regex - Simulate php array language construct or parse with regexp?

Question

From an external source I'm getting strings like

array(1,2,3)

but also a larger arrays like

array("a", "b", "c", array("1", "2", array("A", "B")), array("3", "4"), "d")

I need them to be an actual array in php. I know I could use eval but since it are untrusted sources I'd rather not do that. I also have no control of the external sources.

Should I use some regular expressions for this (if so, what) or is there some other way?

Answer 1

Whilst writing a parser using the Tokenizer which turned out not as easy as I expected, I came up with another idea: Why not parse the array using eval, but first validate that it contains nothing harmful?

So, what the code does: It checks the tokens of the array against some allowed tokens and chars and then executes eval. I do hope I included all possible harmless tokens, if not, simply add them. (I intentionally didn't include HEREDOC and NOWDOC, because I think they are unlikely to be used.)

function parseArray($code) {
    $allowedTokens = array(
        T_ARRAY                    => true,
        T_CONSTANT_ENCAPSED_STRING => true,
        T_LNUMBER                  => true,
        T_DNUMBER                  => true,
        T_DOUBLE_ARROW             => true,
        T_WHITESPACE               => true,
    );
    $allowedChars = array(
        '('                        => true,
        ')'                        => true,
        ','                        => true,
    );

    $tokens = token_get_all('<?php '.$code);
    array_shift($tokens); // remove opening php tag

    foreach ($tokens as $token) {
        // char token
        if (is_string($token)) {
            if (!isset($allowedChars[$token])) {
                throw new Exception('Disallowed token ''.$token.'' encountered.');
            }
            continue;
        }

        // array token

        // true, false and null are okay, too
        if ($token[0] == T_STRING && ($token[1] == 'true' || $token[1] == 'false' || $token[1] == 'null')) {
            continue;
        }

        if (!isset($allowedTokens[$token[0]])) {
            throw new Exception('Disallowed token ''.token_name($token[0]).'' encountered.');
        }
    }

    // fetch error messages
    ob_start();
    if (false === eval('$returnArray = '.$code.';')) {
        throw new Exception('Array couldn't be eval()'d: '.ob_get_clean());
    }
    else {
        ob_end_clean();
        return $returnArray;
    }
}

var_dump(parseArray('array("a", "b", "c", array("1", "2", array("A", "B")), array("3", "4"), "d")'));

I think this is a good comprimise between security and convenience - no need to parse yourself.

For example

parseArray('exec("haha -i -thought -i -was -smart")');

would throw exception:

Disallowed token 'T_STRING' encountered.