how to use a variable inside a nginx “if” regular expression

It’s not exactly what I asked, but I think it’ll work for my purposes. I’m still curious how to interpolate a variable inside a nginx PCRE regex if anyone else knows!

set $chk == "need"; 
set $me "kevin"; 
if ($uri ~ /by-([^-]+)/) { set $by $1; }
if ($by = $me) {set $chk "";}