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This script is using the PHP function preg_replace() on user input. This is not recommended as it can lead to various vulnerabilities. Consult "Web references" for more information about this problem.
The e modifier makes preg_replace() treat the replacement parameter as PHP code after the appropriate references substitution is done. If the regex pattern and the replacement strings are controlled by the user this can conduct to PHP code execution.
It is not recommended to use preg_replace() on user input.