import re
 
s = """TEST IC ITMI12CR01 GRATH1CR01, ALTIR1AB02
TEST-ICITMI12CR01 BE5253, BE2178 and BE40035 towards GRATH1CR01, ROTIM1CR01 and ALTIR1AB02 are down
TEST-IC Multiple links are TRIST1AB01<><>ROTIM1CR01<>GRATH1CR01<>ITMI12CR01
TEST IC ITMI12CR01 links to GRATH1CR01, ALTIR1AB02"""
 
 
l = [match.group(1) if match.group(1) else match.group(2)
     for match in re.finditer(r'\b(?:([A-Z]{4})(?:[0-9][A-Z0-9]{5})|([A-Z]{5})(?:[0-9][A-Z0-9]{4}))\b', s)
    ]
print(l)
 

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