更換爲B，或B對於A

Question

有使用preg_replace更換A爲B如果A存在的一種方式，或爲B如果AB存在？

喜歡的東西：

preg_replace('/ORDER BY field (ASC|DESC)/', 'ORDER BY field (***the alternative not matched***)'); 

變換任何ORDER BY field ASC爲ORDER BY field DESC，任何ORDER BY field DESC到ORDER BY field ASC。

Answer 1

您正在尋找的功能是preg_replace_callback。

例子：

function replace_asc_desc($matches) 
{ 
    return 'ORDER BY field ' . ($matches[1] == 'ASC' ? 'DESC' : 'ASC'); 
} 

$string = 'ORDER BY field DESC'; 

echo preg_replace_callback('/ORDER BY field (ASC|DESC)/', 'replace_asc_desc', $string);