The "FilterFSA" method in Handler et al. 2016.
	Returns token position spans of valid ngrams.

def extract_ngram_filter(pos_seq, regex=SimpleNP, minlen=1, maxlen=8):
	"""The "FilterFSA" method in Handler et al. 2016.
	Returns token position spans of valid ngrams."""
	ss = coarse_tag_str(pos_seq)

	def gen():
		for s in xrange(len(ss)):
			for n in xrange(minlen, 1 + min(maxlen, len(ss) - s)):
				e = s + n
				substr = ss[s:e]
				if re.match(regex + "$", substr):
					yield (s, e)

	return list(gen())