Think about your code, what happen in your code when the textbox is empty (length=0)
bool b = qr.HasArabicCharacters(textBox1.Text);
if (b == false)
textBox1.Text = textBox1.Text.Remove(textBox1.Text.Length - 1);
When you don't understand what your code is doing or why it does what it does, the answer is debugger
Use the debugger to see what your code is doing. It allow you to execute lines 1 by 1 and to inspect variables as it execute, it is an incredible learning tool.
Debugger - Wikipedia, the free encyclopedia
Mastering Debugging in Visual Studio 2010 - A Beginner's Guide
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.