Are square brackets permitted in URLs? - apache

Question

Are square brackets in URLs allowed?
I noticed that Apache commons HttpClient (3.0.1) throws an IOException, wget and Firefox however accept square brackets.
URL example:
http://example.com/path/to/file[3].html
My HTTP client encounters such URLs but I'm not sure whether to patch the code or to throw an exception (as it actually should be).

Answer 1

RFC 3986 states
A host identified by an Internet
Protocol literal address, version 6
[RFC3513] or later, is distinguished
by enclosing the IP literal within
square brackets ("[" and "]"). This
is the only place where square bracket
characters are allowed in the URI
syntax.
So you should not be seeing such URI's in the wild in theory, as they should arrive encoded.

Answer 2

Square brackets [ and ] in URLs are not often supported.
Replace them by %5B and %5D:
Using a command line, the following example is based on bash and sed:
url='http://example.com?day=[0-3][0-9]'
encoded_url="$( sed 's/\[/%5B/g;s/]/%5D/g' <<< "$url")"
Using Java URLEncoder.encode(String s, String enc)
Using PHP rawurlencode() or urlencode()
<?php
echo '<a href="http://example.com/day/',
rawurlencode('[0-3][0-9]'), '">';
?>
output:
<a href="http://example.com/day/%5B0-3%5D%5B0-9%5D">
or:
<?php
$query_string = 'day=' . urlencode('[0-3][0-9]') .
'&month=' . urlencode('[0-1][0-9]');
echo '<a href="http://example.com?',
htmlentities($query_string), '">';
?>
Using your favorite programming language... Please extend this answer by posting a comment or editing directly this answer to add the function you use from your programming language ;-)
For more details, see the RFC 3986 specifying the URL syntax. The Appendix A is about %-encoding in the query string (brackets as belonging to “gen-delims” to be %-encoded).

Answer 3

I know this question is a bit old, but I just wanted to note that PHP uses brackets to pass arrays in a URL.
http://www.example.com/foo.php?bar[]=1&bar[]=2&bar[]=3
In this case $_GET['bar'] will contain array(1, 2, 3).

Answer 4

Pretty much the only characters not allowed in pathnames are # and ? as they signify the end of the path.
The uri rfc will have the definative answer:
http://www.ietf.org/rfc/rfc1738.txt
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.
The answer is that they should be hex encoded, but knowing postel's law, most things will accept them verbatim.

Answer 5

Any browser or web-enabled software that accepts URLs and is not throwing an exception when special characters are introduced is almost guaranteed to be encoding the special characters behind the scenes. Curly brackets, square brackets, spaces, etc all have special encoded ways of representing them so as not to produce conflicts. As per the previous answers, the safest way to deal with these is to URL-encode them before handing them off to something that will try to resolve the URL.

Answer 6

For using the HttpClient commons class, you want to look into the org.apache.commons.httpclient.util.URIUtil class, specifically the encode() method. Use it to URI-encode the URL before trying to fetch it.

Answer 7

StackOverflow seems to not encode them:
https://stackoverflow.com/search?q=square+brackets+[url]

Answer 8

Best to URL encode those, as they are clearly not supported in all web servers. Sometimes, even when there is a standard, not everyone follows it.

Answer 9

According to the URL specification, the square brackets are not valid URL characters.
Here's the relevant snippets:
The "national" and "punctuation" characters do not appear in any
productions and therefore may not appear in URLs.
national { | } | vline | [ | ] | \ | ^ | ~
punctuation < | >

Answer 10

Square brackets are considered unsafe, but majority of browsers will parse those correctly. Having said that it is better to replace square brackets with some other characters.