import re
regex = r"\bDOB ((?:(?:3[01]|[12][0-9]|0?[1-9]) [A-Za-z]+ )?\d{4})\b"
test_str = ("DOB 23 Jun 1959; alt. DOB 23 Jun 1958\n"
"DOB 1959; alt. DOB 1958")
print(re.findall(regex, test_str))
aW1wb3J0IHJlCgpyZWdleCA9IHIiXGJET0IgKCg/Oig/OjNbMDFdfFsxMl1bMC05XXwwP1sxLTldKSBbQS1aYS16XSsgKT9cZHs0fSlcYiIKdGVzdF9zdHIgPSAoIkRPQiAyMyBKdW4gMTk1OTsgYWx0LiBET0IgMjMgSnVuIDE5NThcbiIKICAgICJET0IgMTk1OTsgYWx0LiBET0IgMTk1OCIpCgpwcmludChyZS5maW5kYWxsKHJlZ2V4LCB0ZXN0X3N0cikp