WPF validation rule preventing decimal entry in textbox?

Question

I have a WPF textbox defined in XAML like this:

        
                      
        
        

          

            
            

              
              

            
    


                                 
              
            
                          
    

        
         

                验证码
                

                  
                
                

                   看不清?
                
              
                                  

                    
   
                 
             
              
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