> So clearly they're allowed, in 1.2_3 the underscore is simply 
> ignored and
> the parser should undestand 1.23. If I fancy to write my own 
> YAML by hand
> and make it easily readable (which is kind of the original 
> purpose) I could
> write 100_000_000.03 which would be a nice looking float. Hence the
> ambiguity with "1.2_3"

Hmmmm... I just noticed that the specification
(http://yaml.org/type/float.html) isn't just unclear, it's inconsistent.

The examples listed include:

exponentioal: 685.230_15e+03

Notwithstanding the spelling of exponential, that example is not
consistent with the regular expression (which allows underscores only if
they preceed the decimal point -
[-+]?([0-9][0-9_]*)?\.[0-9.]*([eE][-+][0-9]+)?).

Dan.