Flex/Bison studying.

lexer.l

%{
#include "parser.h"
%}

%option noyywrap

%%

"+"     { return ADD; }
"-"     { return SUB; }
"*"     { return MUL; }
"/"     { return DIV; }
"|"     { return ABS; }
"!"     { return NEGATE; }
"("     { return OP; }
")"     { return CP; }
-?[0-9]+  { yylval = atof(yytext); return NUMBER;  }
\n      { return EOL; }
[ \t]   { /* ignore */ }
.       { /* ignore */ }

%%

Makefile

all: calc

calc: parser lexer
	CC lexer.c parser.c -o calc

lexer:
	flex -o lexer.c lexer.l

parser:
	bison -d -o parser.c parser.y

clean:
	rm -f parser.[h,c]
	rm -f lexer.c

parser.y

%{

#include <stdio.h>

extern int yylex(void);
extern int yyerror(char *s);

%}

%token NUMBER
%token ADD SUB MUL DIV 
%token ABS NEGATE
%token EOL
%token OP CP

%%

calclist: /*nothing*/
  | calclist exp EOL { printf("= %d\n", $2); }
  ;

exp: factor
  | exp ADD factor { $$ = $1 + $3; }
  | exp SUB factor { $$ = $1 - $3; }
  ;

factor: term
  | factor MUL term { $$ = $1 * $3; }
  | factor DIV term { $$ = $1 / $3; } 
  ;

term: NUMBER 
  | ABS term { $$ = $2 >= 0 ? $2 : - $2; }
  | NEGATE term { $$ = $2 * -1; }
  | OP exp CP { $$ = $2; }
  ;

%%

int main(int argc, char **argv)
{
  yyparse();
  return 0;
}

int yyerror(char *s)
{
  fprintf(stderr, "error: %s\n", s);
  return 0;
}