import re
s = """limk ab1
limk ab2 helo
rest helo
ab3 limk helo
ab4 limk"""
matches = [x[0] if x[0] != '' else x[1] for x in re.findall(r'(?m)^([0-9a-zA-Z]{3})\s+limk$|^limk\s+([0-9a-zA-Z]{3})$', s)]
for match in matches:
print(match)
aW1wb3J0IHJlCgpzID0gIiIibGltayBhYjEKbGltayBhYjIgaGVsbwpyZXN0IGhlbG8KYWIzIGxpbWsgaGVsbwphYjQgbGltayIiIgoKbWF0Y2hlcyA9IFt4WzBdIGlmIHhbMF0gIT0gJycgZWxzZSB4WzFdIGZvciB4IGluIHJlLmZpbmRhbGwocicoP20pXihbMC05YS16QS1aXXszfSlccytsaW1rJHxebGlta1xzKyhbMC05YS16QS1aXXszfSkkJywgcyldCmZvciBtYXRjaCBpbiBtYXRjaGVzOgogICAgcHJpbnQobWF0Y2gp