diff --git a/BFR/Operations/Number.cs b/BFR/Operations/Number.cs
index d51f34b..e24b98b 100644
--- a/BFR/Operations/Number.cs
+++ b/BFR/Operations/Number.cs
@@ -1,10 +1,12 @@
-using System.Xml.Serialization;
+using System.Linq;
+using System.Xml.Serialization;
 using System.Collections.Generic;
 
 using Avalonia;
 
 using BFR.Helpers;
 using BFR.DataModels;
+using System;
 
 namespace BFR.Operations
 {
@@ -36,6 +38,16 @@ namespace BFR.Operations
                 ? (StartIndex + Increment * (files.Count - 1)).ToString().Length
                 : Padding;
 
+            // Fail conditions: Out of bounds insert
+            var shortestLength = files.Min(x => FullName ? x.FullName.Length : x.Name.Length);
+            if (Mode == NumberMode.Insert && InsertPosition > shortestLength) throw new OperationException($"First N can't be greater than the shortest file name length ({shortestLength}).");
+
+            // Regex Failure
+            if (Mode == NumberMode.Replace && UseRegex)
+                try { "".Replace(Pattern, "", true, false); }
+                catch (Exception) { throw new OperationException("Invalid Regex Pattern."); }
+
+            // Apply operation
             foreach (var file in files)
             {
                 var newName = FullName ? file.FullName : file.Name;