databinding and commands

js/components/expression.js

@@ -4,7 +4,12 @@
     app.service('expression', {
         parse: function(string) {
             var matches = twoOperandsRegex.exec(string);
+            if(matches == null) {
+                return null;
+            }
+
             console.log(matches);
+
             return {
                 string:matches[0],
                 operand1: parseInt(matches[1], 10),