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Lex

Lex. Compiling Sequence. Lex. letter(letter|digit)* start: goto state0 state0: read c if c = letter goto state1 goto state0 state1: read c if c = letter goto state1 if c = digit goto state1 goto state2 state2: accept string. Lex Structure.

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Lex

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  1. Lex

  2. Compiling Sequence

  3. Lex • letter(letter|digit)* start: goto state0 state0: read c if c = letter goto state1 goto state0 state1: read c if c = letter goto state1 if c = digit goto state1 goto state2 state2: accept string

  4. Lex Structure … definitions … %% … rules … %% … subroutines … • example %% /* match everything except newline */ . ECHO; /* match newline */ \n ECHO; %% int yywrap(void) { return 1; } int main(void) { yylex(); return 0; }

  5. Example %{ int yylineno; %} %% ^(.*)\n printf("%4d\t%s", ++yylineno, yytext); %% int main(int argc, char *argv[]) { yyin = fopen(argv[1], "r"); yylex(); fclose(yyin); } gdueck@ash lex]$ lex lino.l [gdueck@ash lex]$ ls -l total 40 -rw-r--r-- 1 gdueck fcsprofs 36053 Apr 9 10:15 lex.yy.c -rw-r--r-- 1 gdueck fcsprofs 215 Apr 9 10:13 lino.l [gdueck@ash lex]$ cc lex.yy.c [gdueck@ash lex]$ ls -l total 64 -rwxr-xr-x 1 gdueck fcsprofs 20556 Apr 9 10:16 a.out -rw-r--r-- 1 gdueck fcsprofs 36053 Apr 9 10:15 lex.yy.c -rw-r--r-- 1 gdueck fcsprofs 215 Apr 9 10:13 lino.l

  6. Count Identifiers digit [0-9] letter [A-Za-z] %{ int count; %} %% /* match identifier */ {letter}({letter}|{digit})* count++; . \n %% int yywrap(void) { return 1; } int main(void) { yylex(); printf("number of identifiers = %d\n", count); return 0; }

  7. quoted strings %{ char *yylval; #include <string.h> %} %% \"[^"\n]*["\n] { yylval = strdup(yytext+1); if (yylval[yyleng-2] != '"') warning("improperly terminated string"); else yylval[yyleng-2] = 0; printf("found '%s'\n", yylval); }

  8. %{ char buf[100]; char *s; %} %x STRING %% \" { BEGIN STRING; s = buf; } <STRING>\\n { *s++ = '\n'; } <STRING>\\t { *s++ = '\t'; } <STRING>\\\" { *s++ = '\"'; } <STRING>\" { *s = 0; BEGIN 0; printf("found '%s'\n", buf); } <STRING>\n { printf("invalid string"); exit(1); } <STRING>. { *s++ = *yytext; }

  9. Yacc yacc -d bas.y # create y.tab.h, y.tab.c lex bas.l # create lex.yy.c cc lex.yy.c y.tab.c -obas.exe # compile/link

  10. calc.y %token INTEGER %% program: program expr '\n' { printf("%d\n", $2); } | ; expr: INTEGER { $$ = $1; } | expr '+' expr { $$ = $1 + $3; } | expr '-' expr { $$ = $1 - $3; } ; %% int yyerror(char *s) { fprintf(stderr, "%s\n", s); return 0; } int main(void) { yyparse(); return 0; }

  11. calc.l %{ #include "y.tab.h" %} %% [0-9]+ { yylval = atoi(yytext); return INTEGER; } [-+\n] return *yytext; [ \t] ; /* skip whitespace */ . yyerror("invalid character"); %% int yywrap(void) { return 1; }

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