URL Rewrite same host different port in IIS 8.5 - regex

Question

I have a server with three websites:
Default website, port 80 (no content)
Website A, port 81
Website B, port 82
I would like to use URL Rewrite to access Website A and Website B like:
http://myserver/A
http://myserver/B
instead of
http://myserver:81
http://myserver:82
I have installed the URL Rewrite and Application Request Routing modules and enabled the proxy for ARR.
On the default website I created two rewrite rules using the regex pattern /A/(.*) and /B/(.*):
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="WebsiteA" stopProcessing="true">
<match url="/A/(.*)" />
<action type="Rewrite" url="http://localhost:81/{R:1}" />
</rule>
<rule name="WebsiteB" stopProcessing="true">
<match url="/B/(.*)" />
<action type="Rewrite" url="http://localhost:82/{R:1}" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>
Using the Test pattern... functionality I could verify that {R:0} contained the entire test URL and {R:1} just the part after A/ or B/. The rewrite URL I sat to point to either port 81 or 82. I still regard regular expressions as pure magic but the test gave me what I wanted. Trying to connect to the websites through /A/ and /B/ gave me 404 errors.
Seeing that this is a very common problem with lots of questions and answers I picked one suggesting to use the regex pattern ^A(.*)$:
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="WebsiteA" stopProcessing="true">
<match url="^A(.*)$" />
<action type="Rewrite" url="http://localhost:81/{R:1}" />
</rule>
<rule name="WebsiteB" stopProcessing="true">
<match url="^B(.*)$" />
<action type="Rewrite" url="http://localhost:82/{R:1}" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>
Using the test functionality my test URLs failed to match that pattern, but when I tried to access the sites using the new pattern, it worked.
Question: I am going with the later solution but is it pure luck that it works and should I perhaps do something different?
Is it normal for the test functionality to not successfully match the same URL's that works when I feed them to the server?