eager.py

from pyparsing import *


def match(text, matcher):
    result = matcher.parseString(text)
    return result


if __name__ == "__main__":
    element = Word(alphas)
    matcher = ZeroOrMore(element + FollowedBy(element)) + Group(element)

    print match("a b c d e f", matcher)     # ['a', 'b', 'c', 'd', 'e', ['f']]
    print match("a b c d e", matcher)       # ['a', 'b', 'c', 'd', ['e']]