commandline session
$ 4.2.20 10 510---> cat worked Bubble sort Selection sort Insertion sort Shell sort Comb sort Merge sort Heapsort Quicksort Counting sort Bucket sort Radix sort Distribution sort Timsort $ 4.2.20 11 511---> cat worked | grep -E ?sort Bubble sort Selection sort Insertion sort Shell sort Comb sort Merge sort Heapsort Quicksort Counting sort Bucket sort Radix sort Distribution sort Timsort $ 4.2.20 12 512---> cat worked | grep ?sort $ 4.2.20 13 513---> $ 4.2.20 17 517---> echo "(555) 123-4567" | grep -E '^(?[0-9][0-9][0-9])? [0-9] [0-9][0-9]$' (555) 123-4567 $ 4.2.20 18 518---> echo "(555) 123-4567" | grep -E '^([0-9][0-9][0-9])? [0-9] [0-9][0-9]$' (555) 123-4567 $ 4.2.20 19 519---> echo "(555) 123-4567" | grep -E '^([0-9][0-9][0-9]) [0-9] [0-9][0-9]$' (555) 123-4567 $ 4.2.20 20 520--->