Character Limit in RichText Box in Infopath 2010 with SharePoint 2010

How to limited character  enter in Rich Textbox in InfoPath 2010 with SharePoint 2010.

1. At first take one rich text box in Infopath 2010.

2. Add Rule in  Rich Textbox, then select "Validation". Rule type=Validation

3. Set a condition,

      The expression= string-length(.)>10

4.Insert screen tip: "Character Limit to 10"

5.Ok