在preg_match php中使用变量模式返回false

Question

I want check if a string contains a variable amount of generated words. This is the code I came up with, it stores the pattern in a variable which I put in the preg_match.

Result of this code: it returns true;

$string = "What a beatiful world";

$pieces = "#\b(" . implode("|",$pieces_arr). ")\b#";

$pieces outputs for example:

#\b(hello|you)\b#

into:

if(preg_match($pieces, $string)) {  

        echo "piece found in string!!";

}