Using grep with regular expression to filter out matches

Question

I\'m trying to use grep with -v for invert-match along with -e for regular expression. I\'m having trouble getting the syntax right.

I\'m trying something like

Answer 1

You need to escape the pipe symbol when -e is used:

tail -f logFile | grep -ve "string one\|string two"

EDIT: or, as @Adam pointed out, you can use the -E flag:

tail -f logFile | grep -vE "string one|string two"

Answer 2

The problem is that by default, you need to escape your |'s to get proper alternation. That is, grep interprets "foo|bar" as matching the literal string "foo|bar" only, whereas the pattern "foo\|bar" (with an escaped |) matches either "foo" or "bar".

To change this behavior, use the -E flag:

tail -f logFile | grep -vE 'string one|string two'

Alternatively, use egrep, which is equivalent to grep -E:

tail -f logFile | egrep -v 'string one|string two'

Also, the -e is optional, unless your pattern begins with a literal hyphen. grep automatically takes the first non-option argument as the pattern.