How to lock a cell without protecting the worksheet

Another idea:

You could select C4:C999, choose Data Validation, Text Length, minimum 22, max 26.

Your formula string would have a min of 22 min characters long, and a max of 26 with the row reference for 999.

The only way they could mess it up is typing something silly like MondayMondayMonday (24 characters long)

Excellent and elegantly simple solution without having to mess with macro.