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function is_valid_date($value, $format = 'dd/mm/yyyy'){
    if(strlen($value) >= 6 && strlen($format) == 10){

        // find separator. Remove all other characters from $format
        $separator_only = str_replace(array('m','d','y'),'', $format);
        $separator = $separator_only[0]; // separator is first character

        if($separator && strlen($separator_only) == 2){
            // make regex
            $regexp = str_replace('mm', '(0?[1-9]|1[0-2])', $format);
            $regexp = str_replace('dd', '(0?[1-9]|[1-2][0-9]|3[0-1])', $regexp);
            $regexp = str_replace('yyyy', '(19|20)?[0-9][0-9]', $regexp);
            $regexp = str_replace($separator, "\\" . $separator, $regexp);
            if($regexp != $value && preg_match('/'.$regexp.'\z/', $value)){

                // check date
                $arr=explode($separator,$value);
                $day=$arr[0];
                $month=$arr[1];
                $year=$arr[2];
                if(@checkdate($month, $day, $year))
                    return true;
            }
        }
    }
    return false;
}