IIS rewrite rule not working in live environment - regex

Question

I have 4 servers in azure, 3 are load balanced and the 4th is for CMS purposes only.
SSL certificate has been added for the main website, but not for the sobdomain that the CMS is on.
I wrote a rule that should find any url that doesnt contain "backoffice" and match any other page to change it to https.
This works on regexr.com but for some reason doesnt work
<rewrite>
<rules>
<rule name="http to https" stopProcessing="true">
<match url="(https?:\/\/(?!backoffice).*)" />
<conditions>
<add input="{HTTPS}" pattern="^OFF$" />
</conditions>
<action type="Redirect" url="https://www.WEBSITENAME.com{R:1}" />
</rule>
</rules>
</rewrite>
Url Rewriting 2.1 is installed on all 4 servers and i have created a load balance set in azure for https.
going to https manually works fine (along with loadbalancing).
Additional information:
I've tried many rules, including the existing answer. I can see things happening, like assets being brought in as https, but the page itself does not redirect.
There are 2 load balance sets, one for port 80 and the other for port 443. I don't know if this is corect, or could be a potential cause in the redirect not happening.

Answer 1

Your rule should be like that:
<rule name="http to https" stopProcessing="true">
<match url=".*" />
<conditions>
<add input="{HTTPS}" pattern="^OFF$" />
<add input="{REQUEST_URI}" pattern="/backoffice" negate="true" />
</conditions>
<action type="Redirect" url="https://www.WEBSITENAME.com{R:0}" />
</rule>
This rule will exclude requests with /backoffice path.
Also for issue of mixing content you need to fix your paths for css/js/images to relatives. Example:
<img src="/path/to/your/image.jpg"/>
Another way to fix mixed content is create outbound rule, which will change your output HTML (replace http: to https:):
<rewrite>
...
<outboundRules>
<rule name="Rewrite external references to use HTTPS" preCondition="IsHTML">
<match filterByTags="Script, Link, Img, CustomTags" customTags="HTML5Tags" pattern="^http://(.*)$" />
<action type="Rewrite" value="https://{R:1}" />
</rule>
<preConditions>
<preCondition name="IsHTML">
<add input="{RESPONSE_CONTENT_TYPE}" pattern="^text/html" />
</preCondition>
</preConditions>
<customTags>
<tags name="HTML5Tags">
<tag name="Video" attribute="src" />
</tags>
</customTags>
</outboundRules>
</rewrite>

Answer 2

Using the previous answer as a starting point, i made a few minor changes, to use HTTP_HOST rather than REQUEST_URI for the pattern negation and it works.
<system.webServer>
<rewrite xdt:Transform="InsertIfMissing">
<rules>
<rule name="http to https" stopProcessing="true">
<match url=".*" />
<conditions>
<add input="{HTTPS}" pattern="^OFF$" />
<add input="{HTTP_HOST}" pattern="^backoffice\.WEBSITENAME\.com$" negate="true" />
</conditions>
<action type="Redirect" url="https://www.WEBSITENAME.com/{R:0}" />
</rule>
</rules>
</rewrite>
</system.webServer>