如何将目录中所有文件的头部和尾部替换为新的头部和尾部字符串，但最多只能替换某个字符？

import os

def replace_header_footer(directory, header, footer, replace_char):
    for root, dirs, files in os.walk(directory):
        for file in files:
            file_path = os.path.join(root, file)
            with open(file_path, 'r') as f:
                content = f.read()
            
            # 替换头部和尾部
            content = content.replace(content[:len(header)], header.replace(header[0], replace_char))
            content = content.replace(content[-len(footer):], footer.replace(footer[0], replace_char))
            
            with open(file_path, 'w') as f:
                f.write(content)

# 示例调用
replace_header_footer('/path/to/directory', 'old_header', 'old_footer', '-')