How do SO URLs self correct themselves if they are mistyped?

If an extra character (like a period, comma or a brack开发者_JAVA技巧et or even alphabets) gets accidentally added to URL on the stackoverflow.com domain, a 404 error page is not thrown. Instead, URLs self correct themselves & the user is led to the relevant webpage.

For instance, the extra 4 letters I added to the end of a valid SO URL to demonstrate this would be automatically removed when you access the below URL - https://stackoverflow.com/questions/194812/list-of-freely-available-programming-booksasdf

I guess this has something to do with ASP.NET MVC Routing. How is this feature implemented?

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Well, this is quite simple to explain I guess, even without knowing the code behind it: The text is just candy for search engines and people reading the URL:

This URL will work as well, with the complete text removed!

The only part really important is the question ID that's also embedded in the "path".

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This is because EVERYTHING after http://stackoverflow.com/questions/194812 is ignored. It is just there to make the link, if posted somewhere, if more speaking.

Internally the URL is mapped to a handler, e.g., by a rewrite, that transforms into something like: http://stackoverflow.com/questions.php?id=194812 (just an example, don't know the correct internal URL)

This also makes the URL search engine friendly, besides being more readable to humans.