Using variable in pattern

Here is how I want to use "HTTP_SOMEVARIABLE" in pattern.

<outboundRules>
                <rule name="ReverseProxyOutboundRule1" preCondition="ResponseIsHtml1">
                    <match filterByTags="A, Form, Img" pattern="^http(s)?://{HTTP_SOMEVARIABLE}/(.*)" />
                    <action type="Rewrite" value="http{R:1}://reverseproxyaddress/{R:3}" />
                </rule>
                <preConditions>
                    <preCondition name="ResponseIsHtml1">
                        <add input="{RESPONSE_CONTENT_TYPE}" pattern="^text/html" />
                    </preCondition>
                </preConditions>
            </outboundRules>

Is it possible to use variable in pattern? if yes how?