I believe this custom formula will work:

=AND(CODE(UPPER(LEFT(F3,1)))=65,CODE(UPPER(LEFT(F 3,1)))<=90,ISNUMBER(--RIGHT(F3,3)),LEN(F3)=4)

Checks if first character is a letter(character code greater than 65 [A] and
less than 90 [Z]), checks if last 3 characters create a number, and if total
length of text is 4.
--
Best Regards,

Luke M
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"yclhk" wrote:

How to set a cell requested to input in the format of A123 (i.e. one letter
plus three digits) by data validation .