Regex brackets - previous element can exist "this many" times

egrep -r '0{3,}' /etc

will match 000 at least

egrep -r '10{,3}' /etc

will match 1 followed by most 000. This will also match 1s following by no 0s

egrep -r '0{3}' /etc

will fins all strings that have exactly three 0s