import re
s = "aba\naaa\naba\n\naba"
rx = r'(?:{0}(?:\n|$))+|(?:\n|^){0}'.format(r'[^b\n]*b.*')
print(rx)
print( re.sub(rx, '', s).replace('\n','\\n') )
aW1wb3J0IHJlCgpzID0gImFiYVxuYWFhXG5hYmFcblxuYWJhIgpyeCA9IHInKD86ezB9KD86XG58JCkpK3woPzpcbnxeKXswfScuZm9ybWF0KHInW15iXG5dKmIuKicpCnByaW50KHJ4KQpwcmludCggcmUuc3ViKHJ4LCAnJywgcykucmVwbGFjZSgnXG4nLCdcXG4nKSApCg==