import re
text = """Pos no
...
... 25/gm
The Text to be
...
excluded
Pos no
...
... 46 kg
The Text to be
...
excluded
Pos no
...
... 46 xunit
End of My Text"""
print(re.findall(r'(?sm)(Pos no\r?\n.+?)[\r\n]+(?:The Text to be|End of My Text)', text))
aW1wb3J0IHJlCnRleHQgPSAiIiJQb3Mgbm8KLi4uCi4uLiAyNS9nbQpUaGUgVGV4dCB0byBiZSAKLi4uCmV4Y2x1ZGVkClBvcyBubwouLi4KLi4uIDQ2IGtnClRoZSBUZXh0IHRvIGJlIAouLi4KZXhjbHVkZWQKUG9zIG5vCi4uLgouLi4gNDYgeHVuaXQKRW5kIG9mIE15IFRleHQiIiIKcHJpbnQocmUuZmluZGFsbChyJyg/c20pKFBvcyBub1xyP1xuLis/KVtcclxuXSsoPzpUaGUgVGV4dCB0byBiZXxFbmQgb2YgTXkgVGV4dCknLCB0ZXh0KSk=