Treetop grammar problem

Ruby
1

I have devised this grammar for roman numerals for treetop.

grammar RomanNumeral

 rule roman_numeral
     ('MMM' / 'MM' / 'M')?
         (('C' [DM]) / ('D'? ('CCC' / 'CC' / 'C')?))?
         (('X' [LC]) / ('L'? ('XXX' / 'XX' / 'X')?))?
         (('I' [VX]) / ('V'? ('III' / 'II' / 'I')?))?
 end

end

It recognizes numbers in the range 1-3999, and rejects all bad input
as far as I can tell. T

But the empty string passes!

The problem is that any part is optional, but not ALL parts. At a
minimum, there must be one digit. Duh.

Treetop is ingenious, and easy to use, except when it’s hard.

Does anyone have a suggestion? I’m new at this, so I beg your patience.

Also, is there a better way of translating “M{0,3}” than “(‘MMM’ /
‘MM’ / ‘M’)?” ?

Bob Schaaf

2

Robert S. wrote:

But the empty string passes!

It’s not a general solution, but you can require that something
follows, using &.

rule roman_numeral
&. ( stuff )
end

There’s no better way to write M{0,3}

Clifford H…

3

Of course! Lookahead non-gobbling. A lesson learned.

Thanks mightily,

Bob Schaaf