<script> 

function getOpPrec(op) {

    switch(op) {

case '*': case '/':

return 5;

case '+': case '-':

return 3;

case '(':

return 1;

}

}

function whoPrecOp(op1, op2) {

var op1Prec = getOpPrec(op1), op2Prec = getOpPrec(op2);

if (op1Prec > op2Prec) 

return 1;

else if (op1Prec < op2Prec) 

return -1;

else 

return 0;

}

function postfix(exp) { 

var dst = [], src = [], op = [],

src = exp.replace(' ','').match(/[\d\.]+|[+\-*()/]/g);

    while (src.length) {

var tok = src.shift();

if (!isNaN(tok)) {

dst.push(tok);

} else {

switch(tok) {

case '(':

op.push(tok);

break;

case ')':

while(1) {

var popOp = op.pop();

if (popOp == '(')

break;

dst.push(popOp);

}

break;

case '+': case '-':

case '*': case '/':

while (op.length && whoPrecOp(op[op.length-1], tok) > 0) 

dst.push(op.pop());

op.push(tok);

break;

}

}

}

while (op.length)

dst.push(op.pop());

return dst;

}

function evalExp(exp) {

var dst = [], op1, op2;

while (exp.length) {

var tok = exp.shift();

if (!isNaN(tok))

dst.push(tok);

else {

op2 = Number(dst.pop());

op1 = Number(dst.pop());

switch (tok) {

case '+':

dst.push(op1 + op2);

break;

case '-':

dst.push(op1 - op2);

break;

case '*':

dst.push(op1 * op2);

break;

case '/':

dst.push(op1 / op2);

break;

}

}

}

return dst;

}

function cal(src) {

document.getElementById('result').value = evalExp(postfix(src));

}

</script>

입력 <input type="text" id="input" name="src"  value="" onblur="cal(this.value)" /> =

결과 <input id="result" type="text" name="result" size="10" value="" />