TYPO3 Core

Typo3 core functions will generate links for extensions of the form

http://www.server.com/index.php?id=12&tx_someext[somevariable]=12

This does not comply with RFC 1738 where you will find the passage.

"Unsafe:

Characters can be unsafe for a number of reasons.  The space
   character is unsafe because significant spaces may disappear and
   insignificant spaces may be introduced when URLs are transcribed or
   typeset or subjected to the treatment of word-processing programs.
   The characters "<" and ">" are unsafe because they are used as the
   delimiters around URLs in free text; the quote mark (""") is used to
   delimit URLs in some systems.  The character "#" is unsafe and should
   always be encoded because it is used in World Wide Web and in other
   systems to delimit a URL from a fragment/anchor identifier that might
   follow it.  The character "%" is unsafe because it is used for
   encodings of other characters.  Other characters are unsafe because
   gateways and other transport agents are known to sometimes modify
   such characters. These characters are "{", "}", "|", "\", "^", "~",
   "[", "]", and "`".

All unsafe characters must always be encoded within a URL. For
   example, the character "#" must be encoded within URLs even in
   systems that do not normally deal with fragment or anchor
   identifiers, so that if the URL is copied into another system that
   does use them, it will not be necessary to change the URL encoding."

Square brackets should therefore always be encoded as %5B and %5D. I came across this issue because the email client Sylpheed Claws cuts of URLs in emails at the first square bracket, considering the rest not part of the link. As this behaviour is correct according to the RFC, the SC developers will not change that.

http://www.ietf.org/rfc/rfc1738.txt
(issue imported from #M3808)