How to extract a value from a string using regex and a shell?

You can do this with GNU grep’s perl mode:

echo "12 BBQ ,45 rofl, 89 lol" | grep -P '\d+ (?=rofl)' -o
echo "12 BBQ ,45 rofl, 89 lol" | grep --perl-regexp '\d+ (?=rofl)' --only-matching

-P and --perl-regexp mean Perl-style regular expression.
-o and --only-matching mean to output only the matching text.