import re
p = re.compile(r'(TAG|CORE)START(?: B)?(.+?)(\1END(?: B)?)', re.DOTALL)
test_str = "intro intro intro intro intro intro\nBEGIN A This is where some text starts\nThat is not interesting or wanted\nCORESTART save text A save text A save text A save text A \nsave text A save text A save text A save text A save text A \nsave text A COREEND\nThis is an addendum that is not needed but is just in the way\nEND A outro outro outro outro outro outro \noutro outro outro outro outro outro outro \n.\n\nintro intro intro intro intro intro\nINIT B This is where some text starts\nThat is not interesting or wanted\nTAGSTART B save text B save text B save text B save text B \nsave text B save text B save text B save text B save text B \nsave text B TAGEND B\nThis is an addendum that is not needed but is just in the way\nTERM B outro outro outro outro outro outro \noutro outro outro outro outro outro outro "
print([x.group(2) for x in p.finditer(test_str)])

aW1wb3J0IHJlCnAgPSByZS5jb21waWxlKHInKFRBR3xDT1JFKVNUQVJUKD86IEIpPyguKz8pKFwxRU5EKD86IEIpPyknLCByZS5ET1RBTEwpCnRlc3Rfc3RyID0gImludHJvIGludHJvIGludHJvIGludHJvIGludHJvIGludHJvXG5CRUdJTiBBIFRoaXMgaXMgd2hlcmUgc29tZSB0ZXh0IHN0YXJ0c1xuVGhhdCBpcyBub3QgaW50ZXJlc3Rpbmcgb3Igd2FudGVkXG5DT1JFU1RBUlQgc2F2ZSB0ZXh0IEEgc2F2ZSB0ZXh0IEEgc2F2ZSB0ZXh0IEEgc2F2ZSB0ZXh0IEEgXG5zYXZlIHRleHQgQSBzYXZlIHRleHQgQSBzYXZlIHRleHQgQSBzYXZlIHRleHQgQSBzYXZlIHRleHQgQSBcbnNhdmUgdGV4dCBBIENPUkVFTkRcblRoaXMgaXMgYW4gYWRkZW5kdW0gdGhhdCBpcyBub3QgbmVlZGVkIGJ1dCBpcyBqdXN0IGluIHRoZSB3YXlcbkVORCBBIG91dHJvIG91dHJvIG91dHJvIG91dHJvIG91dHJvIG91dHJvIFxub3V0cm8gb3V0cm8gb3V0cm8gb3V0cm8gb3V0cm8gb3V0cm8gb3V0cm8gXG4uXG5cbmludHJvIGludHJvIGludHJvIGludHJvIGludHJvIGludHJvXG5JTklUIEIgVGhpcyBpcyB3aGVyZSBzb21lIHRleHQgc3RhcnRzXG5UaGF0IGlzIG5vdCBpbnRlcmVzdGluZyBvciB3YW50ZWRcblRBR1NUQVJUIEIgc2F2ZSB0ZXh0IEIgc2F2ZSB0ZXh0IEIgc2F2ZSB0ZXh0IEIgc2F2ZSB0ZXh0IEIgXG5zYXZlIHRleHQgQiBzYXZlIHRleHQgQiBzYXZlIHRleHQgQiBzYXZlIHRleHQgQiBzYXZlIHRleHQgQiBcbnNhdmUgdGV4dCBCIFRBR0VORCBCXG5UaGlzIGlzIGFuIGFkZGVuZHVtIHRoYXQgaXMgbm90IG5lZWRlZCBidXQgaXMganVzdCBpbiB0aGUgd2F5XG5URVJNIEIgb3V0cm8gb3V0cm8gb3V0cm8gb3V0cm8gb3V0cm8gb3V0cm8gXG5vdXRybyBvdXRybyBvdXRybyBvdXRybyBvdXRybyBvdXRybyBvdXRybyAiCnByaW50KFt4Lmdyb3VwKDIpIGZvciB4IGluIHAuZmluZGl0ZXIodGVzdF9zdHIpXSk=