init backend

dotnet/AipsCore/Domain/Models/User/Validation/UsernameCharsetRule.cs

@@ -0,0 +1,26 @@
+using AipsCore.Domain.Common.Validation.Rules;
+using AipsCore.Utility.Text;
+
+namespace AipsCore.Domain.Models.User.Validation;
+
+public class UsernameCharsetRule : CharsetRule
+{
+    public UsernameCharsetRule(string stringValue) 
+        : base(stringValue, GetUsernameCharset())
+    {
+    }
+
+    private static char[] GetUsernameCharset()
+    {
+        var alphanumericCharset = Charset.GetAlphanumericCharset();
+
+        char[] usernameCharset = [..alphanumericCharset, '_'];
+
+        return usernameCharset;
+    }
+
+    protected override string ErrorCode => "username_charset";
+
+    protected override string ErrorMessage =>
+        "Username contains invalid characters, only alphanumeric characters and '_' are allowed";
+}