refactor(select): simplify the ngOptions regular expression

\w matches [a-zA-Z0-9_] and \d matches [0-9], using both in a character set is
simply redundant.

Closes #3903

src/ng/directive/select.js

@@ -126,8 +126,8 @@
 
 var ngOptionsDirective = valueFn({ terminal: true });
 var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
-                         //0000111110000000000022220000000000000000000000333300000000000000444444444444444440000000005555555555555555500000006666666666666666600000000000000007777000000000000000000088888
-  var NG_OPTIONS_REGEXP = /^\s*(.*?)(?:\s+as\s+(.*?))?(?:\s+group\s+by\s+(.*))?\s+for\s+(?:([\$\w][\$\w\d]*)|(?:\(\s*([\$\w][\$\w\d]*)\s*,\s*([\$\w][\$\w\d]*)\s*\)))\s+in\s+(.*?)(?:\s+track\s+by\s+(.*?))?$/,
+                         //0000111110000000000022220000000000000000000000333300000000000000444444444444444000000000555555555555555000000066666666666666600000000000000007777000000000000000000088888
+  var NG_OPTIONS_REGEXP = /^\s*(.*?)(?:\s+as\s+(.*?))?(?:\s+group\s+by\s+(.*))?\s+for\s+(?:([\$\w][\$\w]*)|(?:\(\s*([\$\w][\$\w]*)\s*,\s*([\$\w][\$\w]*)\s*\)))\s+in\s+(.*?)(?:\s+track\s+by\s+(.*?))?$/,
       nullModelCtrl = {$setViewValue: noop};
 
   return {