PHP preg_match返回false

在下面的代码中，为什么PHP preg_match返回false？注：本人已阅读过相同标题一些老问题，如我的问题，但背景是不同的PHP preg_match返回false

<html> 
    <head> 
    </head> 
    <body> 
     <?php 
     $str = "This is a test for PCRE."; 
     $ptrn = "This"; 
     $preg_match_result_arr; 
     $preg_match_result = preg_match($ptrn, $str, $preg_match_result_arr); 

     if (1 === $preg_match_result) 
     { 
      echo "The pattern has matched in the string:<br/>". 
       "the match is $preg_match_result_arr[0]<br/>"; 
     } 
     elseif (0 === $preg_match_result) 
     { 
      echo "The pattern has not matched in the string:<br/>"; 
     } 
     elseif (false === $preg_match_result) 
     { 
      echo "An error has occured in matching<br/>"; 
     } 

     ?> 
    </body> 
</html>

来源

2012-08-05 MTVS