preg_match help

I am currently devoloping a php quation plot system, as shown here:
http://w3senteret.com/ymse/kalkulator/graf.php

I need a preg match expression to help me deal with some cases of math:

x^2, x^3, x⁴ and xⁿ need to be pow(x, n)
2x, 3x, 4x and nx need to be n*x

Can anyone help me out with theese two expressions?

Like this?

$regexp = array(
   '#x\^(\d+)#' => 'pow(x, $1)',
   '#(\d+)x#' => '$1*x'
);
$text = preg_replace(array_keys($regexp), $regexp, $text);

Thank you very much🙂
Thats amazing=)