<?php  
$str1 = '%100'; 
$str2 = '%100\%';
$str3 = "100\\\\%";
$str4 = "%100\\%%";
$str5 = '100\%';
$str6 = "100\\\\\\\\%";
$str7 = "100\\\\\\%";
$str8 = "100\\\\\\\\\\\\%";
$str9 = "100\\\\\\\\\\\\\\%";
 
$reg_exp=  '/^%|[^\x5C]%|[^\x5C](\x5C\x5C)+%/';
 
echo $str1.' = '.preg_match($reg_exp, $str1).', ';
echo $str2.' = '.preg_match($reg_exp, $str2).', ';
echo $str3.' = '.preg_match($reg_exp, $str3).', ';
echo $str4.' = '.preg_match($reg_exp, $str4).', ';
echo $str5.' = '.preg_match($reg_exp, $str5).', ';
echo $str6.' = '.preg_match($reg_exp, $str6).', ';
echo $str7.' = '.preg_match($reg_exp, $str7).', ';
echo $str8.' = '.preg_match($reg_exp, $str8).', ';
echo $str9.' = '.preg_match($reg_exp, $str9).', ';
?>

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