MAGETWO-31764: Function _underscore didn't work with keys like SKeyName ('s_key_name')

Author: Pechenushko

dev/tests/unit/testsuite/Magento/Framework/ObjectTest.php

@@ -453,10 +453,11 @@ public function underscoreDataProvider()
         return [
             'Test 1' => ['Stone1Color', 'stone_1_color'],
             'Test 2' => ['StoneColor', 'stone_color'],
-            'Test 3' => ['StoneToXML', 'stone_to_xml'],
+            'Test 3' => ['StoneToXml', 'stone_to_xml'],
             'Test 4' => ['1StoneColor', '1_stone_color'],
             'Test 5' => ['getCcLast4', 'get_cc_last_4'],
-            'Test 6' => ['99Bottles', '99_bottles']
+            'Test 6' => ['99Bottles', '99_bottles'],
+            'Test 7' => ['XApiLogin', 'x_api_login']
         ];
     }
 }

lib/internal/Magento/Framework/Object.php

@@ -544,7 +544,7 @@ protected function _underscore($name)
         if (isset(self::$_underscoreCache[$name])) {
             return self::$_underscoreCache[$name];
         }
-        $result = strtolower(trim(preg_replace('/([A-Z]+|[0-9]+)/', "_$1", $name), '_'));
+        $result = strtolower(trim(preg_replace('/([A-Z]|[0-9]+)/', "_$1", $name), '_'));
         self::$_underscoreCache[$name] = $result;
         return $result;
     }