How to terminal output command pipe to input of grep and using regular expression?

Question

I thought getting an IP address on OSX or Linux would be rather easy to learn how to use regular expression with grep, but it looks like I either have a syntax error or misunderstanding of what is required.

I know this regex is correct, although I know it may not be a valid IP address I'm not considering that at this point.

(\d{1,3}\.){3}\d{1,3}

so I'm not sure why this doesn't work.

ifconfig | grep -e "(\d{1,3}\.){3}\d{1,3}"

Answer 1

Two things:

First, there is a difference between -e and -E : -e just says "the next thing is an expression", while -E says: "use extended regular expressions". Depending on the exact version of grep you are using, you need -E to get things to work properly.

Second, as was pointed out before, -d isn't recognized by all versions of grep. I found that the following worked: since ip addresses are "conventional" numbers, we don't need anything more fancy than [0-9] to find them:

ifconfig | grep -E '([0-9]{1,3}\.){3}[0-9]{1,3}'

No need to escape other characters (in my version of grep - using OSX 10.7.5)

Incidentally, when testing your regular expression you can consider using something like

echo "10.1.15.123" | grep -E '([0-9]{1,3}\.){3}[0-9]{1,3}'

to confirm that your regex is working - regardless of the exact output of ifconfig . It breaks the problem into two smaller problems, which is usually a good strategy.

Answer 2

\\d is not understood by grep, instead you could use [0-9] or [[:digit:]] . Unfortunately there are many dialects of regular expressions. You will also have to escape { , } , ( and ) . The following works for me

/sbin/ifconfig | grep -e "\([[:digit:]]\{1,3\}\.\)\{3\}[[:digit:]]\{1,3\}"