Fix #10, add test

dist/index.js

@@ -219,7 +219,7 @@ var parseExpression = function parseExpression(expression) {
         stack.push('-' + cur);
 
         // 2 - 5 is also OK, pass
-      } else if (_.isNumber(beforeSign)) {
+      } else if (_.isNumber(beforeSign) || isVariable(beforeSign)) {
         stack.push(cur);
       } else {
         stack[past] += cur;
@@ -478,8 +478,10 @@ var fixFloat = function fixFloat(number) {
   * @return {Boolean}
   *         true if variable, else false
   */
+
+var SPECIALS = '()[]{}'.split('');
 var isVariable = function isVariable(a) {
-  return typeof a === 'string' && !_.isNumber(a) && !_operators2['default'][a] && a === a.toLowerCase();
+  return typeof a === 'string' && !_.isNumber(a) && !_operators2['default'][a] && a === a.toLowerCase() && SPECIALS.indexOf(a) === -1;
 };
 
 exports.isVariable = isVariable;

dist/tests/equation.js

@@ -13,6 +13,9 @@ describe('Equations', function () {
     var equation = _M2['default'].equation('x+2');
 
     _expect.expect(equation(2)).to.equal(4);
+
+    var subtraction = _M2['default'].equation('x - 3');
+    _expect.expect(subtraction(10)).to.equal(7);
   });
 
   it('should work with multiple variables', function () {

lib/index.js

@@ -182,7 +182,7 @@ const parseExpression = expression => {
         stack.push(`-${cur}`);
 
         // 2 - 5 is also OK, pass
-      } else if (_.isNumber(beforeSign)) {
+      } else if (_.isNumber(beforeSign) || isVariable(beforeSign)) {
         stack.push(cur);
       } else {
         stack[past] += cur;
@@ -379,9 +379,11 @@ const fixFloat = number => {
   * @return {Boolean}
   *         true if variable, else false
   */
+
+const SPECIALS = '()[]{}'.split('');
 export const isVariable = a => {
   return typeof a === 'string' && !_.isNumber(a) &&
-         !operators[a] && a === a.toLowerCase();
+         !operators[a] && a === a.toLowerCase() && SPECIALS.indexOf(a) === -1;
 };
 
 export default Equation;

package.json

@@ -1,6 +1,6 @@
 {
   "name": "equations",
-  "version": "1.3.2",
+  "version": "1.3.3",
   "description": "",
   "main": "dist/index.js",
   "directories": {

tests/equation.js

@@ -6,6 +6,10 @@ describe('Equations', () => {
     let equation = M.equation('x+2');
 
     expect(equation(2)).to.equal(4);
+
+    // Issue #10
+    let subtraction = M.equation('x - 3');
+    expect(subtraction(10)).to.equal(7);
   });
 
   it('should work with multiple variables', () => {