[TxMt] Dynamic includes in language grammars

I'm updating my Markdown bundle to better handle GitHub-style fenced code blocks like so:

```ruby foo = { bar: true } ```

The grammar is simple enough for one language:

ruby_block = { begin = '^```ruby.*$'; end = '^```.*$'; patterns = ( { include = 'source.ruby'; } ); };

However, I'd like to support any language. I tried the following, but it doesn't appear to work (and I'm not sure why):

code_block = { begin = '^```(\w+).*$'; end = '^```.*$'; patterns = ( { include = 'source.\1'; } ); };

Is something similar possible / supported?

Thanks!

-Tyson