How to use a remembered pattern in a GNU sed embedded shell command?

I am using GNU sed version 4.2.2.
My default shell is GNU bash, version 4.3.11(1)-release (x86_64-pc-linux-gnu).

For days, on and off, I have been trying to get variants of the following sed command to run without errors:

sed -i 's@<span class="math">$\(.*\)$</span>@echo \1 \@ge' $1
 

 Above variant fails with the following error:
 sed: -e expression #1, char 49: unterminated `s' command
 By the way, character 49 is the letter o of echo.
 

 What am I doing wrong?
 

 PS: For the moment I am using the shell command echo as a surrogate for calling a more complex shell command. I figured as long as I do not get to run the echo, I should not bother doing more difficult stuff.

 





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