Which characters make a URL invalid?

I am implementing an old HTTP (0.9, 1.0, 1.1) request and response reader/writer. The request URI is the most problematic place.

You can't just use RFC 1738, 2396 or 3986 as it is. There are many old HTTP clients and servers that allow more characters. So I've made research based on accidentally published web server access logs: "GET URI HTTP/1.0" 200.

I've found that the following non-standard characters are often used in URIs:

\ { } < > | ` ^ "

These characters were described in RFC 1738 as unsafe.

If you want to be compatible with all old HTTP clients and servers - you have to allow these characters in the request URI.

Please read more information about this research in oghttp-request-collector.

I can't comment on the above answers, but wanted to emphasize the point (in another answer) that allowed characters aren't allowed everywhere. For example, domain names can't have underscores, so http://test_url.com is invalid.