In addition to @timl's Notify function, if you want to actually put a message on your form, you can add a label with your message and set the Visible property to:
Or even change the color of the textbox, by changing the Fill property of the DataCardValue5 to:
If(!IsMatch(DataCardValue5.Text, Match.Email), Red, White) (or whatever colors you want.)
Hope this adds a little help.
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