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compiler-constructionflex-lexerlexical-analysis

A weird situation about matching in flex

I am writing a scanner in flex, and I have the following two definitions:

%%
"int" printf("JUST_INT");
"int"[ \t\n]+"matrix" printf("MATRIX_INT");
[A-Za-z][A-Za-z0-9]*  printf("IDENTIFIER");
%%

When the input to the scanner is int matrixM = 3; the output is MATRIX_INT IDENTIFIER. It sees the input as int matrix M=3. But actually, this is not a matrix, the name of identifier is matrixM. Output should be JUST_INT IDENTIFIER. Why is this happening? Is that because of my definitions?

Solution

  • It's not a good idea to have rules that scan across spaces like "int"[ \t\n]+"matrix". Just return "int", "matrix", and IDENTIFIER as three separate tokens and let the parser sort it out. It's much better at that than flex is. flex just does the first longest match.