diff --git a/Interfaces/ZUGFeRDInterface/FileGroups.vb b/Interfaces/ZUGFeRDInterface/FileGroups.vb
index 63fbae79..53c07dc0 100644
--- a/Interfaces/ZUGFeRDInterface/FileGroups.vb
+++ b/Interfaces/ZUGFeRDInterface/FileGroups.vb
@@ -71,7 +71,7 @@ Public Class FileGroups
         ' See also: https://stackoverflow.com/questions/3968500/regex-to-validate-a-message-id-as-per-rfc2822
         'Dim oRegex = "(((([a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+(\.[a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+)*)|(""(([\x01-\x08\x0B\x0C\x0E-\x1F\x7F]|[\x21\x23-\x5B\x5D-\x7E])|(\\[\x01-\x09\x0B\x0C\x0E-\x7F]))*""))@(([a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+(\.[a-zA-Z0-9!#$%&'*+/=?^_`{|}~-]+)*)|(\[(([\x01-\x08\x0B\x0C\x0E-\x1F\x7F]|[\x21-\x5A\x5E-\x7E])|(\\[\x01-\x09\x0B\x0C\x0E-\x7F]))*\]))))~.+"
 
-        Dim oRegex = "[A-Z0-9]+~ATTM\d+\..*"
+        Dim oRegex = "([A-Z0-9]+)~ATTM\d+\..*"
         Dim oMatch = Regex.Match(Filename, oRegex, RegexOptions.IgnoreCase)
 
         If oMatch.Success Then