How to use grep to retrieve matching text from a file?

Question

I have a txt file like this:

asdfasdf
ffaCover 91adf
ffffa

I want to use grep or some linux tool to capture the 1 or 2 digits following the space after 'Cover'. In some programming languages I would use the regex library to match /^Cover (\\d\\d?)$/. Then there would be some way to get at the 1 or 2 digits inside the parenthesis. Is there a way to do this using grep?

Answer 1

Just pipe to grep -oP :

grep -Po 'Cover \K[0-9]{1,2}'
91

\\K in PCRE regex resets all previously matched information.

Answer 2

I had always done something like this:

 grep -Po '(?<=Cover\s)(\d\d?)'

The -o option to grep makes it print out only the matching part of the pattern, which does not include the zero-width lookbehind assertion.

But, I did not know about the \\K option in anubhava's answer before, which seems cleaner.