How to remove part of url between characters in Google Sheets?

Question

Google Sheets

There is a column in Google Spreadsheet with links to posted posts, you need to convert them in such a way that the text from the question mark to the last equal sign with a number is replaced with showtopic
Yes

http://testingipb.ipb.su/index.php?act=ST&f=2&t=2068
http://lili.ipb.su/index.php?act=ST&f=1&t=2785
http://majdan.ipb.su/index.php?act=ST&f=20&t=345
http://psychotronic.ipb.su/index.php?act=ST&f=2&t=251

Necessary

http://testingipb.ipb.su/index.php?showtopic=2068
http://lili.ipb.su/index.php?showtopic=2785
http://majdan.ipb.su/index.php?showtopic=345
http://psychotronic.ipb.su/index.php?showtopic=251

Such links are interspersed with others, it is desirable that only these are equal in the table.

Answer 1

Your formula will look quite long, you can write it, but it's too easy for you))))
I suggest the direction.
What will come in handy for you:
-IFERROR (the condition in case the link is incorrect. will be at the very beginning)
-IF (general condition, will go after IFERROR)
-LEFT(RIGHT(RIGHT)(DLSTR-SEARCH("?"...-(number cut off characters)
Then similar conditions that cut off unnecessary
text.In general, first cut off and get 2 values: - What
's before?
http://testingipb.ipb.su/index.php
- And what's after?
act=ST&f=2&t=2068