gistfile1.php

<?php

$prefix = $blog_prefix = '';
if ( ! got_url_rewrite() )
	$prefix = '/index.php';
if ( is_multisite() && !is_subdomain_install() && is_main_site() )
	$blog_prefix = '/blog';

if ( isset($_POST['permalink_structure']) || isset($_POST['category_base']) ) {
	check_admin_referer('update-permalink');

	if ( isset( $_POST['permalink_structure'] ) ) {
		if ( isset( $_POST['selection'] ) && 'custom' != $_POST['selection'] )
			$permalink_structure = $_POST['selection'];
		else
			$permalink_structure = $_POST['permalink_structure'];

		if ( ! empty( $permalink_structure ) ) {
			$permalink_structure = preg_replace( '#/+#', '/', '/' . str_replace( '#', '', $permalink_structure ) );
			if ( $prefix && $blog_prefix )
				$permalink_structure = $prefix . preg_replace( '#^/?index\.php#', '', $permalink_structure );
			else
				$permalink_structure = $blog_prefix . $permalink_structure;
		}
		$wp_rewrite->set_permalink_structure( $permalink_structure );
	}
//...........
}