import re
text = "the asbestos fiber <unk> <unk| is < unusually <unk once it enters the <<unk>$% with "
replacement = " "
pattern: str = r'(?:(?!<unk>)[\W_](?<!<unk>))+'
print( re.sub(pattern, replacement, text, flags=re.I) )
aW1wb3J0IHJlCnRleHQgPSAidGhlIGFzYmVzdG9zIGZpYmVyIDx1bms+IDx1bmt8IGlzIDwgdW51c3VhbGx5IDx1bmsgb25jZSBpdCBlbnRlcnMgdGhlIDw8dW5rPiQlIHdpdGggIgpyZXBsYWNlbWVudCA9ICIgIgpwYXR0ZXJuOiBzdHIgPSByJyg/Oig/ITx1bms+KVtcV19dKD88ITx1bms+KSkrJwpwcmludCggcmUuc3ViKHBhdHRlcm4sIHJlcGxhY2VtZW50LCB0ZXh0LCBmbGFncz1yZS5JKSApCg==