php中怎么使用正则表达式的反向引用

[sunshaobei1]

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？

[xuzuning]

preg_match("/([ab])\\1/", "abcdebbcde", $m);
print_r($m);

Array
(
    [0] => bb
    [1] => b
)