Theoretical view.
S = {a, b}, L = {anbn: n ³ 0}
Question: aabb Î L ?
Practical view.
S = {begin, end, id, :=, *, ; , ...},
Question: begin id := id * id; end; Î LPascal ?
LPascal = Programming Language Pascal
YES: Program is OK Þ
Create a target program  
NO: Program is not OK Þ
Handle the errors
Languages and Compilers
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