CODEWITHSUNDEEP
javascriptparsingpegpegjs
fadedbee
fadedbee

Reputation: 44827

How to transform a simple grammar into something which works in PEG.js (expected "a" but "a" found)

I've just started playing with PEG.js and have a problem with a grammar (vastly simplified for debugging):

start
  = presingle single
  / preplural plural

presingle
  = "a"
  / "b"

preplural
  = "b"
  / "c"

single
  = "d"
  / "e"

plural
  = "dd"
  / "ee"

I'm using https://pegjs.org/online

This grammar fails to parse bdd. 

Line 1, column 3: Expected "a" but "d" found.

Is this something which PEGs cannot do, or can I transform my grammar into something which will parse this?

P.S. If I try to parse the (erroneously advised?) bda I get the nonsensical error:

Line 1, column 3: Expected "a" but "a" found.

Upvotes: 1

Views: 641

Answers (2)

Paul Wagland
Paul Wagland

Reputation: 29179

The good news is that modern versions of pegjs give the error message: “Line 1, column 3: Expected end of input but "d" found.” when given the input bdd.

This is what you would expect, since it matches the single match first, and since the "b" and the "d" match, it assumes that it is good to go. Inverting the order, as suggested by @HBP forces it to match the plurals first, and only if they don't exist, does it try the single, which gives the result that you expect.

Upvotes: 1

HBP
HBP

Reputation: 16063

This grammar changes only the order of the clauses in start and works for bdd

start = 
   preplural plural /
    presingle single

presingle
  = "a"
  / "b"

preplural
  = "b"
  / "c"

single
  = "d"
  / "e"

plural
  = "dd"
  / "ee"

and for bda shows error Line 1, column 3: Expected "dd" or "ee" but "a" found.

Upvotes: 1

Related Questions