Formatting breaks early on CSS selectors that contain underscores

```
What steps will reproduce the problem?
1. create a CSS selector such as div#main_content or h2.sub_heading

What is the expected output? What do you see instead?
highlighting for class or id only matches up to the underscore, leaves the
underscore as plain text node, and matches the rest of the class or id as a
keyword

What version of the product are you using? On what operating system?
Using r52 straight from SVN, Windows 7, PHP 5.2.9

Please provide any additional information below.

```

Original issue reported on code.google.com by `AshleyJS...@gmail.com` on 2 May 2012 at 1:00

Start by reproducing highlighting for the selectors div#main_content and h2.sub_heading, then trace the CSS selector highlighting entry point. Confirm that the complete class or id, including its underscore, is highlighted consistently rather than splitting at the underscore and treating the remainder as a keyword.

Written by the indexing model from the issue text.