A compact set of PEG parser combinators in ES6.

peg.js

// PEG parser combinators
const lookup = x => typeof x == 'string' ? eval(x) : x
const match  = (x, y, z) => x ? { c:y, v:z } : { e: true }
const eof    = s => match(s.length, 0, '')
const char   = s => match(s.length, 1, s[0])
const not    = g => s => match(g(s).e, 0, '')
const has    = g => s => match(!g(s).e, 0, '')
const oneof  = t => s => match(t.indexOf(s[0]) >= 0, 1, s[0])
const lit    = t => s => match(!s.indexOf(t), t.length, t)
const option = g => choose(g, lit(''))
const prod   = (g, f) => s => (r => r.e ? r : { c:r.c, v:f(r.v) })(g(s))
const choose = (...a) => s => a.reduce((x, y) => x.e ? y(s) : x, { e: true })
const seq    = (...a) => s => a.reduce(function(x, y) {
		var r = x.e ? x : lookup(y)(s.slice(x.c))
		return r.e ? r : { c:x.c+r.c, v:x.v.concat([r.v]) }
	}, { c:0, v:[] })

// usage example:
const noun   = oneof('abcdefghijklmnopqrstuvwxyz')
const xcolon = x => prod(seq(oneof(x), option(lit(':'))), x => x.join(''))
const verb   = xcolon('+-*%!&|<>=~,^#_$?@.')
const adverb = xcolon('\'\\/')
const term   = choose(prod(seq(lit('('), 'kexpr', lit(')')), x => x[1]), noun)
const kexpr  = choose(
	prod(seq(term, adverb, 'kexpr'), x => [x[1], x[0], x[2]]),
	prod(seq(term, verb, 'kexpr'),   x => [x[1], x[0], x[2]]),
	seq(adverb, 'kexpr'),
	seq(verb, 'kexpr'),
	term
)

const test = x => console.log(JSON.stringify(x.v))
test(kexpr('+b'))
test(kexpr('a+b'))
test(kexpr('(a+b)*c'))
test(kexpr('a+b*c'))
test(kexpr('f/b+c'))
test(kexpr('a+f/b+c'))
test(kexpr('(a+f)/b+c'))
test(kexpr('a+f/*:\'z-w'))