Hi Burt
Wordpress determine the content in the shortcode by the first end tag. For example,
[a]hello [a]world[/a] another[/a]
the first a shortcode end with the first [/a]. The result is the right string will not be considered a shortcode.
So the issue with columns is that one cannot nest columns of the same type as per your example. We do not know a way to get around this by php as this is how WP behaves as a core function.
But you can try use
if it turns out you really need to nest (and are not just testing to find holes!)
I think this matter has come up once or twice on the forum in the past as well.
So an example would be:
Morbi in sem quis dui placerat ornare. Pellentesque odio nisi, euismod in, pharetra a, ultricies in, diam. Sed arcu. Cras consequat. Vivamus magna. Cras in mi at felis aliquet. 
Alfonsino platy blue catfish trumpetfish gizzard shad cichlid swampfish, golden trout Canthigaster rostrata. Ridgehead drum hog sucker threadtail sucker rockweed gunnel spinyfin roosterfish New Zealand sand diver. Kuhli loach Atlantic eel whale catfish houndshark deep sea bonefish powen, lizardfish, whitetip reef shark, Black pickerel. Discus Cornish Spaktailed Bream olive flounder, candiru croaker climbing gourami driftfish bonytail chub monkfish kanyu yellowtail horse mackerel opah sunfish clown loach Redfish, bluefish Pacific argentine North American darter. Wolf-eel dwarf gourami grunt Rainbowfish marlin snake mackerel roanoke bass lancetfish convict blenny torrent fish beaked salmon Razorback sucker.
Surgeonfish Celebes rainbowfish, three spot gourami barbelless catfish, sandfish Atlantic cod, eelpout. Tidewater goby false brotula. Hamlet huchen sábalo grunt duckbill eel tench archerfish green swordtail lemon shark orange roughy frogfish sabertooth velvet catfish wrasse conger eel. Sand tiger, Rabbitfish: prickly shark tube-eye northern sea robin sábalo Pacific albacore Indian mul surgeonfish lionfish longfin dragonfish. Pacific trout manefish sand tilefish long-finned char silver dollar. Blue gourami swordfish longnose whiptail catfish Lost River sucker bocaccio goldeye megamouth shark louvar smoothtongue, barbelless catfish smooth dogfish medaka.
Yellowtail snapper shark roosterfish, sailfin silverside houndshark, Ganges shark porcupinefish, “weeverfish archerfish; convict cichlid: goldspotted killifish.” Thorny catfish ray southern flounder lionfish burbot neon tetra tube-snout triggerfish. Raccoon butterfly fish sleeper; hake Bombay duck tompot blenny sardine platyfish gunnel Mozambique tilapia, ridgehead Cherubfish anemonefish redlip blenny.
Tui chub sabertooth grouper, frogmouth catfish archerfish zebra lionfish, “eelpout ayu collared carpetshark, pirarucu turbot squirrelfish,” killifish. Hairtail yellowfin surgeonfish wobbegong flagtail threadfin pike conger bigscale fish eel-goby sandfish yellow perch, yellowbelly tail catfish. Giant sea bass, gouramie.”
Molly Miller airbreathing catfish mouthbrooder mrigal redtooth triggerfish lancetfish dartfish, armored searobin Canthigaster rostrata dorab angler, Antarctic icefish. Fathead sculpin cow shark priapumfish cuchia Atlantic trout bangus pearl perch amur pike alfonsino threadtail tilefish, pink salmon. X-ray tetra cat shark garpike weever butterflyfish Redhorse sucker flagfin Rattail, redtooth triggerfish. Southern sandfish chum salmon frogmouth catfish: duckbilled barracudina hatchetfish goby amago prickly shark gray reef shark, “aruana tube-eye olive flounder.” Redtooth triggerfish bristlemouth pigfish salamanderfish, sand dab–pipefish sand goby, pike conger mojarra ghost carp. Orbicular batfish redtooth triggerfish ray monkeyface prickleback snoek, flying characin snubnose eel lighthousefish wrymouth.
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