Return key pressed inside a text box must maintain no of TABs in the previous line

The attached code changes the behavior of the TAB key inside a text box, so that pressing the TAB key produces an indent, as opposed to changing focus. (I have added the following attribute to the textbox:
onkeydown="return interceptTabs(event, this);")

Now, when I hit the return key in a text box, the indent should be set to the same number of TABs as to what is now in the previous line.

Please tell me how this can be done.

function insertAtCursor(myField, myValue) {
        //IE support
        if (document.selection) {
            myField.focus();
            sel = document.selection.createRange();
            sel.text = myValue;
        }

        //MOZILLA/NETSCAPE support
        else if (myField.selectionStart || myField.selectionStart == '0') {
            var startPos = myField.selectionStart;
            var endPos = myField.selectionEnd;
            restoreTop = myField.scrollTop;

            myField.value = myField.value.substring(0, startPos) + myValue + myField.value.substring(endPos, myField.value.length);

            myField.selectionStart = startPos + myValue.length;
            myField.selectionEnd = startPos + myValue.length;

            if (restoreTop > 0) {
                myField.scrollTop = restoreTop;
            }
        }
        else {
            myField.value += myValue;
        }
    }

    function interceptTabs(evt, control) {
        key = evt.keyCode ? evt.keyCode : evt.which ? evt.which : evt.charCode;
        if (key == 9) {
            insertAtCursor(control, '\t');
            return false;
        }
        else {
            return key;
        }
    }

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