Possible bug in std vmod, using backtrack and tolower, 4.0.2

[T. Pascal]

I am trying to lowercase the query parameters and encounter a very strange
non-lowercasing bug in std. Consider:

sub vcl_rec {
  set req.url = regsub(req.url,"([?].*)",std.tolower("\1"));
}

`curl 'http://localhost:8080/get?TEST=none'`
...

`varnishlog -d`
...
-   ReqMethod      GET
-   ReqURL         /get?TEST=none
-   ReqProtocol    HTTP/1.1
....
-   ReqHeader      Host: localhost:8080
-   VCL_call       RECV
-   ReqURL         /get?TEST=none
...

Not the expected lower case (should be /get?test=none)

Please note that the following works correctly:

sub vcl_rec {
  set req.url = regsub(req.url,"([?].*)",std.tolower("?TEST=none"));
}
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