'g' command in 'ed', and a programming class converted the 1800 lines of C code for 'ed' into 'grep' 25 years ago.
YOUTUBE NTfOnGZUZDk
The Origins of GREP - Unveiling the UNIX Command eightify.app 
The grep utility searches any given input files, selecting lines that match one or more patterns. By default, a pattern matches an input line if the regular expression (RE) in the pattern matches the input line without its trailing newline. An empty expression matches every line. Each input line that matches at least one of the patterns is written to the standard output.
Brian Kernighan Remembers the Origins of ‘grep’ thenewstack.io
YOUTUBE QFK6RG47bww The Factory of Ideas: Working at Bell Labs - Computerphile
How Grep Got Its Name? The History Behind Grep's Creation makeuseof.com