import re
text = "#MainauDeclaration2015 watch out guys.. This is HUGE!! #LindauNobel #SemST"
print( re.sub(r'#(\w+)', lambda x: '#hashtag ' + re.sub(r'(?!^)(?=[A-Z])|(?<=\D)(?=\d)|(?<=\d)(?=\D)', ' ', x.group(1)), text) )
# => #hashtag Mainau Declaration 2015 watch out guys.. This is HUGE!! #hashtag Lindau Nobel #hashtag Sem S T
aW1wb3J0IHJlCnRleHQgPSAiI01haW5hdURlY2xhcmF0aW9uMjAxNSB3YXRjaCBvdXQgZ3V5cy4uIFRoaXMgaXMgSFVHRSEhICNMaW5kYXVOb2JlbCAjU2VtU1QiCnByaW50KCByZS5zdWIocicjKFx3KyknLCBsYW1iZGEgeDogJyNoYXNodGFnICcgKyByZS5zdWIocicoPyFeKSg/PVtBLVpdKXwoPzw9XEQpKD89XGQpfCg/PD1cZCkoPz1cRCknLCAnICcsIHguZ3JvdXAoMSkpLCB0ZXh0KSApCiMgPT4gI2hhc2h0YWcgTWFpbmF1IERlY2xhcmF0aW9uIDIwMTUgd2F0Y2ggb3V0IGd1eXMuLiBUaGlzIGlzIEhVR0UhISAjaGFzaHRhZyBMaW5kYXUgTm9iZWwgI2hhc2h0YWcgU2VtIFMgVA==