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

All valid characters that can be used in a URI (a URL is a type of URI) are defined in RFC 3986.

All other characters can be used in a URL provided that they are "URL Encoded" first. This involves changing the invalid character for specific "codes" (usually in the form of the percent symbol (%) followed by a hexadecimal number).

This link, HTML URL Encoding Reference, contains a list of the encodings for invalid characters.