Egrep

1. Look for one string:

grep "string"
egrep "string"
grep -E "string"

1. Look for either of two strings:

egrep "string1|string2"
grep -E "string1|string2"

1. Look for either of four strings:

egrep "string1|string2|string3|string4"
grep -E "string1|string2|string3|string4"