String Input, via tellraw
So it actually is possible to get textual input in minecraft, though you couldnt do much with it. WARNING: THE FOLLOWING IS COMPLEX.
Lets start with the actual string input. This can be achieved through a simple keyboard tellraw command (like the one in this video). Each key can be linked to a trigger objective where the key determines the value of the score in that objective. Command blocks would then determine what key it is, and summon an entity with that name at a certain position. The system would then detect an entity there, and move it over by one block. This repeats, moving each entity over by one block until a row of "character" entities is made. Next, to use this information, you can issue specific testfors and executes to determine the string.
In this case, a command could be
/execute @e[0,0,0,r=0,type=Chicken,name=A] ~ ~ ~1 execute @e[r=0,type=Chicken,name=B] ~ ~ ~1 execute @e[r=0,type=Chicken,name=e] ~ ~ ~1 execute @e[r=0,type=Chicken,name=2] ~ ~ ~ say TRUE, for example.
Create a tellraw keyboard that spawns in an entity chain, adding to the chain in each click, and perform execute commands to test string qualities.
Note: If this answer gets enough votes I will make a fully working example system to prove this concept.