diff --git a/src/engines/json/joinPath.js b/src/engines/json/joinPath.js
index 4a0a10f..887fae2 100644
--- a/src/engines/json/joinPath.js
+++ b/src/engines/json/joinPath.js
@@ -12,9 +12,9 @@ Validation.prototype.joinPath = function(path, property) {
   // Converts the ‘property’ to a string
   property = property + '';
 
-  if (property.match(/^[a-zA-Z][a-zA-Z0-9]*$/)) {
+  if (property.match(/^[a-zA-Z_][a-zA-Z0-9_]*$/)) {
     return (path) ? (path + '.' + property) : property;
-  } else if (property.match(/\d+/)) {
+  } else if (property.match(/^\d+$/)) {
     return path + '[' + property + ']';
   } else  {
     return path + '["' + property + '"]';