Reading numbers from a text line in bash shell

Question

I'm trying to write a bash shell script, that opens a certain file CATALOG.dat, containing the following lines, made of both characters and numbers:

event_0133_pk.gz
event_0291_pk.gz
event_0298_pk.gz
event_0356_pk.gz
event_0501_pk.gz

What I wanna do is print the numbers (only the numbers) inside a new file NUMBERS.dat, using something like > ./NUMBERS.dat, to get:

0133
0291
0298
0356
0501

My problem is: how do I extract the numbers from the text lines? Is there something to make the script read just the number as a variable, like event_0%d_pk.gz in C/C++?

Answer 1

A grep solution:

grep -oP '[0-9]+' CATALOG.dat >NUMBERS.dat

A sed solution:

sed 's/[^0-9]//g' CATALOG.dat >NUMBERS.dat

And an awk solution:

awk -F"[^0-9]+" '{print $2}' CATALOG.dat >NUMBERS.dat