simple rewrite rules

Hello,

I need to understand the concept of rewrite_rules pf wordpress developped in page :
https://codex.www.ads-software.com/Class_Reference/WP_Rewrite

exemple I need to write a new rule : put string/ before post to have something like that :
https://www.mydomain.com/mystring/%postname%/

what’s the code to add ? (line to change)

—-
// flush rules
function my_insert_rewrite_rules( $rules )
{
$newrules = array();
$newrules[‘(project)/(\d*)$’] = ‘index.php?pagename=$matches[1]&id=$matches[2]’; // line to change
return $newrules + $rules;
}

(…)

thanks a lot