string1 = "this is a test string : that behaves as I expectn"
string2 = "this string does not behave as I expectn"

field = CharsNotIn(":n")
line = field + ZeroOrMore(Literal(":") + field) + LineEnd()

print line.parseString(string1)
print line.parseString(string2)

['this is a test string ', ':', ' that behaves as I expect', 'n']
['this string does not behave as I expect']

string1 = "this is a test string : that behaves as I expect*"
string2 = "this string also behaves as I expect*"

field = CharsNotIn(":*")
line = field + ZeroOrMore(Literal(":") + field) + Literal("*")

print line.parseString(string1)
print line.parseString(string2)

['this is a test string ', ':', ' that behaves as I expect', '*']
['this string also behaves as I expect', '*']

>>> print line
{!W:(:
) [{":" !W:(:
)}]... LineEnd}

>>> print line.parseString('xyzyy')
['xyzyy']