I am trying to write a simple grammer for PEG.js that would match something like this:

some text;
arbitrary other text that can also have µnicode; different expression;
let's escape the \; semicolon, and \not recognized escapes are not a problem;
possibly last expression not ending with semicolon

So basically these are some texts separated by semicolons. My simplified grammer looks like this:

start
= flow:Flow

Flow
= instructions:Instruction*

Instruction
= Empty / Text

TextCharacter
= "\\;" /
.

Text
= text:TextCharacter+ ';' {return text.join('')}

Empty
= Semicolon

Semicolon "semicolon"
= ';'

The problem is that if I put anything other than a semicolon in the input, I get:

SyntaxError: Expected ";", "\\;" or any character but end of input found.

How to solve this? I've read that PEG.js is unable to match end of input.

I am trying to write a simple grammer for PEG.js that would match something like this:

some text;
arbitrary other text that can also have µnicode; different expression;
let's escape the \; semicolon, and \not recognized escapes are not a problem;
possibly last expression not ending with semicolon

So basically these are some texts separated by semicolons. My simplified grammer looks like this:

start
= flow:Flow

Flow
= instructions:Instruction*

Instruction
= Empty / Text

TextCharacter
= "\\;" /
.

Text
= text:TextCharacter+ ';' {return text.join('')}

Empty
= Semicolon

Semicolon "semicolon"
= ';'

The problem is that if I put anything other than a semicolon in the input, I get:

SyntaxError: Expected ";", "\\;" or any character but end of input found.

How to solve this? I've read that PEG.js is unable to match end of input.

• javascript
• parsing
• peg
• pegjs

• 5 FWIW, you can match end of input with !. – ebohlman Commented Oct 6, 2012 at 3:32

1 Answer 1

You have (at least) 2 problems:

Your TextCharacter should not match any character (the .). It should match any character except a backslash and semi-colon, or it should match an escaped character:

TextCharacter
 = [^\\;]
 / "\\" .

The second problem is that your grammar mandates your input to end with a semi-colon (but your input does not end with a ;).

How about something like this instead:

start
 = instructions

instructions
 = instruction (";" instruction)* ";"?

instruction
 = chars:char+ {return chars.join("").trim();}

char
 = [^\\;]
 / "\\" c:. {return ""+c;}

which would parse your input as follows:

[
   "some text",
   [
      [
         ";",
         "arbitrary other text that can also have µnicode"
      ],
      [
         ";",
         "different expression"
      ],
      [
         ";",
         "let's escape the ; semicolon, and not recognized escapes are not a problem"
      ],
      [
         ";",
         "possibly last expression not ending with semicolon"
      ]
   ]
]

Note that the trailing semi-colon is optional now.