Matching a date in a file on Unix

Question

I have the following unix command, which I'm using to try finding a date in the format yyyy-mm-dd in a file:

grep -i -w [\d]{4}-[\d]{2}-[\d]{2}? <filename> 

but for some reason I'm getting an empty answer. Am I matching the regex correctly for grep?

Answer 1

The following is working, using bash extended regex (-E, --extended-regexp):

grep -E -i -w "[0-9]{4}-[0-9]{2}-[0-9]{2}" <filename> 

But, in this case you should use [0-9] instead of \d.

If you want to use \d, you need to specify the PERL regex (-P, --perl-regexp):

grep -P -i -w "\d{4}-\d{2}-\d{2}" <filename>