first test: match on empty file
test 2: print a file wit a line
test 3: match a line in a file
test 3: refactor grep to make more readable
test 4: grep in multiple files
test 4: refactor tests, make them DRY
test 5: matching using regexes
test 6: add 'ignore_case' option
test 7: print line numbers
test 7: refactor grep, make smaller functions
test 8: make sure empty lines are not printed