为什么这个htaccess规则将两个匹配分配给一个$ _GET变量？

Here's the htaccess rule for my site's profile page:

RewriteRule ^users/([0-9]+)/?(.+)?/?(.+)? profile.php?id=$1&slug=$2&tab=$3&%{QUERY_STRING}

When you click on a tab on a user's profile, such as favorites:

/users/83028/nathan-johnson/favorites

The $_GET variable with the name of slug is receiving nathan-johnson/favorites instead of just nathan-johnson and the tab variable is blank, according to a var_dump:

array(3) { 
      ["id"]=> string(5) "83028" 
      ["slug"]=> string(24) "nathan-johnson/favorites" 
      ["tab"]=> string(0) "" }

It looks like the issue is with my htaccess regexp (above), but I don't see the issue with it.

Thanks in advance.

Because your forward slashes are optional (via /?), the (.+) grouping is greedy and will gobble them all up. You need to make them non-greedy by adding a ?: (.+?)

Also, instead of (.+)? you should just use a * which means none, one, or more.

Try:

RewriteRule ^users/([0-9]+)/?([^/]*)/?(.*?)?$ profile.php?id=$1&slug=$2&tab=$3&%{QUERY_STRING}

Also, instead of using %{QUERY_STRING}, you can just use the QSA flag:

RewriteRule ^users/([0-9]+)/?([^/]*)/?(.*?)?$ profile.php?id=$1&slug=$2&tab=$3 [L,QSA]