Orbeon Forms how to throw an alert

I'm using orbeon forms (PE 4.8) and I have created a output field which is counting the some other fields values and I want to show the alert/error message if the value of output field is less then 100.how can i show alert or error if value is not 100. Please help me I am struggling with this issue for last one week.

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Hi,

Also, with the upcoming 4.10, you'll be able to set the minimum length directly in the Control Settings, without having to type an XPath expression, and even have a character counter shown next to the field.

http://blog.orbeon.com/2015/06/how-new-form-builder-appearance.html

Alex