Regex适用于在线测试，但不适用于preg_match_all

Question

I have the following code to extract parameter name and value from string, here it is : (yes, regex has to be this long, it has other purposes)

    $sample = 'href="http://google.com/"';
    $reg = "#([a-zA-Z\-\/]+)\s*(?:=\s*(?:\"([^\">]*)\"?|'([^'>]*)'?|([^'\"\s]*)))?#S";
    preg_match_all($reg, $sample, $m);
    $result = print_r($m, true);
    echo $result;

which returns this:

Array ( [0] => Array ( [0] => href="http://google.com/ )
        [1] => Array ( [0] => href )
        [2] => Array ( [0] => http://google.com/ ) 
        [3] => Array ( [0] => )
        [4] => Array ( [0] => ) )

And it works fine.The problem is that I can also have strings with parameter values escaped, something like this:

$sample = 'href="\http://google.com/\"';

So I had to modify the regex, adding "\?" to allow one backalash before the quotes, and it looks something like this:

$sample = 'href="http://google.com/"';
$reg = "#([a-zA-Z\-\/]+)\s*(?:=\s*(?:\\?\"([^\">]*)\"?|'([^'>]*)'?|([^'\"\s]*)))?#S";
preg_match_all($reg, $sample, $m);
$out = print_r($m, true);
echo $out;

So I tried this new regex in a few online testers, and all of them returned correct result. However, preg_match_all returns this:

 Array ( [0] =>
          Array ( [0] => href= 
                  [1] => http 
                  [2] => //google [3] => com/ ) 
         [1] => Array ( 
                  [0] => href 
                  [1] => http 
                  [2] => //google
                  [3] => com/ ) 
          [2] => Array ( 
                  [0] => 
                  [1] => 
                  [2] =>
                  [3] => ) 
         [3] => Array ( 
                 [0] => 
                 [1] => 
                 [2] => 
                 [3] => ) 
         [4] => Array (
                 [0] => 
                 [1] => 
                 [2] => 
                 [3] => ) )

So why this second regex doesn't work as expected, but it works in online testing tools?

Answer 1

Without trying to rewrite your expression, to match a literal backslash you must use four backslashes:

\\\\?