String function

Is there a function that return how many occurrence of a substring are in a string?
Or how can i do by myself?
Example:

    ST := 'THIS IS A STRING';

    N := XFunction ( ST, 'S' );    // return 3
    N := XFunction ( ST, 'IS' );   // return 2
    N := XFunction ( ST, 'X' );   // return 0