Isapi_rewrite second-level domain name

Ask me a strange question. I use isapi_rewrite as the real second-level domain name system.
Expected results: the actual address of boy119.16.com is www.16.com/u/boy119, and the address of boy119.16.com/123.htmis www.16.com/u/boy119/123.htm.
The regular expression I wrote is as follows:
Rewritecond HOST :(?! Www \.) (. *) \. 16 \. com
Rewriterule/(. *)/u/$1/[I]
Result: The actual address of boy119.16.com is www.16.com/u/boy119, but the actual address of boy119.16.com/123.htmis still www.16.com/u/boy119.

I changed the regular expression:
Rewritecond HOST :(?! Www \.) (. *) \. 16 \. com
Rewriterule/(. *)/u/$1/& 2 [I]
The actual address of boy119.16.com is www.16.com/u/boy119/u/boy119, and the address of boy119.16.com/123.htmis www.16.com/u/boy119/u/boy119/123.htm.
How can I modify the regular expression to achieve my goal? Thank you 1