[Linux 常見問題] How to extract a value from a string using regex and a shell?

Source From Here 
Question 
I am in shell and I have this string: 

12 BBQ ,45 rofl, 89 lol

Using the regexp: \d+ (?=rofl), I want 45 as a result. Is it correct to use regex to extract data from a string? The best I have done is to highlight the value in some of the online regex editor. Most of the time it remove the value from my string. 

I am investigating expr, but all I get is syntax errors. How can I manage to extract 45 in a shell script? 

How-To 
You can do this with GNU grep's perl mode: 

// -P, --perl-regexp: Interpret PATTERN as a Perl regular expression. 
// -o, --only-matching: Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line. 
# echo "12 BBQ ,45 rofl, 89 lol"|grep -P '\d+ (?=rofl)' -o