Remove part form input

Hi all,

I am building a site that requires users to input a url into a text field, however the output only needs one variable from the url string.
Is there a way to make the form strip everything else using js as soon as they paste into the textfield?

the variable is v=

my form code is

Code:

<input type='text' id='userInput' value='Paste your url' class="formtext" />
<input type='image' onclick="changeText2();document.getElementById('box').style.height = '272px';"  class="formbutton" src="images/download2.png"/>

i have tried loads of ideas but im kind of new to js.

Let me know,

Thanks.

<input type='text' id='userInput' value='Paste your url' class="formtext" />

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