Help me to improve HTML to BBcode parse

[kevin28]

Hello!

To convert the string (with BBcode) to HTML I use preg_replace below

$string="[B]Bold [/B]Text Normal Text";
$pattern = '/\[b\](.*?)\[\/b\]/i';					
$replace = '<b>$1</b>';
$output_html = preg_replace($pattern, $replace, $string);
print $output_html ; //Out put <b>Bold </b>Text Normal Text

But what if user write like this

$string="[B]Bold [B]Text[/B] Extra[/B] Normal Text";

my preg_replace code above will output

<b>Bold [B]Text</b> Extra[/B] Normal Text

I expect the html code output like this

<b>Bold Text Extra</b> Normal Text

Below is what I want

If user enter string: 

[b]Bold [b]Text[/b] Extra[/b] Normal Text

Will output HTML: 

<b>Bold Text Extra</b> Normal Text

If user enter string

[b]Bold Text[/b] Extra Normal Text [b]Bold again[/b] Normal Text

will output to 

<b>Bold Text</b> Extra Normal Text <b>Bold again</b> Normal Text

Can you guys help me ?

Thanks