import re
subject = 'keyword1 keyword2 title:hello title:world "title:quoted" keyword3'
regex = re.compile(r'"[^"]*"|(\btitle:\S+)')
def myreplacement(m):
    if m.group(1):
        return ""
    else:
        return m.group(0)
replaced = regex.sub(myreplacement, subject)
print(replaced)