Thank you. Trying this, the problem I encounter here is that because I'm using a non-breaking space (the underscore was just an example for my post), I only want the non-breaking spaces between e.g. the word "page" and a number that comes afterwards, but not when there's a word after "page". So that method works to replace all instances where it's "page 1", "page 2", etc., but then it also adds non-breaking spaces after all instances of page, and I can't then remove them easily for those instances where there's a word after page, because ^$ isn't a valid character for the replace box.

So basically I want (as examples):
Before: page 1 -> page_1
Before: page 20 -> page_20
But not:
page of -> page_of
page that -> page_that

But i'm not sure how to achieve that easily.