This is a simple version of the Unix tool
grep
% man grep 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.
go run main.go "" test.txt | diff test.txt -
go run main.go J rockbands.txt
go run main.go -r "Nirvana" "*"
go run main.go -r Nirvana * | go run main.go -v Madonna
go run main.go "\d" test-subdir/BFS1985.txt
go run main.go "\w" symbols.txt
go run main.go A rockbands.txt | wc -l
go run main.go -i A rockbands.txt | wc -l