Regexp literal backslash getting stripped

[Neurogami]

Run rdoc over enum.c

The comments in the source code have this:

 e = lines.slice_after(/(?<!\\)\n\z/)

but the resulting rdoc output has this:

e = lines.slice_after(/(?<!\)\n\z/)

Note the missing backslash after the exclamation point.

[aycabta]

This is fixed by #471.