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Lex

Lex. Lex: a lexical analyzer. A Lex program recognizes strings For each kind of string found the lex program takes an action. Output. Identifier: Var Operand: = Integer: 12 Operand: + Integer: 9 Semicolumn: ; Keyword: if Parenthesis: ( Identifier: test. Input. Var = 12 + 9;

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Lex

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  1. Lex Costas Busch - LSU

  2. Lex: a lexical analyzer • A Lex program recognizes strings • For each kind of string found the lex program takes an action Costas Busch - LSU

  3. Output Identifier: Var Operand: = Integer: 12 Operand: + Integer: 9 Semicolumn: ; Keyword: if Parenthesis: ( Identifier: test .... Input Var = 12 + 9; if (test > 20) temp = 0; else while (a < 20) temp++; Lex program Costas Busch - LSU

  4. In Lex strings are described with regular expressions Lex program Regular expressions “+” “-” “=“ /* operators */ “if” “then” /* keywords */ Costas Busch - LSU

  5. Lex program Regular expressions (0|1|2|3|4|5|6|7|8|9)+ /* integers */ (a|b|..|z|A|B|...|Z)+ /* identifiers */ Costas Busch - LSU

  6. integers (0|1|2|3|4|5|6|7|8|9)+ [0-9]+ Costas Busch - LSU

  7. identifiers (a|b|..|z|A|B|...|Z)+ [a-zA-Z]+ Costas Busch - LSU

  8. Each regular expression has an associated action (in C code) Examples: Regular expression Action linenum++; \n prinf(“integer”); [0-9]+ [a-zA-Z]+ printf(“identifier”); Costas Busch - LSU

  9. Default action: ECHO; Prints the string identified to the output Costas Busch - LSU

  10. A small lex program %% [ \t\n] ; /*skip spaces*/ [0-9]+ printf(“Integer\n”); [a-zA-Z]+ printf(“Identifier\n”); Costas Busch - LSU

  11. Output Input Integer Identifier Identifier Integer Integer Integer 1234 test var 566 78 9800 Costas Busch - LSU

  12. %{ int linenum = 1; %} Another program %% [ \t] ; /*skip spaces*/ \n linenum++; prinf(“Integer\n”); [0-9]+ printf(“Identifier\n”); [a-zA-Z]+ . printf(“Error in line: %d\n”, linenum); Costas Busch - LSU

  13. Output Input Integer Identifier Identifier Integer Integer Integer Error in line: 3 Identifier 1234 test var 566 78 9800 + temp Costas Busch - LSU

  14. Lex matches the longest input string Regular Expressions “if” “ifend” Example: ifend if Input: Matches: “ifend” “if” Costas Busch - LSU

  15. Internal Structure of Lex Lex Minimal DFA Regular expressions NFA DFA The final states of the DFA are associated with actions Costas Busch - LSU

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