What are the security implications of using the 'e' modifier in PHP regular expressions?

The 'e' modifier in PHP regular expressions allows for the execution of arbitrary PHP code within the regex, which can pose a significant security risk if user input is involved. To solve this issue, it is recommended to avoid using the 'e' modifier and instead use other methods to achieve the desired functionality, such as using preg_replace_callback(). 

// Example of using preg_replace_callback() instead of 'e' modifier
$pattern = '/\b(\w+)\b/e';
$replacement = 'strtoupper("$1")';
$input = 'hello world';
$output = preg_replace_callback($pattern, function($matches) use ($replacement) {
    return eval('return ' . $replacement . ';');
}, $input);
echo $output;

Keywords

PHP regular expressions 'e' modifier security implications code injection

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