grep ? (regex)

from grep info. GNU FDL

`?’ The preceding item is optional and will be matched at most once.

$cat bu.txt
bugy
buggy
bugggy
$

$grep -E bug\?y  bu.txt
bugy
$grep -E bugg\?y  bu.txt
bugy
buggy
$grep -E buggg\?y  bu.txt
buggy
bugggy
$