Despite this being a rather blatant attempt to abuse the Parry and Riposte ability, it will work - but you have to remember a few things:
Casting in combat specifically says that if something interrupts you then you must make a concentration check in order to continue the cast.
Attacks of Opportunity also very specifically say that they are an interrupt to whatever actions are happening.
Regardless of the outcome of the Attack of Opportunity, you will have to make a concentration check, with the DC decided by your GM.
The GM should be putting a fairly serious DC penalty on you, or denying this outright (as an ex-GM, this would be my decision), given that you are essentially dropping the spell casting in favour of doing physical damage, only to try and pick up the spell again straight after.
If I was still GMing you, I would only allow this if you were happy to be essentially burning up spells as a way to lure people in to attack you. You would lose the spell (save for very basic verbal ones), but you would gain an opening that you want.
Another thing to remember is that while Parry and Riposte is using up one of your own Attacks of Opportunity (and needs at least 2 panache points available - one to spend on Parry, one to keep in the bag for Riposte), the Riposte part is counted as an Immediate Action and therefore can only happen once per round - regardless of any Attacks of Opportunity stacking you may do. Parry as often as your panache points allow, but Riposte only the once.
One more thing, given that the roleplay element of the RPG seems to have been lost on you here - imagine you are actually in this fight for a moment. You are casting a spell and suddenly someone attacks you. Your spell has a somatic component - oh no, your new movement for parrying and riposte utterly destroys that spell process. Spell is now over and you have to start fresh.
Your spell has a material component - oh dear, that material that you already started using is now gone, or if it involved throwing things around you will have to pick it up again (unless you have more) to start the spell again.
Uh oh, there is a focus element to the spell. That specific prop that you need is suddenly in the wrong position from where you needed it in the spell casting - gotta start over fresh again.
The spell is all verbal?? Totally go for it! It's like shouting insults at someone in a fight, you're not going to stop just because someone swings at you. Unless you need to say something in some complex language that only you as some magic user know, and trying to do a different language in the middle of frantic combat may be somewhat taxing. Concentration check time!