@TXDIGIT[string]: Returns the number of characters in string that are hexadecimal digits (0-9 and A-F or a-f).
See also: @TALNUM, @TALPHA, @TASCII, @TCNTRL, @TDIGIT, @TLOWER, @TPRINT, @TPUNCT, @TSPACE, and @TUPPER.
Example:
echo %@txdigit[123abc]
6
echo %@txdigit[123 ttt]
3