fix issue #50

In issue #50 pre-existing shortcodes weren't matching the regex, take the following:

```
[pullquote]4 score and 7...[/pullquote]
```

The shortcode tag was matched as `pullquote]4`.

I made the match for non white space and no closing square brackets. Also the test for `matches[2]` was always coming back true as you can't use the `!=` operator with undefined.