sed - insert line after X lines after match

sed '/function_1(/,/^[[:space:]]*}/ {
 ,/^[[:space:]]*}/ a\
Line that\
you want to\
insert (append) here
   }' YourFile

• insert the line after the } (alone in the line with eventually some space before) from the section starting with function_1(
• i assume there is no } alone in your internal code like in your sample

be carreful on selection based on function name because it could be used (and normaly it is) as a call to the function itself in other code section so maybe a /^void function_1()$/ is better

Don't count, match:

sed -e '/^void function_1()/,/^}$/ { /^}$/a\
TEXT TO INSERT
}' input

This looks at the block between the declaration and the closing brace, and then append TEXT_TO_INSERT after the closing brace.

Use awk:

awk '1;/function_1/{c=4}c&&!--c{print "new text"}' file

• 1 is a shorthand for {print}, so all lines in the file are printed
• when the pattern is matched, set c to 4
• when c reaches 1 (so c is true and !--c is true), insert the line

You could just use !--c but adding the check for c being true as well means that c doesn't keep decreasing beyond 0.

Try this with GNU sed:

sed "/function_1/{N;N;N;a new_text
}" filename