Why is AliasMatch not working?

Question

Here is what we have:

 Alias /assets/ "/home/virtual/public_assets/"
 AliasMatch ^/~([a-zA-Z0-9]+)/assets/(.*)$ /home/virtual/public_assets/$2

 <Directory "/home/virtual/public_assets">
      AllowOverride All
 </Directory>

Here is the URI we are trying to match to:

 /~admin30/assets/js/tests.js

The Alias directive works for our live sites, however, the AliasMatch which is supposed to match user directories does not.