How to use regex for numbers in the string?

Question

For example I have files that have names like

• book1_volume1.txt
• book1_volume2.txt
• book1_review.txt
• book2_volume1.txt

How can I find all 'book%NUMBER%_volume' files in one search? How do I mask the number after 'book'?

Answer 1

If there can be more than one digit, use an extended pattern:

shopt -s extglob nullglob
books=( book_+([0-9])_volume* )

Answer 2

The simplest way to do so is to use wildcards, specifically book[0-9]_volume* matches your example.