Using Cookie as RewriteRule variable in Apache ReverseProxy

If have a cookie named myServerName, and want to check for its availability and use its content for building a URL

Code:
RewriteCond %{REQUEST_URI} ^/
RewriteCond %{HTTP_COOKIE} ^.*myServerName=([^;]+)
RewriteRule /(.*) http://%1.test.com.au/$1 [P]

Re Write to get the content from AWS S3 Bucket

RewriteEngine On
RewriteCond %{HTTP_COOKIE} ^.*mycookie.*$ [NC] 
RequestHeader set Host 's3-ap-southeast-2.amazonaws.com'
RewriteRule (.*)/files/(.*) http://s3-ap-southeast-2.amazonaws.com/mybucket/$2 [P]
ProxyPassReverse /files/ http://s3-ap-southeast-2.amazonaws.com/mybucket/

#If no Cookies Found
RewriteRule (.*)/files/(.*) http://s3-ap-southeast-2.amazonaws.com/mybucket//error_url/error.html [P]
ProxyPassReverse /files/ http://s3-ap-southeast-2.amazonaws.com/mybucket/