SED INSERT block of code BEFORE matched block

Question

I have a text file I want to edit during a script excecution. I want to pass from the text on left to the text on right using sed insert:

INSERT BEFORE: from:

[...]
[ placeholder ]
Categ         #n
ItemName      1
[...]
[ placeholder ]
Categ         #n
ItemName      2
[...]

to:

[...]
[ placeholder ]
Categ         #n
ItemName      1
[...]
**new_line1**        <<<--- INSERT BEFORE
**new_line2**        |||
**new_line3**        |||
[ placeholder ]      <<<--- 3-lines match
Categ         #n     |||
ItemName      2      |||
[...]

I have tried to adapt sed append lines to sed insert. The appending works, the insert does not. The match to append was 2 lines one after the other. In the insert, the match it is 3 lines and I tried to match the first and the last, as I don't know how to match the entire 3-lines block.

APPEND AFTER:

sed -e '/^\[ placeholder \]\/ItemName      2/a \\nnew_line1\nnew_line2\nnew_line3' input.txt

and it works from:

[...]
[ placeholder ]
ItemName      1
[...]
[ placeholder ]
ItemName      2
[...]

to:

[...]
[ placeholder ]
ItemName      1
[...]
[ placeholder ]     <<<--- 2-lines match
ItemName      2     |||
**new_line1**       <<<--- APPENDED AFTER
**new_line2**       |||
**new_line3**       |||
[...]

I have tried to use the syntax examples in this post, but with no success. The sed - Stream Editor

Answer 1

If your input does not contain NUL bytes you could try the following with GNU sed:

$ from='\[ placeholder \]\nCateg         #n\nItemName      2\n'
$ to='**new_line1**\n**new_line2**\n**new_line3**\n'
$ sed -Ez "s/(^|\n)($from)/\1$to\2/" input.txt
[...]
[ placeholder ]
Categ         #n
ItemName      1
[...]
**new_line1**
**new_line2**
**new_line3**
[ placeholder ]
Categ         #n
ItemName      2
[...]

With the -z option the input is treated as a set of lines, each terminated by a zero byte (the ASCII NUL character) instead of a newline. If the input does not contain NUL bytes this allows to process the entire file at once. Of course you must properly escape special characters in from and to.