python regular expression to split paragraphs

the_str = 'paragraph1nnparagraph2'
# splitting should yield ['paragraph1', 'paragraph2']

the_str = 'p1ntnp2tntstill p2t   n     ntp3'
# should yield ['p1', 'p2tntstill p2', 'p3']

the_str = 'p1nnntp2'
# should yield ['p1', 'ntp2']

from itertools import groupby

def paragraph(lines) :
    for group_separator, line_iteration in groupby(lines.splitlines(True), key = str.isspace) :
        if not group_separator :
            yield ''.join(line_iteration)

for p in paragraph('p1ntnp2tntstill p2t   n     ntp'):
    print repr(p)

'p1n'
'p2tntstill p2t   n'
'tp3'

s*?ns*?ns*?