Disable BBcode encoding within a latex tag

Hello, I have the following bb function

function bb($string) {
  $string = trim($string);
  $search = array(
    '@\<(?i)latex\>(.*?)\</(?i)latex\>@si',
    '@\<(?i)b\>(.*?)\</(?i)b\>@si',
    '@\<(?i)i\>(.*?)\</(?i)i\>@si',
    '@\<(?i)u\>(.*?)\</(?i)u\>@si',
    '@\<(?i)blockquote\>(.*?)\</(?i)blockquote\>@si',
    '@\<(?i)pre\>(.*?)\</(?i)pre\>@si'
  );
  $replace = array(
    '<img src="cgi/mathtex.cgi?\\1">',
    '<b>\\1</b>',
    '<i>\\1</i>',
    '<u>\\1</u>',
    '<blockquote>\\1</blockquote>',
    '<pre>\\1</pre>'
  );
  $string = strtr($string, array("\n" => '<br>'));
  $string = preg_replace($search ,$replace, $string);
  return $string;
}

The problem is that if one enters newlines within latex tags (to make latex more clearly arranged), the function replaces the \n by <br>, which then becomes of course invalid latex code. How to disable the encoding of code which is within latex tags?