Which characters make a URL invalid?

Question

Which characters make a URL invalid?

Are these valid URLs?

• example.com/file[/].html
• http://example.com/file[/].html<

Answer 1

In your supplementary question you asked if www.example.com/file[/].html is a valid URL.

That URL isn't valid because a URL is a type of URI and a valid URI must have a scheme like http: (see RFC 3986).

If you meant to ask if http://www.example.com/file[/].html is a valid URL then the answer is still no because the square bracket characters aren't valid there.

The square bracket characters are reserved for URLs in this format: http://[2001:db8:85a3::8a2e:370:7334]/foo/bar (i.e. an IPv6 literal instead of a host name)

It's worth reading RFC 3986 carefully if you want to understand the issue fully.