Why doesn't this regex work for grep?

Question

I'm trying to remove "Packet number 624 doesn't match" from a response so the obvious thing to try is

cat somefile.txt | grep -v "Packet number \d+ doesn't match"

If I remove the -v, just for testing, then it returns nothing. So maybe the command line is doing something with the \d or + first. So I have tried various combinations such as \\d+ \\d\+ \\\\d+ \\\\d\+ [0-9]+ [0-9]\+. Bingo!! That last one worked. Can someone explain what is going on here? If this is getting modified by the command line why does echo "\d+" still return \d+?

possible duplicate of [Is \d not supported by grep's basic expressions?](http://stackoverflow.com/questions/6901171/is-d-not-supported-by-greps-basic-expressions) — CDahn, Jun 17 '14 at 01:44
Thanks, that works and answers that question. The question still remains as to why `[0-9]+` didn't work — MikeKulls, Jun 17 '14 at 01:51
Most probably you have to avoid cat command. Your command should be `grep -v "Packet number [0-9]\+ doesn't match" somefile.txt` — Avinash Raj, Jun 17 '14 at 02:18
^ Yes, swinging that cat around is unnecessary — `grep ... file` should work. — l'L'l, Jun 17 '14 at 02:21
The cat is just for demonstrating the problem, the output actually comes from radsniff which we use to sniff radius traffic — MikeKulls, Jun 17 '14 at 02:36

hwnd · Accepted Answer · 2015-01-24T22:25:54.923

5

By default, grep uses basic regular expression and \d is (PCRE) syntax. It is not supported so you'll need to use ( [0-9] ) or ( [[:digit:]] ) instead, or use grep with option -P

Why doesn't [0-9]+ work?

In BRE, meta-characters like + lose their meaning and need to be escaped.

You can fix this by using one of the following:

grep -v "Packet number [0-9]\+ doesn't match"

OR

grep -v "Packet number [[:digit:]]\+ doesn't match"

edited Jan 24 '15 at 22:25

answered Jun 17 '14 at 01:57

hwnd

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Thanks everyone for the answers. This clears up something that has confused me for a long time which is why regex "sort of work" in some cases. I'd always assumed that non PCRE was basically the same with some subtle differences that I would likely never encounter. – MikeKulls Jun 17 '14 at 02:38

Joe Tse · Answer 2 · 2014-06-17T02:07:06.007

1

-v means inverse, any lines not matching pattern will be listed -E is the right parameter [Correction] -E is the right parameter WORKS IN Mac OS X -P is the right parameter, TESTED IN Ubuntu

try this

//in somefile.txt:
Packet number 100 doesn't match -E
Pck number 100 doesn't match -v

use cat somefile.txt | grep -v "Packet number \d+ doesn't match" both lines appears use cat somefile.txt | grep -E "Packet number \d+ doesn't match" in Mac OS X use cat somefile.txt | grep -P "Packet number \d+ doesn't match" in Linux only "Packet number 100 doesn't match -E" appears, which is what you want.

edited Jun 17 '14 at 02:07

answered Jun 17 '14 at 01:53

Joe Tse

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I can confirm that the `-P` option works but the `-E` option returns nothing (or everything with `-v`) – MikeKulls Jun 17 '14 at 01:55
@MikeKulls, when you use -v, `[0-9]+` won't work. try -E for `[0-9]+` – Joe Tse Jun 17 '14 at 01:56
-v, in this case, nothing match the pattern, (\d does not work in default mode), so everything returns. – Joe Tse Jun 17 '14 at 02:01
@MikeKulls just tested. see my correction in the answer. – Joe Tse Jun 17 '14 at 02:08
More specifically, `-E` works in most BSDs and `-P` works in most linuxes – Wolph Jun 17 '14 at 07:54
Even more specifically, `-E` works on any POSIX platform, whereas `-P` is a nonstandard extension. – tripleee Mar 19 '19 at 18:54

score 0 · Answer 3 · answered Jun 17 '14 at 01:54

0

grep's re syntax didn't support notations like \d. There is a option to use Perl's syntax, try this,

cat somefile.txt | grep -v -P "Packet number \\d+ doesn't match"

Please note you need to escaspe the backslash.

answered Jun 17 '14 at 01:54

fwu

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A better fix is to use single quotes around your regex. Also, lose the [useless use of `cat`.](/questions/11710552/useless-use-of-cat) – tripleee Mar 19 '19 at 18:55

Why doesn't this regex work for grep?

3 Answers3