Better C++ regex

Based upon this comment: https://github.com/github/linguist/issues/1626#issuecomment-66371622
I've never seen `:blank:`. I can't get it to match anything in an `irb` console. What am I missing?

```ruby
>> /[:blank:]/.match(" ")
=> nil
>> /[:blank:]/.match("\t")
=> nil
>> /[:blank:]/.match("")
=> nil
```