$patterns = array(
'#[b]s*(.*?)s*[/b]#s',
'#[u]s*(.*?)s*[/u]#s',
'#[strike]s*(.*?)s*[/strike]#s',
'#[em]s*(.*?)s*[/em]#s',
'#[red]s*(.*?)s*[/red]#s',
'#[green]s*(.*?)s*[/green]#s',
'#[yellow]s*(.*?)s*[/yellow]#s',
);

$replacements = array(
'<b>$1</b>',
'<u>$1</u>',
'<strike>$1</strike>',
'<em>$1</em>',
'<red>$1</red>',
'<green>$1</green>',
'<yellow>$1</yellow>',
);

$text = preg_replace($patterns, $replacements, $text);