Convert a string to a numeric value
string - a literal string or string function result that can be parsed as an integer or decimal number
nth - the instance of a number to find within the string (optional)
If nth is not specified then this function only looks for a number at the beginning of the string.
If nth is 1 then the first number found within the string (or at the beginning) is returned.
If nth is 2 the second number found is returned, and so on.
A return value of 0 means either the number found was 0 or no number was found.