Description:
------------
The perl-compatible regex matcher somehow matches a negated character class if the string to match contains the same characters, and in the same order, as the character class.

I don't see why /[^ab]/ should match "ab" but not "ba". Perl agrees:
$ perl -e 'for("ab","ba") { print "$_: "; if( /[^ab]/ ) {print "match\n"} else {print "no match\n"}}'
ab: no match
ba: no match


Test script:
---------------
<?php

echo "expecting 0, 0, 0, 0, 0\n";

echo preg_match( '[^ab]', 'ba' ) . "\n";        // expected no match, passes
echo preg_match( '[^ab]', 'ab' ) . "\n";        // expected no match, fails
echo preg_match( '([^ab])', 'ab' ) . "\n";      // expected no match, passes

echo preg_match( '[^ab]', 'aba' ) . "\n";    // expected no match, fails
echo preg_match( '[^ab]', 'abb' ) . "\n";    // expected no match, fails
echo preg_match( '([^ab])', 'abb' ) . "\n";    // expected no match, passes

?>

Expected result:
----------------
expecting 0, 0, 0, 0, 0, 0
0
0
0
0
0
0

Actual result:
--------------
expecting 0, 0, 0, 0, 0, 0
0
1
0
1
1
0