# Data validation must begin with

Data validation rules are triggered when a user adds or changes a cell value.

In this formula, the LEFT function is used to extract the first 3 characters of the input in C5.

Next, the EXACT function is used to compare the extracted text to the text hard-coded into the formula, "MX-". EXACT performs a case-sensitive comparison. If the two text strings match exactly, EXACT returns TRUE and validation will pass. If the match fails, EXACT will return FALSE, and input will fail validation.

### Non case-sensitive test with COUNTIF

If you don't need a case-sensitive test, you can use a simpler formula based on the COUNTIF function with a wildcard:

=COUNTIF(C5,"MX-*")

The asterisk (*) is a wildcard that matches one or more characters.

*Note: Cell references in data validation formulas are relative to the upper left cell in the range selected when the validation rule is defined, in this case C5.*

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