Re: [jflex-users] Lookahead expression must have match with at least length 1

[Steve R. <sa...@od...>]

Hi Ken,

Ken Williams wrote:
> ---------------------------
> ROMANPAT="m"*
>          ("d"?"c"{0,3}|"c"("d"|"m"))
>          ("l"?"x"{0,3}|"x"("l"|"c"))
>          ("v"?"i"{0,3}|"i"("v"|"x"))
> ...
> {ROMANPAT} / [^a-z]    { return newTok("ROMAN"); }
> {ROMANPAT} $        { return newTok("ROMAN"); }
> ---------------------------
> ---------------------------
> Lookahead expression must have match with at least length 1.
> {ROMANPAT} / [^a-z]    { return newTok("ROMAN"); }
> ---------------------------
> 
> Does the error refer to the lookahead (what’s after the slash), or the 
> main regex (before the slash)?  IIUC the lookahead itself matches 1 
> character, so that shouldn’t be an issue.  The ROMANPAT regex can indeed 
> match a zero-length string, so perhaps that’s the problem.

Yes, the problem is the ROMANPAT macro.

> Is there some way to force this rule to have a non-zero-length match?

How about spelling out all of the alternatives such that at least one
character is mandatory? Something like (warning: untested):

ROMANPAT = "m"+ ("d"? "c"{0,3} |            "c" [dm])
                 ("l"? "x"{0,3} |            "x" [lc])
                 ("v"? "i"{0,3} |            "i" [vx])
          |      ("d"  "c"{0,3} | "c"{1,3} | "c" [dm])
                 ("l"? "x"{0,3} |            "x" [lc])
                 ("v"? "i"{0,3} |            "i" [vx])
          |      ("l"  "x"{0,3} | "x"{1,3} | "x" [lc])
                 ("v"? "i"{0,3} |            "i" [vx])
          |      ("v"  "i"{0,3} | "i"{1,3} | "i" [vx])

Steve

P.S.: Wikipedia says that the formulation you've specified is modern,
and that things like IIIII (5) and VV (10) and XIIII (14) were once used.