diff --git a/src/dotnet-ylt/Commands/AddCalculatorCommand.cs b/src/dotnet-ylt/Commands/AddCalculatorCommand.cs
index 6e18623..85288a9 100644
--- a/src/dotnet-ylt/Commands/AddCalculatorCommand.cs
+++ b/src/dotnet-ylt/Commands/AddCalculatorCommand.cs
@@ -34,6 +34,10 @@ public async Task<int> OnExecute()
             if (Namespace != null)
             {
                 Namespace = Namespace.Replace('/', '.').Replace('\\', '.');
+                if (!ValidateNamespace(Namespace))
+                {
+                    return 1;
+                }
             }
 
             XDocument xProj = XDocument.Load(projectFile);
diff --git a/src/dotnet-ylt/Commands/AddParserCommand.cs b/src/dotnet-ylt/Commands/AddParserCommand.cs
index 83dff36..e8417af 100644
--- a/src/dotnet-ylt/Commands/AddParserCommand.cs
+++ b/src/dotnet-ylt/Commands/AddParserCommand.cs
@@ -35,6 +35,10 @@ public async Task<int> OnExecute()
             if (Namespace != null)
             {
                 Namespace = Namespace.Replace('/', '.').Replace('\\', '.');
+                if (!ValidateNamespace(Namespace))
+                {
+                    return 1;
+                }
             }
 
             XDocument xProj = XDocument.Load(projectFile);
diff --git a/src/dotnet-ylt/Commands/CommandBase.cs b/src/dotnet-ylt/Commands/CommandBase.cs
index 0c1ebca..d1f95aa 100644
--- a/src/dotnet-ylt/Commands/CommandBase.cs
+++ b/src/dotnet-ylt/Commands/CommandBase.cs
@@ -3,6 +3,7 @@
 using System.Diagnostics;
 using System.IO;
 using System.Linq;
+using System.Text.RegularExpressions;
 using System.Xml.Linq;
 using YaccLexTools.Utilities;
 
@@ -13,6 +14,21 @@ namespace DotnetYaccLexTools.Commands
     public abstract class CommandBase
     {
 
+        protected readonly Regex _namespaceRegex = new Regex(@"^[a-zA-Z_][a-zA-Z\d_]*(\.[a-zA-Z_][a-zA-Z\d_]*)*$", RegexOptions.Compiled);
+
+
+        protected bool ValidateNamespace(string @namespace)
+        {
+            if (!_namespaceRegex.IsMatch(@namespace))
+            {
+                Console.Error.WriteLine("Namespace is not valid.");
+                return false;
+            }
+
+            return true;
+        }
+
+
         protected string? GetProjectFile()
         {
             var projectFiles = Directory.GetFiles(".", "*.csproj");