Note that RFC 1738 has been amended:
The "[" and "]" are no longer considered unsafe, but instead are now considered "reserved", meaning that they CAN be used in URLs!
Currently this usage has only been allowed in the hostname part, but there are some proposals to allow such use in some URL schemes. Similar extensions are now found that use the "{}" characters as "reserved" characters with special semantics, instead of "unsafe" characters that must be URL encoded...
Note also that some characters are currently "reserved" but should have instead been considered as "unsafe": this includes the parenthesis "()" which are clearly unsafe when a URL is used in MIME headers.
Because of this, if a valid URL contains "()" characters, one should use an upper-level encoding to either enclose the URL with a pair of "unsafe" characters defined in the upper-level protocol (for example a "<>" pair in MIME headers, because these characters cannot be part of a valid URL)...
