import re
LIST1=["AR BR_18_0138249", "AR R_16_01382649", "BR 16 0138264", "R 16 01382679" ]
pattern = r"\b(B?R(?=([\s_]))(?:\2\d+)+)\b(?!.*\b\1\b)"
for i in LIST1:
rest = re.search(pattern, i)
if rest:
print(rest.group(1))
aW1wb3J0IHJlCkxJU1QxPVsiQVIgQlJfMThfMDEzODI0OSIsICAiQVIgUl8xNl8wMTM4MjY0OSIsICAiQlIgMTYgMDEzODI2NCIsICJSIDE2IDAxMzgyNjc5IiBdCnBhdHRlcm4gPSByIlxiKEI/Uig/PShbXHNfXSkpKD86XDJcZCspKylcYig/IS4qXGJcMVxiKSIKZm9yIGkgaW4gTElTVDE6CiAgcmVzdCA9IHJlLnNlYXJjaChwYXR0ZXJuLCBpKQogIGlmIHJlc3Q6CiAgICBwcmludChyZXN0Lmdyb3VwKDEpKQ==