Question

This does not generate any output. How come?

$ echo 'this 1 2 3' | grep '\d\+'

But these do:

$ echo 'this 1 2 3' | grep '\s\+'
this 1 2 3

$ echo 'this 1 2 3' | grep '\w\+'
this 1 2 3

Answer 1

As specified in POSIX, grep uses basic regular expressions, but \d is part of a Perl-compatible regular expression (PCRE).

If you are using GNU grep, you can use the -P option, to allow use of PCRE regular expressions. Otherwise you can use the POSIX-specified [[:digit:]] character class in place of \d.

echo 1 | grep -P '\d'
# output: 1
echo 1 | grep '[[:digit:]]'
# output: 1