Making a substring out of a line read from file

So I am trying to read through a .txt file and find all instances of html tags, push opening tags to a stack, and then pop it when I find a closing tag. Right now I am getting String out of bounds exception for the following line:

    if(scan.next().startsWith("</", 1))
            {
                toCompare = scan.next().substring(scan.next().indexOf('<'+2), scan.next().indexOf('>'));
                tempString = htmlTag.pop();
                if(!tempString.equals(toCompare))
                {
                    isBalanced = false;
                }
            }
            else if(scan.next().startsWith("<"))
            {
                tempString = scan.next().substring(scan.next().indexOf('<'+1), scan.next().indexOf('>'));
                htmlTag.push(tempString);
            }

It is telling me that the index of the last letter is -1. The problem I can think of is that all of the scan.next() calls are moving onto the next string. If this is the case, do I need to just write

    toCompare = scan.next()

and then so my comparisons?