Yes, you can use regular limes to make a key lime pie, although it will not have the exact same flavor. Key limes have a slightly sweeter and more distinct flavor than regular limes. Regular limes may create a slightly more bitter tasting filling. If you want the authentic key lime pie taste, it's best to use key limes or store-bought key lime juice. I highly recommend using the key lime juice as this makes this recipe much simpler to make. By using store-bought juice, you don’t have to juice a bunch of limes by hand. Remember that we need 3/4 cup of key lime juice for this recipe. That would be a lot of limes to squeeze to get the same amount of lime juice!