I'm not sure what is intended by the command you mentioned:
echo hello | sed '/hello/i world'
However, I presume that you want to perform certain action on lines matching the pattern hello. Lets say you wanted to change the lines matching the pattern hello to world. In order to accomplish that, you can say:
$ echo -e "something\nhello" | sed '\|hello|{s|.*|world|}'
something
world
In order to match lines using a regexp, the following forms can be used:
/regexp/
\%regexp%
where % may be replaced by any other single character (note the preceding \ in the second case).
The manual provides more details on this.