Hello J D,
Thanks for writing.
The purpose of RadSpinEditor is to restrict user input in range of two numeric values. However, this control does not provide information for which character does not pass validation and is rejected, which implies that the RadSpinEditor is not appropriate for your purposes. I can suggest trying to implement this functionality with a regular text box - if you succeed, it can be applied to the hosted text box into RadSpinEditor.
Another suggestion is to try the MaskedEdit control, which may fit your needs.
Do not hesitate to contact us if you have other questions.
All the best,
the Telerik team
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