[SOLVED] Grep - how to output only the content of a capturing group

Grep - how to output only the content of a capturing group

I am trying to find a way for grep to output only the content of a capturing group. If I have this file:

hello1, please match me
hello2, please do not match me

I would like

grep -Eo '(hello[0-9]+), please match me' file

To output hello1. However it outputs hello1, please match me.

I know that grep -Po 'hello[0-9]+(?=, please match me)' will do the trick, but I'm thinking there must be a way to simply return a capturing group.

Is it possible, or are capturing groups only meant to be backrefenced?

Solution

If you have either pcregrep or pcre2grep you can use the -o1 command-line flag to request that only capture group 1 is output. (Or change 1 to some other number if there are more captures in the regex.)

You can use the -oN command more than once if you want to output more than one capture group.

As far as I know, grep -P does not implement this extension. You'll find pcre2grep in Debian/Ubuntu package pcre2-utils. pcregrep is in package pcregrep.