Incorrect behaviour (Markdown 1.0b4)

[Jesper]

> I accidentally forgot to quote an inlined code segment 
> yesterday during Markdown use and observed that Markdown is 
> recognising underscored delimiters even without whitespace.
> 
> A bug I think ...
> 
> ---- Example input ----
> 
> ... recovery procedure is detailed in the 
> command_reference_guide.pdf, App. B
> 
> ---- Example output ----
> 
> <p>&#8230; recovery procedure is detailed in the 
> command<em>reference</em>guide.pdf, App. B</p>

A qualified guess at what's causing this, without even looking. 

John's probably checking for 'word breaks'; that is, characters that 'end'
the current word, so that this could match:
    *foo*. _foo_.
without whitespace afterwards. 

But I think it's now taking the _ themselves as word breaks; because noone
in their right mind would say:
    Foo_bar_baz.
is one word.

/Jesper