Complex IF statements in Excel

I have been given a spreadsheet with an IF statement which I am trying to ammend.

The current if statement is:

=IF(ISERR(FIND("tag=",F3)),"-",MID(F3,FIND("tag=",F3)+4,7))

This is referiing to the following hyperlink:

http://mkge.englishtown.com/online/lp/PrivateTeacher_CRM043.aspx?lng=ar& ;etag=E112546&first_name=%%First Name%%&last_name=%%Last Name%%&email=%%Email%%

And is basically trying to extract the E112546 out.

This currently works as the IF statement is based on a fixed width of 7.

I have just been told that the hyperlink is now changing and that the want to use freeform text which could be longer then 7 characters and up to 25 characters.

EXAMPLE 2:

http://mkge.englishtown.com/online/lp/OnlineStudent_CRM043.aspx?lng=pt& ;ctr=br&etag=mkgepm_303_gre_oo_lpgr&first_name=%%First Name%%&last_name=%%Last Name%%&email=%%Email%%

So the code in the above example would be:

mkgepm_303_gre_oo_lpgr

So we basically need a statement which captures anything after the "etag=" but before the "&first_name="

Now to make matters even worse, I have heard that the "&first_name=" will not always appear after the code.

Sometimes there will be nothing, and other times there could be:

"&ptn="
"&email="

I think there is basically 4 combinations.

What I urgently need to know, is, Can a comprehesive IF statement to capture any size code (up to 25 characters) be written?

If you can help with this then that would be great.

Kind regards
Nick