[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. For instance, if I have the following 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.

Now, 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, but I couldn't find any info (on the net and in man grep).

Is it possible, or are capturing groups only meant to be backrefenced ? It would seem weird to me if there was no way of doing that.

Thank you for your time, and feel free to critique the way this post is constructed!

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.