BASH: How to extract substring that is surrounded by specific text

Question

I am trying to extract numbers from file names that follow a specific pattern:

file-8923489_something.txt
another_file-8923489_something.txt
some-other_file-8923         


        

Answer 1

Using pre BASH regex:

x="some-other_file-8923489_something.txt"
[[ "$x" =~ file-([0-9]*)_something ]] && echo ${BASH_REMATCH[1]}
8923489

OR this grep -P will also work:

grep -oP "file-\K\d+(?=_something)" file
8923489
8923489
8923489

OR using awk:

awk -F 'file-|_something' '{print $2}' file
8923489
8923489
8923489