Grammar for extracting special blocks in text

[pegguy]

Hello again,

my example is of course simplified. Given the following text I want to extracting all lines beginning with '#':
ab
#cd
ef
#gh

My grammar:
start = ( ( ! block . ) / block ) +

block = Result:( "#" identifier ) { return( Result ) }                                        // 1
// block = Result:( "#" identifier ) { return( "Block: " + Result ) }                         // 2
// block = Result:( "#" identifier ) { return( "Block: " + Result[0] + Result[1].join("") ) } // 3

identifier = [a-zA-Z_]+

The result:
[
  [
    [
      "",
      "a"
    ] 
  ]

  [
    [
      "",
      "b"
    ] 
  ]

  ...

  [
    "#",   // hint, not in output: Result[0]
    [
      "g", // hint, not in output: Result[1][0]
      "h"  // hint, not in output: Result[1][1]
    ] 
  ]
]

How can I avoid all the in my case unnecessary output?
When I use the commented line 2 of my code, I get
[
  ...
  "Block: #,g,h"
]
or with commented line 3
[
  ...
  "Block: #gh"
]

And that "Block ..." is the only part I need.

Any hints/suggestions?


Many thanks and greetings
pegguy

[Guilherme Vieira]

Hi,

You're getting unnecessary data because your start rule consumes unnecessary stuff but doesn't discard it, returning it all in an array.

The unnecessary data is in the (!block .) expression, so you can make it just return nothing instead of the matched data. Then, in the start rule action, you can filter out the undefined values in the array, like this:

start = data:( ( ! block . ) { return; } / block ) + { return data.filter(function (d) { return d !== undefined; }); }

block = Result:( "#" identifier ) { return( Result ) }

identifier = [a-zA-Z_]+

Results:

[
   [
      "#",
      [
         "a",
         "b"
      ]
   ],
   [
      "#",
      [
         "c",
         "b"
      ]
   ]
]

I hope this helps.

-- 

Atenciosamente / Sincerely,
Guilherme Prá Vieira

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[Guilherme Vieira]

Oh, and also, if you want the full strings after the hash character, you can do this:

start = data:( ( ! block . ) { return; } / block ) + { return data.filter(function (d) { return d !== undefined; }); }

block = ( "#" Result:identifier ) { return( Result ) }

identifier = c:[a-zA-Z_]+ { return c.join(''); }

[Zak Greant]

Also, in some cases like this, people may find this small function helpful – it recursively joins arrays that contain a mix of strings and arrays.

Array.prototype.rjoin = function( glue ){
glue = glue || ''; // join with an empty string by default

return this.map( function( item ){
return (item instanceof Array) ? item.rjoin( glue ) : item;
}).join( glue );
}



Cheers!
--zak