1. Select the range A2:A7.
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2. On the Data tab, click Data Validation.
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3. In the Allow list, click Custom.
4. In the Formula box, enter the formula shown below and click OK.
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Explanation: this AND function has three arguments. LEFT(A2)="C" forces the user to start with the letter C. LEN(A2)=4 forces the user to enter a string with a length of 4 characters. ISNUMBER(VALUE(RIGHT(A2,3))) forces the user to end with 3 numbers. RIGHT(A2,3) extracts the 3 rightmost characters from the text string. The VALUE function converts this text string to a number. ISNUMBER checks whether this value is a number. The AND Function returns TRUE if all conditions are true. Because we selected the range A2:A7 before we clicked on Data Validation, Excel automatically copies the formula to the other cells.
5. To check this, select cell A3 and click Data Validation.
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As you can see, this cell also contains the correct formula.
6. Enter an incorrect product code.
Result. Excel shows an error alert.
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Note: to enter an input message and error alert message, go to the Input Message and Error Alert tab.
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