My Textbox content get trimmed.

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C#

WPF

I have a TextBox Bound to a Double type variable.

<TextBox Text="{Binding Path=OldRegCost,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"  Grid.Row="0" Grid.Column="1" Height="25" Width="50" Margin="10,0,10,0"></TextBox>

In the code behind i binded the textbox to a Double type variable.

private Double? _OldRegCost;

public Double? OldRegCost
  {
     get { return _OldRegCost; }
     set
         {
            OldRegCost = value;
            RaisePropertyChanged("OldRegCost");
          }
  }

When the user try to enter a symbol (@,#,$,%,& etc) the TextBox automatically turns RED colour.
It also trims the entered special character.

Since am doing a separate validation using Regex in code behind:

1) Is there any way in code behind to know whether the text field turned RED (an invalid character entered)

2) or can i remove the RED effect and get the entire text (with invalid character) to code behind.

Please give me a solution