import re
txt = """
Some initial text....
Heading1
Some other text.....
Heading2
Some more text....
"""
new_text = re.sub('(Heading1)(.*)(Heading2)', r"\1 replaced with python script \3", txt, flags=re.DOTALL)
print (new_text)
aW1wb3J0IHJlCnR4dCA9ICIiIgpTb21lIGluaXRpYWwgdGV4dC4uLi4KSGVhZGluZzEKU29tZSBvdGhlciB0ZXh0Li4uLi4KSGVhZGluZzIKU29tZSBtb3JlIHRleHQuLi4uCiIiIgpuZXdfdGV4dCA9IHJlLnN1YignKEhlYWRpbmcxKSguKikoSGVhZGluZzIpJywgciJcMSByZXBsYWNlZCB3aXRoIHB5dGhvbiBzY3JpcHQgXDMiLCB0eHQsIGZsYWdzPXJlLkRPVEFMTCkKcHJpbnQgKG5ld190ZXh0KQ==