多行 regexp 替換為無條件分支

假設我有一個名為 in.txt 的檔案：

$ cat in.txt
a
b
a
c
a
d

我只想用 deleted 替換 a\nc，而不是 a\nb 或 a\nd。

$ sed -e ':loop         # create a branch/label named `loop`
 $!{
 N                      # append the next line of input into the pattern space
 /\n$/!b loop           # If it is not the last line go to the `loop` branch again
 }
 s/a\nc/"deleted"/' in.txt    # do replacing in the pattern space

a
b
"deleted"    # see! succeed
a
d