如何替换文本文件中字符串的前半部分

import re

# 1. 打开文本文件
file_path = "example.txt"
with open(file_path, "r") as file:
    # 2. 读取文件内容
    content = file.read()

# 3. 定位目标字符串
target_string = "old_string"
match = re.search(target_string, content)
if match:
    # 4. 替换字符串
    start_index = match.start()
    end_index = match.end()
    new_string = "new" + content[start_index+len(target_string):]
    replaced_content = content[:start_index] + new_string + content[end_index:]
else:
    replaced_content = content

# 5. 保存修改后的内容
modified_content = replaced_content

# 6. 写入文件
with open(file_path, "w") as file:
    file.write(modified_content)