import re
s = """some text here 45 [check {xyz}]
more text 12 [check {pqr[25]}
some more text 56 [check {efg[4]}]
still more text 78 [check { jkl[12]}
more text 123 [check abc]
more text 34 ghi
"""
regex = r"(?<=check )(?:{[^{}]+}|[^\[\]]+)|\w+$"
l = re.findall(regex, s)
print(l)
aW1wb3J0IHJlCnMgPSAiIiJzb21lIHRleHQgaGVyZSA0NSBbY2hlY2sge3h5en1dCm1vcmUgdGV4dCAxMiBbY2hlY2sge3BxclsyNV19CnNvbWUgbW9yZSB0ZXh0IDU2IFtjaGVjayB7ZWZnWzRdfV0Kc3RpbGwgbW9yZSB0ZXh0IDc4IFtjaGVjayB7IGprbFsxMl19Cm1vcmUgdGV4dCAxMjMgW2NoZWNrIGFiY10KbW9yZSB0ZXh0IDM0IGdoaQoiIiIKCnJlZ2V4ID0gciIoPzw9Y2hlY2sgKSg/OntbXnt9XSt9fFteXFtcXV0rKXxcdyskIgoKbCA9IHJlLmZpbmRhbGwocmVnZXgsIHMpCnByaW50KGwp