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+?

Answer 1

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.

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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"