import re
pattern = r"^# .*$"
text = "# Title\n## Chapter\n### sub-chapter#### What a lovely day.\n"
header = re.search(pattern, text, re.M)
print(header.group())
aW1wb3J0IHJlCgpwYXR0ZXJuID0gciJeIyAuKiQiCnRleHQgPSAiIyBUaXRsZVxuIyMgQ2hhcHRlclxuIyMjIHN1Yi1jaGFwdGVyIyMjIyBXaGF0IGEgbG92ZWx5IGRheS5cbiIKaGVhZGVyID0gcmUuc2VhcmNoKHBhdHRlcm4sIHRleHQsIHJlLk0pCnByaW50KGhlYWRlci5ncm91cCgpKQ==