How to write a regex to remove the last parameter from a url

I am trying to create a URL rewrite rule for IIS to return the URL before a specific parameter.

Edited: I should have indicated that authProvider is always the last parameter.

Example:

http://localhost/WebAccess/Default.ashx?accessionNumber=009&authProvider=Bypass

I want to trim & authProvider = crawl from end of URL

I tried:

.*(?=&authProvider)

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <system.webServer>
        <rewrite>
            <rules>
                <rule name="Test" enabled="true">
                    <match url="(.*)&amp;authProvider.+" />
                    <action type="Rewrite" url="{R:0}" logRewrittenUrl="true" />
                </rule>
            </rules>
        </rewrite>

    </system.webServer>
</configuration>