PHP正则表达式的帮助。 的preg_replace

I have four words A,B,C,D. I want to replace all occurrences of A B or A C with A D in given sentence.

I have written this preg_replace("/([A])\s[C|B]/i", "\1 D",$sentence);

But it is not giving correct output. Where I am going wrong?

\ within " are escaping the string special characters, not the regexp special characters. Either you ' as string delimiter or double \ characters:

preg_replace('/([A])\s[C|B]/i', '\1 D', $sentence);