grep:how to use non-capturing group?

Question

I am confused about the following command

$ cat num.txt 1 2 3 1st 2nd 3th $ cat num.txt | grep -Eo '[0-9](?:st|nd|th)?'

I think it should output as

1 2 3 1 2 3

But it output as

1 2 3 1 2nd 3th

What am I doing wrong here?Thanks for any help.

A non-capturing group doesn't prevent any substring to be in the whole match result. It only avoids to create a capture (a separate substring accessible with the group number). — Casimir et Hippolyte
– Casimir et Hippolyte, Commented Jul 9, 2015 at 10:22

anubhava · Accepted Answer · 2015-07-09 10:21:55Z

4

You can use:

grep -Eo '^[0-9]+' file 1 2 3 1 2 3

Or using lookahead in grep -P:

grep -Po '[0-9]+(?=st|nd|th)?' file 1 2 3 1 2 3

answered Jul 9, 2015 at 10:21

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