Adding rewrite rule dynamically

Question

I'm trying to dynamically add a rewrite rule that allows one to access any page under an alternate permalink structure of my choosing.

So for example, if the alternate permalink structure includes the slug foo/ in the first postion, I want to add a rewrite rule such that:

example.com/foo/my-page rewrites to example.com/my-page

example.com/foo/my-cool/blog/post rewrites to example.com/my-cool/blog/post

So basically, no matter whether the query is for a page, a post, or any other type of custom content, I want this content to be accessible under the /foo/ slug as well as the original non-foo URL structure.

I have written a working function that accomplishes this by..

1. reading the URI
2. removing the slug from he URI
3. looping through all the existing rewrite rules
4. matching the URI with the slug removed to an existing rewrite rule
5. grabbing the matching groups for matched the rewrite rule
6. compiling a new rewrite rule with the slugged URI as the source and the matched rewrite rule endpoint with matching groups replaced with their values as the destination.

Here is the code:

// functions.php

add_filter('rewrite_rules_array', function($rules) {

    $slug = 'foo';

    // get the request URI
    $uri = $_SERVER['REQUEST_URI'];
    
    // determine whether URI has slug in the first position
    if(preg_match('/^(\/' . $slug . ')/', $uri)){

        // get the base URI by removing the slug
        $base_uri = str_replace('/'.$slug,'',$uri);

        // loop through existing rewrite rules
        foreach($rules as $src => $dest){

            // find the rewrite rule for which the base URI would have matched
            $regex_to_match = '/' . str_replace('/','\/',$src) . '/';
            preg_match_all($regex_to_match, $base_uri, $matches, PREG_SET_ORDER);

            if(count($matches) > 0){

                // get the specific matching groups
                $matches = $matches[0]; 

                // compile valid regex from URI with slug to create new rewrite source
                $new_src = ($uri[0] == '/' ? substr($uri, 1) : $uri) . '?$';
                
                // replace match variables with their string values to create new rewrite destination
                for($i=1; $i<count($matches)+1; $i++){
                    $replacement = isset($matches[$i]) ? $matches[$i] : '';
                    $dest = str_replace('$matches[' . $i . ']', $replacement, $dest);
                }
                $new_dest = $dest;

                // add new rewrite rule to $wp_rewrite rules array
                $rules[$new_src] = $new_dest;

                // add the write rule
                add_rewrite_rule($new_src,$new_dest,'top');

                break;
            }
        }
    }

    return $rules;

});

flush_rewrite_rules();

This works but only for URLs with multiple slugs.

example.com/foo/my-page/123 rewrites correctly!

example.com/foo/abc/defg/bar rewrites correctly!

example.com/foo/abc DOES NOT rewrite correctly. Instead it redirects to example.com/abc

same for example.com/foo/my-page -- it redirects to example.com/my-page

Answer 1

The issue with your updated code is that because you are using add_rewrite_rule() inside the filter, it can't properly add it to the top of the list. You can see this if you dump out the $rules array. Because it gets added to the end of the $rules list, WP steps in beforehand to match on slug alone using some magic in the background.

In order to circumvent that process, you have to add it to the top of the list manually and return that from the filter.

Specifically, you need to change the add_rewrite_rule() call to this:

$rules = [$new_src => $new_dest] + $rules;

I included the full updated code below.

add_filter('rewrite_rules_array', function($rules) {

    $slug = 'foo';

    // get the request URI
    $uri = $_SERVER['REQUEST_URI'];
    
    // determine whether URI has slug in the first position
    if(preg_match('/^(\/' . $slug . ')/', $uri)){

        // get the base URI by removing the slug
        $base_uri = str_replace('/'.$slug,'',$uri);

        // loop through existing rewrite rules
        foreach($rules as $src => $dest){

            // find the rewrite rule for which the base URI would have matched
            $regex_to_match = '/' . str_replace('/','\/',$src) . '/';
            
            preg_match_all($regex_to_match, $base_uri, $matches, PREG_SET_ORDER);


            if(count($matches) > 0){

                // get the specific matching groups
                $matches = $matches[0]; 

                // compile valid regex from URI with slug to create new rewrite source
                $new_src = ($uri[0] == '/' ? substr($uri, 1) : $uri) . '?$';
                
                // replace match variables with their string values to create new rewrite destination
                for($i=1; $i<count($matches)+1; $i++){
                    $replacement = isset($matches[$i]) ? $matches[$i] : '';
                    $replacement = strpos($replacement, '/') === 0 ? substr($replacement, 1) : $replacement;
                    $dest = str_replace('$matches[' . $i . ']', $replacement, $dest);
                }
                $new_dest = $dest;

                // add new rewrite rule to $wp_rewrite rules array
                $rules = [$new_src => $new_dest] + $rules;

                break;
            }
        }
    }

    

    return $rules;

});

This still feels like a weird way of doing this, and I'm sure there is a better one but without better understanding your use case I can't really provide an alternate suggestion. The biggest issue here to me is you are flushing the rewrite rules on every request, which can be an expensive operation.

Original Answer

I wouldn't hook into generate_rewrite_rules for this. You can just use the regular rewrite rules API.

Tested the below, and it works for redirecting http://example.com/foo/test/123 to https://example.com/test/123. Can customize it to your needs. Drop it in functions.php or a functionality plugin.

add_action( 'init',  function() {
    add_rewrite_rule( 'foo/([a-z0-9-/]+)[/]?$', 'index.php?foo_redirect_to=$matches[1]', 'top' );
} );

add_filter( 'query_vars', function( $query_vars ) {
    $query_vars[] = 'foo_redirect_to';
    return $query_vars;
} );

add_action('template_redirect', function() {
    if ( ( $redirect_to = get_query_var( 'foo_redirect_to' ) ) == false ) {
        return;
    }

    wp_redirect("https://example.com/$redirect_to");
});