Re: lpeg as a part of lua (was: An introduction to Lua)
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- Subject: Re: lpeg as a part of lua (was: An introduction to Lua)
- From: Sean Conner <sean@...>
- Date: 2017年10月22日 22:24:52 -0400
It was thus said that the Great Sean Conner once stated:
> >
> > Also, unless I misunderstood something or something has changed since I
> > last looked at it, LPeg cannot do anything requiring backtracking, so there
> > are things matchable by patterns but unmatchable by LPeg. Possible example
> > being "capture the three characters between a question mark and a dollar
> > sign, immediately before the dollar sign", for which the (untested) pattern
> > "%?.-([^$][^$][^$])%$" should match easily, but AFAICT, there is no
> > equivalent LPeg pattern since the only way is to match up to the $ and then
> > back up three characters.
>
> Yes it can:
>
> qd = lpeg.P"?" -- match the question mark
> * lpeg.P(1)^3 -- at least three characters, could be more
> * lpeg.Cmt( -- at match time
> lpeg,P"$", -- when we hit the dollar sign, call this
> function(subject,position)
> -- position is one past the dollar sign,
> -- so we return the three characters prior to the
> -- dollar sign (position-4 to position-2 of subject)
> return position,subject:sub(position-4,position-2)
> end
> )
I realized the code has a bug in that the three characters matched should
*not* match the dollar sign. Easiest fix is in the function:
qd = lpeg.P"?"
* lpeg.P(1)^3
* lpeg.Cmt(lpeg.P"$",function(subject,position)
local ret = subject:sub(position-4,position-2)
if not ret:find("$",1,true) then
return position,ret
end
end
)
OR, you could fix it:
no_dollar = lpeg.R("0円#","%255円") -- look ma! No $
qd = lpeg.P"?"
* (lpeg.P(1) - (no_dollar * no_dollar * no_dollar * lpeg.P"$"))^0
* lpeg.C(no_dollar * no_dollar * no_dollar)
* lpeg.P"$"
and get rid of the call to lpeg.Cmt(). Okay, this one:
lpeg.R() - define ranges of characters to match
lpeg.C() - return pattern matches as a capture
patt1 - patt2 - match pattern1 if pattern2 does not match
That should take care of
?$$$$123ドル$ - returns 123
?$$$$ - returns nil
?123$ - returns 123
?987654123$ - returns 123
?12$ - returns nil
?12$$ - returns nil
-spc (Think of 'patt1 - patt2' as look ahead ... )