How to find a partial String in another String with JavaScript

I've got a string lets say its "/s200/" which is part of a URL, and this part is responsible for resizing an image on demand. So whenever you change s200 to a higher or lower number following the s, then the picture that is returned by the complete URL will be either increased or decreased in size. Now i want to programmatically find this part in the url and replace it by lets say "/s800/" in order to make the returned image bigger.
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