UPC is a FORTRAN90 library which can compute the check digit associated with a uniform product code (UPC), or it can report whether a 12-digit UPC is actually valid.
The computer code and data files made available on this web page are distributed under the GNU LGPL license.
UPC is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.
ISBN, a FORTRAN90 library which can determine the check digit for an International Standard Book Number or report whether a given ISBN is valid.
LUHN, a FORTRAN90 program which computes the Luhn check digit for a string, and can validate a string.
You can go up one level to the FORTRAN90 source codes.