ADFS custom rule - Truncate Email Domain, anything after @ sign, custom rule

Trying this but it still sends (in SAML assertion) complete email address. Can someone help what I am missing here? Any help in this regard would be greatly appreciated. The goal here is that if the email address for an individual is John.Doe@contoso.com, we only want to send John.Doe, stripping @contoso.com completely out. 

Thanks in advance.

c:[Type == "http://schemas.microsoft.com/ws/2008/06/identity/claims/windowsaccountname", Issuer == "AD AUTHORITY"]
 => issue(store = "Active Directory", types = ("http://schemas.xmlsoap.org/ws/2005/05/identity/claims/nameidentifier"), query = ";mail;{0}", param = RegexReplace(c.Value, "$@", ""), param = c.Value);

Thanks, Feddie