import re
s = '''
COMMENTS:
first comment /user_x
second comment
two lines /user_y
Here is some unimportant text.
KEYWORD:
Don't match comments after keyword like this /user1
'''
pattern = re.compile(r'COMMENTS:\s*((?:.+?/(?:user_x|user_y))+)(?=.+?KEYWORD:)', flags=re.DOTALL)
match = re.search(pattern, s)
if match:
print(match.group(1))
aW1wb3J0IHJlCgpzID0gJycnCkNPTU1FTlRTOiAgCmZpcnN0IGNvbW1lbnQgL3VzZXJfeCAgCnNlY29uZCBjb21tZW50CnR3byBsaW5lcyAvdXNlcl95CkhlcmUgaXMgc29tZSB1bmltcG9ydGFudCB0ZXh0LiAgCktFWVdPUkQ6CkRvbid0IG1hdGNoIGNvbW1lbnRzIGFmdGVyIGtleXdvcmQgbGlrZSB0aGlzIC91c2VyMSAgCicnJwpwYXR0ZXJuID0gcmUuY29tcGlsZShyJ0NPTU1FTlRTOlxzKigoPzouKz8vKD86dXNlcl94fHVzZXJfeSkpKykoPz0uKz9LRVlXT1JEOiknLCBmbGFncz1yZS5ET1RBTEwpCm1hdGNoID0gcmUuc2VhcmNoKHBhdHRlcm4sIHMpCmlmIG1hdGNoOgogICAgcHJpbnQobWF0Y2guZ3JvdXAoMSkpCgo=