a lexer is a program that runs through text and converts it into something you can use in your program an example would be a program that lets you input something like 5x^5 - 3x^3 + 20x^2 +5x -15. it would convert all of those into tokens that your program can use to output a solution. at least this is what i believe it is.
>So you mean it have nothing to do with human readable part.
Pretty much. We're talking about individual tokens such as operators, numbers, identifiers, etc... The parser should simply be able to take a token and work with it rather than try to recognize what it represents first.