php 替换字符串中第N次出现的字符代码

<?php 
/* 
 * $text是输入的文本； 
 * $word是原来的字符串； 
 * $cword是需要替换成为的字符串； 
 * $pos是指$word在$text中第N次出现的位置，从1开始算起 
 *
 */
function changeNstr($text,$word,$cword,$pos=1){ 
 $text_array=explode($word,$text); 
 $num=count($text_array)-1; 
 if($pos>$num){ 
  return "the number is too big!or can not find the $word"; 
 } 
 $result_str=''; 
 for($i=0;$i<=$num;$i++){ 
  if($i==$pos-1){ 
   $result_str.=$text_array[$i].$cword; 
  }else{ 
   $result_str.=$text_array[$i].$word;
  } 
 } 
 return rtrim($result_str,$word); 
} 
$text='hello world hello pig hello cat hello dog hello small boy'; 
$word='hello'; 
$cword='good-bye'; 
echo changeNstr($text,$word,$cword,3); 
//输出：hello world hello pig good-bye cat hello dog hello small boy
?> 

<?php
$regex = "/(,|，|||｜)/i";
$test = "河北，石家庄，北京,上海|天津｜｜｜重庆｜保定,,,河南， ";
$result = preg_replace($regex," ",$test);
print_r($result);
?>