Embed BBCode within itself not working

Question

So I was working on a simple php bbcode system, though doing something like [center][b][i]Text[/i][/b][/center] Just doesn't parse. It leaves a couple out. I was curious if someone could give me a tip? or point me in the direction to learn how to fix this issue.

Sample of code:

function bbcode($str) {
  $patterns = array();
  $patterns = array(
    '#\[b\](.*?)\[/b\]#',
    '#\[i\](.*?)\[/i\]#',
    '#\[u\](.*?)\[/u\]#',
    '#\[color=(.*?)\](.*?)\[/color\]#',
    '#\[img\](.*?)\[/img\]#',
    '#\[center\](.*?)\[/center\]#',
    '#\n#'

  );

  $replacements = array();
  $replacements = array(
    '<strong>$1</strong>',
    '<em>$1</em>',
    '<u>$1</u>',
    '<span style="color:$1">$2</span>',
    '<img src="$1" style="max-width:45%;"></img>',
    '<center>$1</center>',
    '<br/>'
  );


  return preg_replace($patterns, $replacements, $str);
}