Description:
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preg_replace offers the e modifier to evaluate a replacement string as PHP code and use the result of that code for the replacement. This is a very powerful feature. There is, however one problem:

If you want to get the string found by preg_replace things can get complicated, dangerous, even impossible:

This should replace anything between a and b by giving it's strlen.

$search[0]="/a(.*)b/e";
$replace[0]="strlen('\\1')";
$result=preg_replace($search, $replace, $_GET['in']);

Obviously it is possible to do very bad things here, because $_GET['in'] might be a string like: "');dosthbad();$a=('".


Expected result:
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It would be good to have a predefined variable available inside the eval'ed PHP code, that just contains all the values for the parenthesis. Assume this is called $found. Then one could write something like this:

$search[0]="/a(.*)b/e"; // same as before
$replace[0]='strlen($found[1])'; // <- changed
$result=preg_replace($search, $replace, $_GET['in']); // same

And this time things would be safe. Possibly it might be useful to introduce this functionality under a different modifier, but I think it would be a significant improvement for many applications.