php中怎么使用正则表达式的反向引用

使用preg_match 函数的话，匹配总是不成功

 <?php  
if (preg_match("/([ab])\1/", "abcdebbcde")) {
    echo "A match was found.";
} else {
    echo "A match was not found.";
}
?>

返回信息
A match was not found. 为什么？

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