Modifying all URLs in a string

I am modifying URLs for amazon products in a string along with other
html data in a real basic script. I can't find the right way to do it.

An Example URL:
http://www.amazon.com/exec/obidos/ASIN/B0013EF17O/ref=br_nf_0_4?pf_rd_p=369756201&pf_rd_s=center-11&pf_rd_t=101&pf_rd_i=496994&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0N5NC0HECYSPF2G37FPK


After  "/ASIN/B0013EF17O/" I want to insert my standard amazon
affiliate code    so it would be like....

/ASIN/B0013EF170/**myaffiliatecodehere***/   <---> chop off the rest
of the URL


So I am basically, searching, inserting my code in the right spot, and
then chopping off part of the URL. I need to do this for every link in
my string.