Reset prefix if no drawing info is found.

ExportDXF/DrawingInfo.cs

@@ -4,7 +4,7 @@ namespace ExportDXF
 {
     public class DrawingInfo
     {
-        private static Regex drawingFormatRegex = new Regex(@"(?<jobNo>[34]\d{3}(-\d+\w{1,2})?)\s?(?<dwgNo>[ABEP]\d+(-?(\d+|[A-Z]))?)", RegexOptions.IgnoreCase);
+        private static Regex drawingFormatRegex = new Regex(@"(?<jobNo>[34]\d{3}(-\d+\w{1,2})?)\s?(?<dwgNo>[ABEP]\d+(-?(\d+[A-Z]?))?)", RegexOptions.IgnoreCase);
 
         public string JobNo { get; set; }
 
@@ -14,7 +14,7 @@ namespace ExportDXF
 
         public override string ToString()
         {
-            return string.Format("{0} {1}", JobNo, DrawingNo);
+            return $"{JobNo} {DrawingNo}";
         }
 
         public override bool Equals(object obj)

ExportDXF/Forms/MainForm.cs

@@ -597,16 +597,20 @@ namespace ExportDXF.Forms
             var model = sldWorks.ActiveDoc as ModelDoc2;
             var isDrawing = model is DrawingDoc;
 
-            if (!isDrawing)
-                return;
-
+            if (isDrawing)
+            {
                 var drawingInfo = DrawingInfo.Parse(activeDocTitleBox.Text);
 
                 if (drawingInfo == null)
                     return;
 
-            prefixTextBox.Text = string.Format("{0} {1} PT", drawingInfo.JobNo, drawingInfo.DrawingNo);
+                prefixTextBox.Text = $"{drawingInfo.JobNo} {drawingInfo.DrawingNo} PT";
                 prefixTextBox.SelectionStart = prefixTextBox.Text.Length;
             }
+            else
+            {
+                prefixTextBox.Text = string.Empty;
+            }
+        }
     }
 }