基于模式解析字符串

I am using php 5 to parse a string. My input string looks like the following:

{Billion is|Millions are|Trillion is} {an extremely |a| a generously | a very} { tiny|little |smallish |short |small} stage in a vast {galactic| |large|huge|tense|big |cosmic} {universe|Colosseum|planet|arena}.

Find below my minimum viable example:

<?php

function process($text)
{
    return preg_replace_callback('/\[(((?>[^\[\]]+)|(?R))*)\]/x', array(
        $this,
        'replace'
    ), $text);
}
function replace($text)
{
    $text  = $this->process($text[1]);
    $parts = explode('|', $text);
    return $parts[array_rand($parts)];
}

$text = "{Billion is|Millions are|Trillion is} {an extremely |a| a generously | a very} { tiny|little |smallish |short |small} stage in a vast {galactic| |large|huge|tense|big |cosmic} {universe|Colosseum|planet|arena}.";

$res = process($text);

echo $res;

As you can see I am trying to parse the following pattern f.ex.: {Billion is|Millions are|Trillion is} using the above regex, /\[(((?>[^\[\]]+)|(?R))*)\]/x.

As a result I am getting the same string as inputted. I would like to get as an output for example:

Billion is a very little stage in a vast huge arena.

Any suggestions what I am doing wrong?

How would your current code generate anything.

Your regex doesn't fit. It matches nested bracketed stuff and not braced. Try{([^}]*)} for capturing everything inside {...} to $m[1] if there are no nested braces.
Read about preg_replace_callback(). The second argument can not be an array.

A working code with some further adjustments could look like this:

function process($text) {
  return preg_replace_callback('/{([^}]*)}/', 'replace', $text);
}

function replace($m) {
  $parts = explode('|', $m[1]);
  shuffle($parts);
  return $parts[0];
}

$text = "{Billion is|Millions are|Trillion is} {an extremely|a|a generously|a very} {tiny|little|smallish|short|small} stage in a vast {galactic||large|huge|tense|big|cosmic} {universe|Colosseum|planet|arena}.";

echo process($text);

Billion is a generously short stage in a vast Colosseum.

Here is a demo at eval.in

(you can also use an anonymous function if PHP >= 5.3)