Simple parsing questions using PEG.js

Question

I'm trying to wrap my head around PEG by entering simple grammars into the PEG.js playground.

Example 1:

Input: "abcdef1234567ghijklmn8901opqrs"
Desired output: ["abcdef", "1234567", "ghijklmn", "8901", "opqrs"]
Actual output: ["abcdef", ["1234567", ["ghijklmn", ["8901", ["opqrs", ""]]]]]

This example pretty much works, but can I get PEG.js to not nest the resulting array to a million levels? I assume the trick is to use concat() instead of join() somewhere, but I can't find the spot.

start
  = Text

Text
  = Numbers Text
  / Characters Text
  / EOF

Numbers
  = numbers: [0-9]+ {return numbers.join("")}

Characters
  = text: [a-z]+ {return text.join("")}

EOF
  = !.

Example 2:

Same problem and code as Example 1, but change the Characters rule to the following, which I expected would produce the same result.

Characters
  = text: (!Numbers .)+ {return text.join("")}

The resulting output is:

[",a,b,c,d,e,f", ["1234567", [",g,h,i,j,k,l,m,n", ["8901", [",o,p,q,r,s", ""]]]]]

Why do I get all these empty matches?

Example 3:

Last question. This doesn't work at all. How can I make it work? And for bonus points, any pointers on efficiency? For example, should I avoid recursion if possible?

I'd also appreciate a link to a good PEG tutorial. I've read (http://www.codeproject.com/KB/recipes/grammar_support_1.aspx), but as you can see I need more help ...

Input: 'abcdefghijklmnop"qrstuvwxyz"abcdefg'
Desired output: ["abcdefghijklmnop", "qrstuvwxyz", "abcdefg"]
Actual output: "abcdefghijklmnop\"qrstuvwxyz\"abcdefg"

start
  = Words

Words
  = Quote
  / Text
  / EOF

Quote
  = quote: ('"' .* '"') Words {return quote.join("")}

Text
  = text: (!Quote . Words) {return text.join("")}

EOF
  = !.

Simple parsing questions using PEG.js

3 Answers