A Lisp(ish) S-Expression parser in ES6, supporting symbols, signed ints, lists, and line comments.

lisparse.js

// S-Expression Parser

const token = /^\s*(?:;[^\n]*\n)*\s*(?:(-?\d+)|([a-z\-!?]+|[+\-*\/<>=()]))/

function parse(text) {
	const term = _ => {
		const x = token.exec(text)
		if (!x) throw 'Reached the end of input while parsing!'
		text = text.slice(x[0].length)
		const here = x[1] ? +x[1] : x[2]
		if (here != '(') return here
		let ret = [], next
		while (')' != (next = term())) ret.push(next)
		return ret
	}
	return term()
}

// tests

function test(x, y) {
	const a = JSON.stringify(x), b = JSON.stringify(y)
	if (a != b) console.log(`got:      ${a}\nexpected: ${b}`)
}

test(parse('(first (list 1 (+ 2 3) 9))'),     ['first',['list',1,['+',2,3],9]])
test(parse('(-234 ; comment\ndashed-atom!)'), [-234,'dashed-atom!'])
test(parse('(3 (4 (5 6)) (7 8))'),            [3,[4,[5,6]],[7,8]])
test(parse('bare-atom?'),                     'bare-atom?')