import re

example = ("OPERATOR: Some text with numbers, special characters and linebreaks.\n\n"
	"NAME, COMPANY, POSITION: Some text with numbers, special characters and linebreaks.\n"
	"NAME: Some text with numbers, special characters and linebreaks.")
r = re.compile(r"^([^a-z\r\n]+):(.*(?:(?!\n[^a-z\r\n]+:)[\r\n].*)*)", flags=re.MULTILINE)
print(re.findall(r, example))