当在 unix 文件的一行中找到字符串 'ny' 时，我想用另一个字符串 'ab' 替换字符串 'xy'。
示例文本：
If we have xy today we can go to ny.
If we have xy tomorrow we can go to ny tomorrow.
If we have mn now we can go to ny now.
输出文本应如下所示：-
If we have ab today we can go to ny.
If we have ab tomorrow we can go to ny tomorrow.
If we have mn now we can go to ny now.