sed - insert a string with backslash at a specific line number - regex

Question

I have a file which contains multiple latex equations like this :
...
\begin{equation}
\beq{x}=x^{1}\beq{e_{1}}+x^{2}\beq{e_{2}}+x^{3}\beq{e_{3}}
\end{equation}
...
\begin{equation}
\beq{y}=y^{1}\beq{e_{1}}+y^{2}\beq{e_{2}}+y^{3}\beq{e_{3}}
\end{equation}
...
I want to insert just before the "\end{equation}" the string "\tag{number}" where I can successfully get number variable.
To insert this string at the line identified by "$(($line)-1)", I do :
gsed -i "$(($line)-1)i \tag{$number}" file
But I get only :
...
\begin{equation}
\beq{x}=x^{1}\beq{e_{1}}+x^{2}\beq{e_{2}}+x^{3}\beq{e_{3}}
tag{1}
\end{equation}
...
\begin{equation}
\beq{y}=y^{1}\beq{e_{1}}+y^{2}\beq{e_{2}}+y^{3}\beq{e_{3}}
tag{2}
\end{equation}
...
As you can see, I can't print the backslash character at the beginning of "\tag" string
I tried with :
gsed -i "$(($line)-1)i '\'tag{$number}" file
or
gsed -i "$(($line)-1)i \\tag{$number}" file
but no good results,
if someone could see what's wrong ...
Thanks
PS: I am on MacOS X, that's why I used gsed

Answer 1

You need five slashes:
gsed -i "$(($line)-1)i \\\\\tag{$number}" file
Let me explain starting with a single quoted command:
gsed -i '1i \\\test'
You would need three slashes in that case:
The first one delimits the i command with the text to be inserted, the second one escapes the slash itself because otherwise it would get expanded as \t. The third, now escaped, slash, will get inserted as literal \ at the start of the new line.
If we additionally using double quotes to enclose the command,
gsed -i "1i\\\\\test"
the string will get additionally subject of parsing by the shell. Both escaping slashes from the single quoted command, would therefore need to get escaped as well. This makes 5 slashes.
So far so good. But since you are interpolating shell variables into the command, you need to make sure that slashes in them would get escaped as well.