How to replace all named groups with a special word? or remove them?

[Hassan Monfared]

I have a rewritten URL like : http://localhost/category/23/seach_kw/foo and I want to replace 23 and foo with '_' character. and what I need is :

http://localhost/category/_/search_kw/_

My problem is that RE2::Replace method requires rewrite argument which I don't have it.

std::string input="http://localhost/category/23/search_kw/foo";

RE2 rx("http://localhost/category/(?P<CAT_ID>\\d+)/search_kw/(?P<SEARCH_KW>\\w+)");

RE2::Replace(&s,rx,?????????????);

I'm not also sure if unnamed group may help me or not.

Is there any approach ?

Appreciate your help.

Hassan