Returns the index of a substring in the specified string. If the substring does not occur within the string then -1 is returned. If you do not specify the index to start at the search begins at the start of the string otherwise it will begin from the position you specify.
indexOf(str, substr)
indexOf(str, substr, start)
[r: indexOf("this is a test", "is")]Returns 2.
[r: indexOf("this is a test", "is", 3)]Returns 5.
[r: indexOf("this is a test", "x")]Returns -1.