Thank you for contacting us, Diego.
The RadMaskedEditBox control does not support the "Regex" mask type. I will put your feature request in our TO DO list and we will consider implementing it in one of the upcoming releases. Unfortunately, I am not able to give you an exact time-frame when the feature will be available.
I think that you can simulate the input by subscribing to the TextChanged event of the RadTextBox and then use the Regex.Match method to test whether the textbox's text matches with the required regex expression. If the text box's string is invalid you can put an error provider component which will pop up an error message and will notify you that the entered string value is invalid.
I hope this helps. If you have more questions, feel free to write us.
the Telerik team
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