add_rewrite_rule - Page Slug from "/foo-bar/" to "/foo/bar/"

Question

I need foo-bar to become foo/bar instead:

//domain.com/foo-bar   »   //domain.com/foo/bar

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I'm rebuilding a website that is currently in a home-brew CMS. They have a few pages with children, but the parent page URL does not match the children. The parent URLs were changed but the children were never updated.

Expected Structure:
//domain.com/parent/
//domain.com/parent/child

Current Structure:
//domain.com/parentpage/     (changed from /parent/)
//domain.com/parent/child

I could just create a page for each but I'm trying to avoid having empty/unused pages.

What I'm hoping to do is just create //domain.com/parent-child/ and rewrite the URL to match, but I can't get my rules to take priority over an existing rule.

Maybe I'm misunderstanding what rewrites can accomplish?

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add_rewrite_rule

Matched Query:

pagename=foo-bar&page=

My Attempts: I expected somethig like one of these to be my solution, but I've tried a dozen different minor variations without success:

add_rewrite_rule( '^foo/bar', 'index.php?pagename=foo-bar', 'top');
add_rewrite_rule( '(foo)/(bar)', 'index.php?pagename=$matches[1]-$matches[2]&page=', 'top');

Default rule my page is matching:

add_rewrite_rule( '(.?.+?)(?:/([0-9]+))?/?$', 'index.php?pagename=$matches[1]&page=$matches[2]')

Answer 1

This rule is what I ended up with after finding this post: Understanding add_rewrite_rule

add_rewrite_rule( '^foo/([^/]*)/?$', 'index.php?pagename=foo-$matches[1]', 'top' );

Answer 2

First of all, you can change rules not only with add_rewrite_rule but also with a filter(s) (one - general I am using in all common cases is rewrite_rules_array). You will receive an array of rewrite rules (and you expect to return it back), but you can change it slightly.

And (minute of advertisement) - you can also use debug bar and debug bar rewrite rules panel to check your rules against your test URLs to see priority/matches etc. I am the author of this plugin (debug bar rewrite rules panel) and will be glad for a feedback.