PEG

#####################
# Parsing framework #
#####################

lue = lambda c: lambda inp: inp[1:] if inp and inp[0] in c else None
tai = lambda va,vb: lambda inp: va(inp) or vb(inp)
ja  = lambda va,vb: lambda inp: (lambda x: x and vb(x))(va(inp))

class P(object):
    def __init__(self, c, name=""):
        self.test = c
        self.desc = name
    def __and__(self, o):
        n = P(self.test, self.desc+"&"+o.desc)
        n.test = ja(n.test, o.test)
        return n
    def __or__(self, o):
        n = P(self.test, '('+self.desc+"|"+o.desc+')')
        n.test = tai(n.test, o.test)
        return n
    def __call__(self, inp):
        print self.desc
        return self.test(inp)

###############################
# Language grammar definition #
###############################

cra    = lambda a,b: set(chr(i) for i in range(ord(a), ord(b)+1))
letter = cra('a','z')|cra('A','Z')|set('_')
digit  = cra('0','9')

term   = lambda a: P(lue(a), '"'+str(a)[:1]+'"')
word   = term(letter) & P(lambda i: word(i)) | term(letter)
digits = term(digit) & P(lambda i: digits(i)) | term(digit)
number = term('-') & digits | digits
ws     = term(' ') & P(lambda i: ws(i)) | term(' ')
code_  = word | number | ws | P(lambda i: block(i))
code   = code_ & P(lambda i: code(i)) | code_
block  = term('[') & code & term(']') | (term('[') & term(']'))

###########
# Testing #
###########

print word("abSDFf_kj%")
print code("-034 num %")
print code("034 kd[dfd [123 dflkj -340] [] 3498]%")