At the core, this formula uses the LEFT function to return text starting from the left. To work out how many characters should be returned, the formula uses this expression:
Here, total characters are calculated with the LEN function. From this number, the result of the following expression is subtracted:
With number of characters not specified, RIGHT will return the last character in the string. If this characters is a forward slash "/", the expression returns TRUE. If not, it returns FALSE.
Because TRUE and FALSE are coerced automatically to 1 and zero in math operations, we subtract zero from the result of LEN when the last characters is not "/" and we subtract 1 when the last characters is "/".
The result is provided to LEFT, which returns the final result.