Data validation rules are triggered when a user adds or changes a cell value. Cell references in data validation formulas are relative to the upper left cell in the range selected when the validation rule is defined.
The ISTEXT function returns TRUE when a value is text and FALSE if not. As a result, all text input will pass validation, but numbers and formulas will fail validation.
Note: Excel has several built-in data validation rules for numbers. This page explains how to create a your own validation rule based on a custom formula. To allow only numbers in a cell, you can use data validation with a custom formula based on...
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