Difference between Peg.js and Regex

Question

I'd like to use Peg.js to parse and validate something I've been doing with Regular Expressions and am struggling with the syntax.

My Peg.js program is :

start = (var / other) cr 
d:var {return d.join('')}

var = [^(?=.{1,32}$)([a-zA-Z_][a-zA-Z0-9_]*)$] {return "OK"}
other = . {return "NOT OK"}
cr = "\n"

And I am testing it with the following text:

test1
no space
1var
_temp
ReallyLongNameisInvalidandLongerthan32
IncorrectChars!!asdf
_
underscore_is_fine_

I expect the results YES, NO, NO, YES, NO, NO, YES, YES, but am going round in circles with the regular expression errors that would normally work fine.

Ahh, sorry, I should say I was testing in https://pegjs.org/online, the error I get is :Line 4, column 44: Expected "!", "$", "&", "(", ".", "/", "/*", "//", ";", character class, code block, comment, end of line, identifier, literal, or whitespace but ")" found. — user5072412, Sep 27 '18 at 09:27

Josh Voigts · Accepted Answer · 2018-09-27T15:04:25.250

When a peg rule fails to match, it will try to match the next rule until it has run out of rules. Try something along these lines:

start = line+

line = d:(var / other)
{
    return d;
}

var = v:$([a-zA-Z_][a-zA-Z0-9_]*) cr
{
    if (v.length > 32) {
        return "no";
    } else {
        return "yes";
    }
}

other = [^\n]+ cr
{
    return "no";
}

cr = "\n"

Output:

[
   "yes",
   "no",
   "no",
   "yes",
   "no",
   "no",
   "yes",
   "yes"
]

Difference between Peg.js and Regex

1 Answers1