Explanation
Data validation rules are triggered when a user adds or changes a cell value.
If found, FIND will return a numeric position (i.e.
2, 4, 5, etc.)
to represent the starting point of the text in the cell.
If the text is not found, FIND will return an error.
The result returned by the FIND function is then evaluated by the ISNUMBER function.
For any numeric result returned by FIND, ISNUMBER will return TRUE and validation will succeed.
When the text is not found, FIND returns a #VALUE error.
