import re
l = ["42. blabla \n foo", "42 blabla \n foo", "422. blabla \n foo"]
regex_header = re.compile(r"^\d+(?![.\d])")
for s in l:
if (regex_header.search(s)):
print(s)
aW1wb3J0IHJlIApsID0gWyI0Mi4gYmxhYmxhIFxuIGZvbyIsICI0MiBibGFibGEgXG4gZm9vIiwgIjQyMi4gYmxhYmxhIFxuIGZvbyJdCnJlZ2V4X2hlYWRlciA9IHJlLmNvbXBpbGUociJeXGQrKD8hWy5cZF0pIikKZm9yIHMgaW4gbDoKICAgIGlmIChyZWdleF9oZWFkZXIuc2VhcmNoKHMpKToKICAgIAlwcmludChzKQ==