Sed replace every nth occurrence

Question

I am trying to use sed to replace every other occurrence of an html element of a file so I can make alternating color rows.

Here is what I have tried and it doesn't work.

sed 's/<tr valign=top>/<tr valign=top bgcolor='#E0E0E0'>/2' untitled.html

Answer 1

I'd solve it with awk:

awk '/<tr valign=top>/&&v++%2{sub(/<tr valign=top>/, "<tr valign=top bgcolor='#E0E0E0'>")}{print}' untitled.html 

First, it verifies if the line contains <tr valign=top>

/<tr valign=top>/&&v++%2

and whether the <tr valign=top> is an odd found instance:

v++%2

If so, it replaces the <tr valign=top> in the line

{sub(/<tr valign=top>/, "<tr valign=top bgcolor='#E0E0E0'>")}

Since all lines are to be printed, there is a block that always will be executed (for all lines) and will print the current line:

{print}