snippet Aristotle <p>I wrote this mainly because someone asked how to do this in PHP. Turns out there is nothing in like the venerable [cpan://URI] module in all of PHP-dom (how pathetic is that for a language as web-centric as it?).</p> <p>First, I tried writing this with mostly string functions, but that was too painful, so I switched to doing it using regexes, even though they’re piss-poorly supported. After a while, my eyes glazed over, so I resorted to writing and debugging the code in Perl first, and then compiling it down to <del>machine code</del> <ins>PHP</ins>.</p> <p>I thought I’d post this byproduct here for posterity, in case someone needs it in some circumstance.</p> <p>Tests included, but I’m pretty sure they’re not extensive enough to be 100% confident in the code. Proceed at your discretion.</p> <c> sub abs_url { my ( $relative, $base ) = @_; return $relative if $relative =~ m{ \A http:// }ix; my ( $host, $hostrelative_abs ) = $base =~ m{ \A http:// # skip scheme ([^/]*) # capture hostname /* # skip front slashes (.*?) # capture everything that follows, but [^/]* # leave out the optional final non-directory component \z }ix; $hostrelative_abs = '' if $relative =~ m!^/!; my $abs_url = join '/', $host, $hostrelative_abs, $relative; # replace '//' or '/./' with '/' 1 while $abs_url =~ s{ / \.? (?=/|\z) }{}x; # remove '/foo/..' (but be careful to skip '/../..') 1 while $abs_url =~ s{ / (?!\.\.) [^/]+ / \.\. (?=/|\z) }{}x; return "http://$abs_url"; } </c> <c> use Test::More; my @test = qw( /foo/bar.gif http://www.example.com/baz/quux/ http://www.example.com/foo/bar.gif foo/bar.gif http://www.example.com/baz/quux/ http://www.example.com/baz/quux/foo/bar.gif ../../bar.gif http://www.example.com/baz/quux/ http://www.example.com/bar.gif foo/bar.gif http://www.example.com/baz/quux http://www.example.com/baz/foo/bar.gif ../foo/bar.gif http://www.example.com/baz/quux http://www.example.com/foo/bar.gif ../../foo/bar.gif http://www.example.com/baz/quux/qux http://www.example.com/foo/bar.gif ); plan tests => @test / 3; do { is abs_url( $test[0], $test[1] ), $test[2]; splice @test, 0, 3 } while @test; </c>