[NG] BBCode überschreibt Text

Code:
function insertHyperlink() {

    var url = prompt("Please enter the URL of the website.", "http://");

    if(url) {

        // New/edited Code Start - only for Mozilla //

        var messageBox = document.input.message;

        if(is_mozilla && messageBox.selectionEnd) {

            var select_start = messageBox.selectionStart;

            var select_end = messageBox.selectionEnd;

            if(select_end <= 2) {

                select_end = messageBox.textLength;

            }

            var start = (messageBox.value).substring(0, select_start);

            var middle = (messageBox.value).substring(select_start, select_end);

            var end = (messageBox.value).substring(select_end, messageBox.textLength);

            if((messageBox.selectionEnd - messageBox.selectionStart > 0)) {

                middle = middle;

            }

            messageBox.value = start+middle+end;

        }

        if(middle == "" || normalmode == true) {

            var urltitle = prompt("If you wish to, you may also insert a title to be shown instead of the URL.", middle);

        } else {

            var urltitle = middle;

        }

        // New/edited Code Start - only for Mozilla //

        if(urltitle) {

            doInsert("[url="+url+"]"+urltitle+"[/url]", "", false);

        } else {

            doInsert("[url]"+url+"[/url]", "", false);

        }

    } else {

        alert("Error!\n\nYou did not enter a URL for the website. Please try again.");

    }

}