Normally, I would recommend using the HTML DOM to do this type of thing as it handles all of the weird things that can occur within 'valid" HTML files.
Also, I assume that you are looking for the exact character sequence "OBJ=123" and that this is not just an example of a string that you want to find, but have not specified.
Given that, try a pattern of
and a replacement string of
(For some reason, the forum software if replacing my uppercase HREF with a lowercase version in the above line)
However, please be aware that this will ONLY find "OBJ=123" and will add in exactly the replacement string with the "OBJ=123" added into it.