The GPT answer is still pretty good. Adding re.S makes it even more accurate. Adding it also matches the newline before the z string, otherwise there would be a few more newlines.
The reference code is as follows, which is expected to be adopted.
z = re.sub(r'.*\d{2}:\d{2}:\d{2}\n', '', z, re.S) # 替换为空字符串
print(z)