我今天开始在StackOverflow的Careers上尝试应聘一些Remote的工作，遇到一道笔试题。

Code a simple Random Sentence Spinning Function(s) Or Class.

Basically you take this following variable Where the | is a delimiter
meaning either word could be shown up:

$rand_sentence = "{Please|Just} make this {cool|awesome|random} test sentence
{rotate {quickly|fast} and random|spin and be random}";

and the end result should be an output of the sentence like:
Please make this cool test sentence rotate fast and random.
OR
Just make this random test sentence spin and be random.
or any other random variation...

然后我就搜索了下“正则表达式嵌套该如何匹配”，找到了一个解决方案，$patten = '/{(?:[^{}]+|(?R))*}/';，然后写了一个程序。

<?php
$rand_sentence = "{Please|Just} make this {cool|awesome|random} test sentence
{rotate {quickly|fast} and random|spin and be random}";
$patten = '/{(?:[^{}]+|(?R))*}/';

echo randspin($rand_sentence);

function randspin($sentence) {
  global $patten;
  return preg_replace_callback($patten, "randpick",$sentence);
}

function randpick($input) {
  global $patten;
  $line = $input[0];
  $line = substr($line,1,strlen($line)-2);
  $matchCount = preg_match_all($patten,$line,$matches);
  if($matchCount>0) {
    return randspin($line);
  }else {

    $parts = explode("|",$line);
    $index = rand(0,count($parts));
    return $parts[$index];
  }
}

代码提交以后，我才发现我的解有点问题，可以匹配简单的{Please|Just}问题以及{rotate {quickly|fast} and be random}问题，但是random|spin这样的外面没有{和}的解决不了，而按照题目的要求其实也是要能解决的。

大家看该如何解决？我尝试了几个写法都不对。