Testing query string unicode handling in Perl - perl

Question

I'm trying to write up an example of testing query string parsing when I got stumped on a Unicode issue. In short, the letter "Omega" (Ω) doesn't seem to be decoded correctly.
Unicode: U+2126
3-byte sequence: \xe2\x84\xa6
URI encoded: %E2%84%A6
So I wrote this test program verify that I could "decode" unicode query strings with URI::Encode.
use strict;
use warnings;
use utf8::all; # use before Test::Builder clones STDOUT, etc.
use URI::Encode 'uri_decode';
use Test::More;
sub parse_query_string {
my $query_string = shift;
my #pairs = split /[&;]/ => $query_string;
my %values_for;
foreach my $pair (#pairs) {
my ( $key, $value ) = split( /=/, $pair );
$_ = uri_decode($_) for $key, $value;
$values_for{$key} ||= [];
push #{ $values_for{$key} } => $value;
}
return \%values_for;
}
my $omega = "\N{U+2126}";
my $query = parse_query_string('alpha=%E2%84%A6');
is_deeply $query, { alpha => [$omega] }, 'Unicode should decode correctly';
diag $omega;
diag $query->{alpha}[0];
done_testing;
And the output of the test:
query.t ..
not ok 1 - Unicode should decode correctly
# Failed test 'Unicode should decode correctly'
# at query.t line 23.
# Structures begin differing at:
# $got->{alpha}[0] = 'â¦'
# $expected->{alpha}[0] = 'Ω'
# Ω
# â¦
1..1
# Looks like you failed 1 test of 1.
Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/1 subtests
Test Summary Report
-------------------
query.t (Wstat: 256 Tests: 1 Failed: 1)
Failed test: 1
Non-zero exit status: 1
Files=1, Tests=1, 0 wallclock secs ( 0.03 usr 0.01 sys + 0.05 cusr 0.00 csys = 0.09 CPU)
Result: FAIL
It looks to me like URI::Encode may be broken here, but switching to URI::Escape and using the uri_unescape function reports the same error. What am I missing?

Answer 1

the URI encoded characters simply represents utf-8 sequences, and URI::Encode and URI::Escape simply decodes them into a utf-8 byte string, and neither of them decode the bytestrings as UTF-8 (which is a correct behavior as a generic URI decoding library).
Put it another way, your code basically does:
is "\N{U+2126}", "\xe2\x84\xa6" and that will fail, since upon comparison, perl upgrades the latter as a 3-character-length latin-1 strings.
You have to manually decode the input value with Encode::decode_utf8 after uri_decode, or instead compare encoded utf8 byte sequence.

Answer 2

URI escaping represents octets and knows nothing about character encodings, so you have to decode from UTF-8 octets to characters yourself, e.g.:
$_ = decode_utf8(uri_decode($_)) for $key, $value;

Answer 3

The problem can be seen in incorrect details in your own explanation of the problem. What you are dealing with is really:
Unicode codepoint: U+2126
UTF-8 encoding of codepoint: \xe2\x84\xa6
URI encoding of UTF-8 encoding of codepoint: %E2%84%A6
The problem is that you only undid one of the encodings.
Solutions have already been presented. I just wanted to present an alternate explanation.

Answer 4

I'd recommend that you have a look at Why does modern Perl avoid UTF-8 by default? for a thorough discussion on this topic.
I would add to the discussion there:
You'll notice a lot of odd glyphs on the page. This was intentional on the part of the author.
I've tried the Symbola font recommended in the thread and it looked horrible on Win 7. YMMV.
Reading Why does modern Perl avoid UTF-8 by default? too frequently may lead to depression and lingering doubts about your life choices.