A great JS formula parser - this is a fork

parser.js

/*
 Based on ndef.parser, by Raphael Graf(r@undefined.ch)
 http://www.undefined.ch/mparser/index.html
 
 Website  http://silentmatt.com/javascript-expression-evaluator/
*/

//  Added by TES 6/13/2011
//  re-define indexOf, because IE doesn't know it ...
//  from http://stellapower.net/content/javascript-support-and-arrayindexof-ie
	if (!Array.indexOf) {
	  Array.prototype.indexOf = function (obj, start) {
	    for (var i = (start || 0); i < this.length; i++) {
	      if (this[i] === obj) {
	        return i;
	      }
	    }
	    return false; //-1;
	  }
	}

var Parser = (function (scope) {
	function object(o) {
		function F() {}
		F.prototype = o;
		return new F();
	}


	function getindexOf( arr, value, start) {
		var istart = ( start === undefined ) ? 0 : start;
		for(var i=istart; i<arr.length; i++)
			if ( arr[i] === value ) return i;
		return -1;	
	}
	
	var TNUMBER = 0;
	var TOP1 = 1;
	var TOP2 = 2;
	var TVAR = 3;
	var TFUNCALL = 4;

	function Token(type_, index_, prio_, number_) {
		this.type_ = type_;
		this.index_ = index_ || 0;
		this.prio_ = prio_ || 0;
		this.number_ = (number_ !== undefined && number_ !== null) ? number_ : 0;
		this.toString = function () {
			switch (this.type_) {
			case TNUMBER:
				return this.number_;
			case TOP1:
			case TOP2:
			case TVAR:
				return this.index_;
			case TFUNCALL:
				return "CALL";
			default:
				return "Invalid Token";
			}
		};
	}

	function Expression(tokens, ops1, ops2, functions) {
		this.tokens = tokens;
		this.ops1 = ops1;
		this.ops2 = ops2;
		this.functions = functions;
	}

	// Based on http://www.json.org/json2.js
    var cx = /[\u0000\u00ad\u0600-\u0604\u070f\u17b4\u17b5\u200c-\u200f\u2028-\u202f\u2060-\u206f\ufeff\ufff0-\uffff]/g,
        escapable = /[\\\'\x00-\x1f\x7f-\x9f\u00ad\u0600-\u0604\u070f\u17b4\u17b5\u200c-\u200f\u2028-\u202f\u2060-\u206f\ufeff\ufff0-\uffff]/g,
        meta = {    // table of character substitutions
            '\b': '\\b',
            '\t': '\\t',
            '\n': '\\n',
            '\f': '\\f',
            '\r': '\\r',
            "'" : "\\'",
            '\\': '\\\\'
        };

	function escapeValue(v) {
		if (typeof v === "string") {
			escapable.lastIndex = 0;
	        return escapable.test(v) ?
	            "'" + v.replace(escapable, function (a) {
	                var c = meta[a];
	                return typeof c === 'string' ? c :
	                    '\\u' + ('0000' + a.charCodeAt(0).toString(16)).slice(-4);
	            }) + "'" :
	            "'" + v + "'";
		}
		return v;
	}

	Expression.prototype = {
		simplify: function (values) {
			values = values || {};
			var nstack = [];
			var newexpression = [];
			var n1;
			var n2;
			var f;
			var L = this.tokens.length;
			var item;
			var i = 0;
			for (i = 0; i < L; i++) {
				item = this.tokens[i];
				var type_ = item.type_;
				if (type_ === TNUMBER) {
					nstack.push(item);
				}
				else if (type_ === TVAR && (item.index_ in values)) {
					item = new Token(TNUMBER, 0, 0, values[item.index_]);
					nstack.push(item);
				}
				else if (type_ === TOP2 && nstack.length > 1) {
					n2 = nstack.pop();
					n1 = nstack.pop();
					f = this.ops2[item.index_];
					item = new Token(TNUMBER, 0, 0, f(n1.number_, n2.number_));
					nstack.push(item);
				}
				else if (type_ === TOP1 && nstack.length > 0) {
					n1 = nstack.pop();
					f = this.ops1[item.index_];
					item = new Token(TNUMBER, 0, 0, f(n1.number_));
					nstack.push(item);
				}
				else {
					while (nstack.length > 0) {
						newexpression.push(nstack.shift());
					}
					newexpression.push(item);
				}
			}
			while (nstack.length > 0) {
				newexpression.push(nstack.shift());
			}

			return new Expression(newexpression, object(this.ops1), object(this.ops2), object(this.functions));
		},

		substitute: function (variable, expr) {
			if (!(expr instanceof Expression)) {
				expr = new Parser().parse(String(expr));
			}
			var newexpression = [];
			var L = this.tokens.length;
			var item;
			var i = 0;
			for (i = 0; i < L; i++) {
				item = this.tokens[i];
				var type_ = item.type_;
				if (type_ === TVAR && item.index_ === variable) {
					for (var j = 0; j < expr.tokens.length; j++) {
						var expritem = expr.tokens[j];
						var replitem = new Token(expritem.type_, expritem.index_, expritem.prio_, expritem.number_);
						newexpression.push(replitem);
					}
				}
				else {
					newexpression.push(item);
				}
			}

			var ret = new Expression(newexpression, object(this.ops1), object(this.ops2), object(this.functions));
			return ret;
		},

		evaluate: function (values) {
			values = values || {};
			var nstack = [];
			var n1;
			var n2;
			var f;
			var L = this.tokens.length;
			var item;
			var i = 0;
			for (i = 0; i < L; i++) {
				item = this.tokens[i];
				var type_ = item.type_;
				if (type_ === TNUMBER) {
					nstack.push(item.number_);
				}
				else if (type_ === TOP2) {
					n2 = nstack.pop();
					n1 = nstack.pop();
					f = this.ops2[item.index_];
					nstack.push(f(n1, n2));
				}
				else if (type_ === TVAR) {
					if (item.index_ in values) {
						nstack.push(values[item.index_]);
					}
					else if (item.index_ in this.functions) {
						nstack.push(this.functions[item.index_]);
					}
					else {
						throw new Error("undefined variable: " + item.index_);
					}
				}
				else if (type_ === TOP1) {
					n1 = nstack.pop();
					f = this.ops1[item.index_];
					nstack.push(f(n1));
				}
				else if (type_ === TFUNCALL) {
					n1 = nstack.pop();
					f = nstack.pop();
					if (f.apply && f.call) {
						if (Object.prototype.toString.call(n1) == "[object Array]") {
							nstack.push(f.apply(undefined, n1));
						}
						else {
							nstack.push(f.call(undefined, n1));
						}
					}
					else {
						throw new Error(f + " is not a function");
					}
				}
				else {
					throw new Error("invalid Expression");
				}
			}
			if (nstack.length > 1) {
				throw new Error("invalid Expression (parity)");
			}
			return nstack[0];
		},

		toString: function (toJS) {
			var nstack = [];
			var n1;
			var n2;
			var f;
			var L = this.tokens.length;
			var item;
			var i = 0;
			for (i = 0; i < L; i++) {
				item = this.tokens[i];
				var type_ = item.type_;
				if (type_ === TNUMBER) {
					nstack.push(escapeValue(item.number_));
				}
				else if (type_ === TOP2) {
					n2 = nstack.pop();
					n1 = nstack.pop();
					f = item.index_;
					if (toJS && f == "^") {
						nstack.push("Math.pow(" + n1 + "," + n2 + ")");
					}
					else {
						nstack.push("(" + n1 + f + n2 + ")");
					}
				}
				else if (type_ === TVAR) {
					nstack.push(item.index_);
				}
				else if (type_ === TOP1) {
					n1 = nstack.pop();
					f = item.index_;
					if (f === "-") {
						nstack.push("(" + f + n1 + ")");
					}
					else {
						nstack.push(f + "(" + n1 + ")");
					}
				}
				else if (type_ === TFUNCALL) {
					n1 = nstack.pop();
					f = nstack.pop();
					nstack.push(f + "(" + n1 + ")");
				}
				else {
					throw new Error("invalid Expression");
				}
			}
			if (nstack.length > 1) {
				throw new Error("invalid Expression (parity)");
			}
			return nstack[0];
		},

		variables: function () {
			var L = this.tokens.length;
			var vars = [];
			for (var i = 0; i < L; i++) {
				var item = this.tokens[i];
				if (item.type_ === TVAR && (vars.indexOf(item.index_) === false)) {
			//	if (item.type_ === TVAR && ( getindexOf(vars,item.index_) == -1)) {
					vars.push(item.index_);
				}
			}

			return vars;
		},

		toJSFunction: function (param, variables) {
			var f = new Function(param, "with(Parser.values) { return " + this.simplify(variables).toString(true) + "; }");
			return f;
		}
	};

	function add(a, b) {
		return Number(a) + Number(b);
	}
	function sub(a, b) {
		return a - b; 
	}
	function mul(a, b) {
		return a * b;
	}
	function div(a, b) {
		return a / b;
	}
	function mod(a, b) {
		return a % b;
	}
	function concat(a, b) {
		return "" + a + b;
	}

	function neg(a) {
		return -a;
	}

	function random(a) {
		return Math.random() * (a || 1);
	}
	function fac(a) { //a!
		a = Math.floor(a);
		var b = a;
		while (a > 1) {
			b = b * (--a);
		}
		return b;
	}

	// TODO: use hypot that doesn't overflow
	function pyt(a, b) {
		return Math.sqrt(a * a + b * b);
	}

	function append(a, b) {
		if (Object.prototype.toString.call(a) != "[object Array]") {
			return [a, b];
		}
		a = a.slice();
		a.push(b);
		return a;
	}

	function Parser() {
		this.success = false;
		this.errormsg = "";
		this.expression = "";

		this.pos = 0;

		this.tokennumber = 0;
		this.tokenprio = 0;
		this.tokenindex = 0;
		this.tmpprio = 0;

		this.ops1 = {
			"sin": Math.sin,
			"cos": Math.cos,
			"tan": Math.tan,
			"asin": Math.asin,
			"acos": Math.acos,
			"atan": Math.atan,
			"sqrt": Math.sqrt,
			"log": Math.log,
			"abs": Math.abs,
			"ceil": Math.ceil,
			"floor": Math.floor,
			"round": Math.round,
			"-": neg,
			"exp": Math.exp
		};

		this.ops2 = {
			"+": add,
			"-": sub,
			"*": mul,
			"/": div,
			"%": mod,
			"^": Math.pow,
			",": append,
			"||": concat
		};

		this.functions = {
			"random": random,
			"fac": fac,
			"min": Math.min,
			"max": Math.max,
			"pyt": pyt,
			"pow": Math.pow,
			"atan2": Math.atan2
		};

		this.consts = {
			"E": Math.E,
			"PI": Math.PI
		};
	}

	Parser.parse = function (expr) {
		return new Parser().parse(expr);
	};

	Parser.evaluate = function (expr, variables) {
		return Parser.parse(expr).evaluate(variables);
	};

	Parser.Expression = Expression;

	Parser.values = {
		sin: Math.sin,
		cos: Math.cos,
		tan: Math.tan,
		asin: Math.asin,
		acos: Math.acos,
		atan: Math.atan,
		sqrt: Math.sqrt,
		log: Math.log,
		abs: Math.abs,
		ceil: Math.ceil,
		floor: Math.floor,
		round: Math.round,
		random: random,
		fac: fac,
		exp: Math.exp,
		min: Math.min,
		max: Math.max,
		pyt: pyt,
		pow: Math.pow,
		atan2: Math.atan2,
		E: Math.E,
		PI: Math.PI
	};

	var PRIMARY  = 1 << 0;
	var OPERATOR = 1 << 1;
	var FUNCTION = 1 << 2;
	var LPAREN   = 1 << 3;
	var RPAREN   = 1 << 4;
	var COMMA    = 1 << 5;
	var SIGN     = 1 << 6;
	var CALL     = 1 << 7;

	Parser.prototype = {
		parse: function (expr) {
			this.errormsg = "";
			this.success = true;
			var operstack = [];
			var tokenstack = [];
			this.tmpprio = 0;
			var expected = (PRIMARY | LPAREN | FUNCTION | SIGN);
			var noperators = 0;
			this.expression = expr;
			this.pos = 0;

			while (this.pos < this.expression.length) {
				if (this.isOperator()) {
					if (this.isSign() && (expected & SIGN)) {
						if (this.isNegativeSign()) {
							this.tokenprio = 2;
							this.tokenindex = "-";
							noperators++;
							this.addfunc(tokenstack, operstack, TOP1);
						}
						expected = (PRIMARY | LPAREN | FUNCTION | SIGN);
					}
					else if (this.isComment()) {

					}
					else {
						if ((expected & OPERATOR) === 0) {
							this.error_parsing(this.pos, "unexpected operator");
						}
						noperators += 2;
						this.addfunc(tokenstack, operstack, TOP2);
						expected = (PRIMARY | LPAREN | FUNCTION | SIGN);
					}
				}
				else if (this.isNumber()) {
					if ((expected & PRIMARY) === 0) {
						this.error_parsing(this.pos, "unexpected number");
					}
					var token = new Token(TNUMBER, 0, 0, this.tokennumber);
					tokenstack.push(token);

					expected = (OPERATOR | RPAREN | COMMA);
				}
				else if (this.isString()) {
					if ((expected & PRIMARY) === 0) {
						this.error_parsing(this.pos, "unexpected string");
					}
					var token = new Token(TNUMBER, 0, 0, this.tokennumber);
					tokenstack.push(token);

					expected = (OPERATOR | RPAREN | COMMA);
				}
				else if (this.isLeftParenth()) {
					if ((expected & LPAREN) === 0) {
						this.error_parsing(this.pos, "unexpected \"(\"");
					}

					if (expected & CALL) {
						noperators += 2;
						this.tokenprio = -2;
						this.tokenindex = -1;
						this.addfunc(tokenstack, operstack, TFUNCALL);
					}

					expected = (PRIMARY | LPAREN | FUNCTION | SIGN);
				}
				else if (this.isRightParenth()) {
					if ((expected & RPAREN) === 0) {
						this.error_parsing(this.pos, "unexpected \")\"");
					}

					expected = (OPERATOR | RPAREN | COMMA | LPAREN | CALL);
				}
				else if (this.isComma()) {
					if ((expected & COMMA) === 0) {
						this.error_parsing(this.pos, "unexpected \",\"");
					}
					this.addfunc(tokenstack, operstack, TOP2);
					noperators += 2;
					expected = (PRIMARY | LPAREN | FUNCTION | SIGN);
				}
				else if (this.isConst()) {
					if ((expected & PRIMARY) === 0) {
						this.error_parsing(this.pos, "unexpected constant");
					}
					var consttoken = new Token(TNUMBER, 0, 0, this.tokennumber);
					tokenstack.push(consttoken);
					expected = (OPERATOR | RPAREN | COMMA);
				}
				else if (this.isOp2()) {
					if ((expected & FUNCTION) === 0) {
						this.error_parsing(this.pos, "unexpected function");
					}
					this.addfunc(tokenstack, operstack, TOP2);
					noperators += 2;
					expected = (LPAREN);
				}
				else if (this.isOp1()) {
					if ((expected & FUNCTION) === 0) {
						this.error_parsing(this.pos, "unexpected function");
					}
					this.addfunc(tokenstack, operstack, TOP1);
					noperators++;
					expected = (LPAREN);
				}
				else if (this.isVar()) {
					if ((expected & PRIMARY) === 0) {
						this.error_parsing(this.pos, "unexpected variable");
					}
					var vartoken = new Token(TVAR, this.tokenindex, 0, 0);
					tokenstack.push(vartoken);

					expected = (OPERATOR | RPAREN | COMMA | LPAREN | CALL);
				}
				else if (this.isWhite()) {
				}
				else {
					if (this.errormsg === "") {
						this.error_parsing(this.pos, "unknown character");
					}
					else {
						this.error_parsing(this.pos, this.errormsg);
					}
				}
			}
			if (this.tmpprio < 0 || this.tmpprio >= 10) {
				this.error_parsing(this.pos, "unmatched \"()\"");
			}
			while (operstack.length > 0) {
				var tmp = operstack.pop();
				tokenstack.push(tmp);
			}
			if (noperators + 1 !== tokenstack.length) {
				//print(noperators + 1);
				//print(tokenstack);
				this.error_parsing(this.pos, "parity");
			}

			return new Expression(tokenstack, object(this.ops1), object(this.ops2), object(this.functions));
		},

		evaluate: function (expr, variables) {
			return this.parse(expr).evaluate(variables);
		},

		error_parsing: function (column, msg) {
			this.success = false;
			this.errormsg = "parse error [column " + (column) + "]: " + msg;
			throw new Error(this.errormsg);
		},

//\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

		addfunc: function (tokenstack, operstack, type_) {
			var operator = new Token(type_, this.tokenindex, this.tokenprio + this.tmpprio, 0);
			while (operstack.length > 0) {
				if (operator.prio_ <= operstack[operstack.length - 1].prio_) {
					tokenstack.push(operstack.pop());
				}
				else {
					break;
				}
			}
			operstack.push(operator);
		},

		isNumber: function () {
			var r = false;
			var str = "";
			while (this.pos < this.expression.length) {
				var code = this.expression.charCodeAt(this.pos);
				if ((code >= 48 && code <= 57) || code === 46) {
					str += this.expression.charAt(this.pos);
					this.pos++;
					this.tokennumber = parseFloat(str);
					r = true;
				}
				else {
					break;
				}
			}
			return r;
		},

		// Ported from the yajjl JSON parser at http://code.google.com/p/yajjl/
		unescape: function(v, pos) {
			var buffer = [];
			var escaping = false;

			for (var i = 0; i < v.length; i++) {
				var c = v.charAt(i);
	
				if (escaping) {
					switch (c) {
					case "'":
						buffer.push("'");
						break;
					case '\\':
						buffer.push('\\');
						break;
					case '/':
						buffer.push('/');
						break;
					case 'b':
						buffer.push('\b');
						break;
					case 'f':
						buffer.push('\f');
						break;
					case 'n':
						buffer.push('\n');
						break;
					case 'r':
						buffer.push('\r');
						break;
					case 't':
						buffer.push('\t');
						break;
					case 'u':
						// interpret the following 4 characters as the hex of the unicode code point
						var codePoint = parseInt(v.substring(i + 1, i + 5), 16);
						buffer.push(String.fromCharCode(codePoint));
						i += 4;
						break;
					default:
						throw this.error_parsing(pos + i, "Illegal escape sequence: '\\" + c + "'");
					}
					escaping = false;
				} else {
					if (c == '\\') {
						escaping = true;
					} else {
						buffer.push(c);
					}
				}
			}
	
			return buffer.join('');
		},

		isString: function () {
			var r = false;
			var str = "";
			var startpos = this.pos;
			if (this.pos < this.expression.length && this.expression.charAt(this.pos) == "'") {
				this.pos++;
				while (this.pos < this.expression.length) {
					var code = this.expression.charAt(this.pos);
					if (code != "'" || str.slice(-1) == "\\") {
						str += this.expression.charAt(this.pos);
						this.pos++;
					}
					else {
						this.pos++;
						this.tokennumber = this.unescape(str, startpos);
						r = true;
						break;
					}
				}
			}
			return r;
		},

		isConst: function () {
			var str;
			for (var i in this.consts) {
				if (true) {
					var L = i.length;
					str = this.expression.substr(this.pos, L);
					if (i === str) {
						this.tokennumber = this.consts[i];
						this.pos += L;
						return true;
					}
				}
			}
			return false;
		},

		isOperator: function () {
			var code = this.expression.charCodeAt(this.pos);
			if (code === 43) { // +
				this.tokenprio = 0;
				this.tokenindex = "+";
			}
			else if (code === 45) { // -
				this.tokenprio = 0;
				this.tokenindex = "-";
			}
			else if (code === 124) { // |
				if (this.expression.charCodeAt(this.pos + 1) === 124) {
					this.pos++;
					this.tokenprio = 0;
					this.tokenindex = "||";
				}
				else {
					return false;
				}
			}
			else if (code === 42) { // *
				this.tokenprio = 1;
				this.tokenindex = "*";
			}
			else if (code === 47) { // /
				this.tokenprio = 2;
				this.tokenindex = "/";
			}
			else if (code === 37) { // %
				this.tokenprio = 2;
				this.tokenindex = "%";
			}
			else if (code === 94) { // ^
				this.tokenprio = 3;
				this.tokenindex = "^";
			}
			else {
				return false;
			}
			this.pos++;
			return true;
		},

		isSign: function () {
			var code = this.expression.charCodeAt(this.pos - 1);
			if (code === 45 || code === 43) { // -
				return true;
			}
			return false;
		},

		isPositiveSign: function () {
			var code = this.expression.charCodeAt(this.pos - 1);
			if (code === 43) { // -
				return true;
			}
			return false;
		},

		isNegativeSign: function () {
			var code = this.expression.charCodeAt(this.pos - 1);
			if (code === 45) { // -
				return true;
			}
			return false;
		},

		isLeftParenth: function () {
			var code = this.expression.charCodeAt(this.pos);
			if (code === 40) { // (
				this.pos++;
				this.tmpprio += 10;
				return true;
			}
			return false;
		},

		isRightParenth: function () {
			var code = this.expression.charCodeAt(this.pos);
			if (code === 41) { // )
				this.pos++;
				this.tmpprio -= 10;
				return true;
			}
			return false;
		},

		isComma: function () {
			var code = this.expression.charCodeAt(this.pos);
			if (code === 44) { // ,
				this.pos++;
				this.tokenprio = -1;
				this.tokenindex = ",";
				return true;
			}
			return false;
		},

		isWhite: function () {
			var code = this.expression.charCodeAt(this.pos);
			if (code === 32 || code === 9 || code === 10 || code === 13) {
				this.pos++;
				return true;
			}
			return false;
		},

		isOp1: function () {
			var str = "";
			for (var i = this.pos; i < this.expression.length; i++) {
				var c = this.expression.charAt(i);
				if (c.toUpperCase() === c.toLowerCase()) {
					if (i === this.pos || c < '0' || c > '9') {
						break;
					}
				}
				str += c;
			}
			if (str.length > 0 && (str in this.ops1)) {
				this.tokenindex = str;
				this.tokenprio = 5;
				this.pos += str.length;
				return true;
			}
			return false;
		},

		isOp2: function () {
			var str = "";
			for (var i = this.pos; i < this.expression.length; i++) {
				var c = this.expression.charAt(i);
				if (c.toUpperCase() === c.toLowerCase()) {
					if (i === this.pos || c < '0' || c > '9') {
						break;
					}
				}
				str += c;
			}
			if (str.length > 0 && (str in this.ops2)) {
				this.tokenindex = str;
				this.tokenprio = 5;
				this.pos += str.length;
				return true;
			}
			return false;
		},

		isVar: function () {
			var str = "";
			for (var i = this.pos; i < this.expression.length; i++) {
				var c = this.expression.charAt(i);
				if (c.toUpperCase() === c.toLowerCase()) {
					if (i === this.pos || c < '0' || c > '9') {
						break;
					}
				}
				str += c;
			}
			if (str.length > 0) {
				this.tokenindex = str;
				this.tokenprio = 4;
				this.pos += str.length;
				return true;
			}
			return false;
		},

		isComment: function () {
			var code = this.expression.charCodeAt(this.pos - 1);
			if (code === 47 && this.expression.charCodeAt(this.pos) === 42) {
				this.pos = this.expression.indexOf("*/", this.pos) + 2;
		//		this.pos = getindexOf( this.expression, "*/", this.pos) + 2;
				if (this.pos === 1) {
					this.pos = this.expression.length;
				}
				return true;
			}
			return false;
		}
	};

	scope.Parser = Parser;
	return Parser
})(typeof exports === 'undefined' ? {} : exports);