REGEX: replace a specific character after the appearence of another character in string

Question

i have this string: owl:Class rdf:about="#other important people"

i want with help of vim regex to replace the space with _ but only after the word starting with #.

I mean i want finally: owl:Class rdf:about="#other_important_people"

sorry for my poor grammar

Answer 1

:s/\(#[^"]*\)\@<= /_/g