I think I was not very clear, it tried to make it as simple as possible, showing that I can search for the string in Regex mode but not in Regex function mode. This just made it impossible to understand what I need...
The initial search is actually wider (<div.*?</div>), and I will do many replacements within the function, one of them is this:
string.replace('</span>\s*<br/>\s*', 'xx')
But it seems I cannot use \s or quantifiers in the function. Is there a way to do this?