# grep key1.*key2 <file> search 2 keywords with logical AND match
# grep -w <key> <file> search with whole word matching
# grep [xyz]key <file> search for "key" prefixed with either x or y or z.
# grep [0-9]key <file> search for "key" prefixed with any char within range specified by [ ]
# grep _[0-9][0-9]_ search _nn_ where n represents any digit between 0 to 9