Example

DIGIT [0-9]

ID [a-z][a-z0-9]*

{DIGIT}+ {printf("An integer:%s(%d)\n",yytext,atoi(yytext));}

{DIGIT}+"."{DIGIT}* {printf("A float: %s (%g)\n",yytext,atof(yytext));}

if|then|begin|end|procedure|function {printf("A keyword: %s\n",yytext);}

{ID} {printf("An identifier %s\n",yytext);}

"+"|"-"|"*"|"/" {printf("An operator %s\n",yytext);}

"{"[^}\n]*"}" {/* eat up one-line comments */}

[ \t\n]+ {/* eat up white space */}

. {printf("Unrecognized character: %s\n", yytext);}

int main(int argc, char *argv[]){

++argv, --argc;

if (argcɬ) yyin = fopen(argv[0],"r"); else yyin = stdin;

yylex();

}