// kein markierter Text
b.value = (b.value.indexOf('+') > -1 ? '-' : '+') + tag;
return (b.value.indexOf('+') > -1) ? '[/' + tag + ']' : '[' + tag +  ']';

// kein markierter Text
b.value = (b.value.indexOf('-') > -1 ? '+' : '-') + tag;
return (b.value.indexOf('-') > -1) ? '[' + tag + ']' : '[/' + tag +  ']';

Code

function getSelection() {
		if(typeof document.selection != 'undefined') {
			var range = document.selection.createRange();
			return range.text;
		} else if(typeof textarea.selectionStart != 'undefined') {
			var start = textarea.selectionStart;
			var end = textarea.selectionEnd;
			return textarea.value.substring(start, end);
		} else {
			return window.prompt('Text:');
		}
	}