import re

pattern = r"(# .+?\n)"

text = r"# Titlen## Chaptern### sub-chapter#### What a lovely day.n"

header = re.search(pattern, text)
print(header.string)

(# .+?\n)(.+)

# coding=utf8
# the above tag defines encoding for this document and is for Python 2.x compatibility

import re

regex = r"(# .+?\n)(.+)"

test_str = "# Title\n## Chapter\n### sub-chapter#### The Bar\nIt was a fall day.\n"

subst = "\1"

# You can manually specify the number of replacements by changing the 4th argument
result = re.sub(regex, subst, test_str, 1)

if result:
    print (result)

# Note: for Python 2.7 compatibility, use ur"" to prefix the regex and u"" to prefix the test string and substitution.