grep -E . ERE . Alternation

commandline session

$ 4.1.5 1 16---> echo  sdf | grep -E 'a|b'
$ 4.1.5 2 17---> echo  sdf | grep -E 'a|b|c'
$ 4.1.5 3 18---> echo  sdf | grep -E 'a|b|c|d'
sdf
$ 4.1.5 4 19---> echo  sdf | grep -E 'a|d'
sdf
$ 4.1.5 5 20--->