import re
s = '''
COMMENTS:
first comment /user_x
second comment
two lines /user_y
Here is some unimportant text.
KEYWORD:
Don't match comments or anything else after first keyword like this /user_x
KEYWORD: <- again
Also ignore same keyword which could appear serveral times.
'''
pattern = re.compile(r'COMMENTS:\s*((?:(?:(?!KEYWORD:).)+?/(?:user_x|user_y))+).+?KEYWORD:', flags=re.DOTALL)
match = re.search(pattern, s)
if match:
print(match.group(1))
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