add_rewrite_endpoint generates incorrect attachment rules

[batmoo]

add_rewrite_endpoint with EP_ALL or EP_ATTACHMENT results in incorrect endpoint matches. The index set for the endpoint value is 2 which ends up returning the slash as well. The index should actually be 3 since we only care about the inner match.

Example:

add_rewrite_endpoint( 'my-endpoint', EP_ALL )

Results in rules like (dump from $wp_rewrite->generate_rules:

  '.?.+?/([^/]+)/my-endpoint(/(.*))?/?$' =>
  string(35) "index.php?attachment=$1&my-endpoint=$2"
  '.?.+?/attachment/([^/]+)/my-endpoint(/(.*))?/?$' =>
  string(35) "index.php?attachment=$1&my-endpoint=$2"

Visiting a my-endpoint URL for an attachment like /2012/01/01/post-name/attachment/my-endpoint/foo/ will return the value of get_query_var( 'my-endpoint' ) as /foo instead of foo.

[duck_]

In 24809:

Drop leading / by retrieving the inner subpattern when matching attachment endpoints.

Fixes #22619