What regular expression can I use in preg_split(); to match a comma but only if it hasn't got a space after it?

I.e. it'd match this: bla,bla no space after comma
..but not match this: bla, ooh look a space after comma

I'm not sure if it'd be easier to try and match the space, or just to check whether or not it has an alphanumeric character either side of the comma?

Me + regular expressions = poop.

Happy new year everyone :)

edit: resolved!