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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!
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How to Exclude a Single Word with grep. The most simple way to exclude lines with a string or syntax match is by using grep and the -v flag. The output will be the example. txt text file but excluding any line that contains a string match with “ThisWord”.
Capturing groups are a way to treat multiple characters as a single unit. They are created by placing the characters to be grouped inside a set of parentheses. For example, the regular expression (dog) creates a single group containing the letters "d", "o", and "g".
Exclude Words and Patterns By default, grep is case-sensitive. This means that the uppercase and lowercase characters are treated as distinct. To ignore the case when searching, invoke grep with the -i option. If the search string includes spaces, you need to enclose it in single or double quotation marks.
This question was asked ten years ago, so I won't mark it as duplicate. Also I noticed no sed solution was given since OP asked an answer without:
sed -nr 's/(hello[0-9]+), please match me/\1/p' test.txt
-n stands for quiet (won't print anything except if explicitly asked)-r allows use of extented regular expressions (avoids here using \ before parenthesis)s/reg/repl/p command means "if regexp reg matches the current line, replace it by captured text by repl, and prints it (/p)"
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