diff --git a/.gitignore b/.gitignore
index 2a324d4..e4db436 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,4 @@ test/output
 test/output.js
 test/output.js.map
 dist
+.idea/
diff --git a/README.md b/README.md
index 646ecb6..cbe75e0 100644
--- a/README.md
+++ b/README.md
@@ -259,6 +259,14 @@ builder.bundle('app/**/* - [app/**/*]', 'dependencies.js', { minify: true, sourc
 
 The above means _take the tree of app and all its dependencies, and subtract just the modules in app_, thus leaving us with just the tree of dependencies of the app package.
 
+#### Example - Multiple Common Bundles
+
+Parentheses are supported, so the following would bundle everything in common with `page1` and `page2`, and also everything in common between `page3` and `page4`:
+
+```javascript
+builder.bundle('(app/page1.js & app/page2.js) + (app/page3.js & app/page4.js)', 'common.js');
+```
+
 #### Example - Direct Trace API
 
 Instead of using the arithmetic syntax, we can construct the trace ourselves.
diff --git a/lib/arithmetic.js b/lib/arithmetic.js
index d55d2a3..5d08378 100644
--- a/lib/arithmetic.js
+++ b/lib/arithmetic.js
@@ -13,37 +13,108 @@ var verifyTree = require('./utils').verifyTree;
 
 function parseExpression(expressionString) {
   expressionString = ' + ' + expressionString;
-  var args = expressionString.split(/ [\+\-\&] /);
 
+  var index = 0;
   var operations = [];
+  var operatorRegex = /[\+\-\&]/;
+  var errorMessagesFromIndex = 3;
 
-  var curLen = 0;
-  for (var i = 0; i < args.length - 1; i++) {
-    var operator = expressionString[curLen + 1];
-    var moduleName = args[i + 1];
+  function getNextIdentifier() {
+    eatWhitespace();
+    var firstChar = expressionString.charAt(index);
 
-    curLen += moduleName.length + 3;
+    if (operatorRegex.test(firstChar)){
+      throw 'Syntax Error: Identifier or sub expression expected after <' + expressionString.slice(errorMessagesFromIndex).substr(0, index - errorMessagesFromIndex) + '> but found <' + firstChar + '> instead';
+    }
 
-    if (operator !== '+' && operator !== '-' && operator !== '&')
-      throw new TypeError('Expected operator before ' + operator);
-    if (!moduleName)
-      throw new TypeError('A module name is needed after ' + operator);
+    if (firstChar === '(') {
+      var closingParenIndex = index,
+          numOpenBeforeSelf = 0;
+
+      while (++closingParenIndex < expressionString.length){
+        if (expressionString.charAt(closingParenIndex) === '('){
+          numOpenBeforeSelf++;
+        } else if (expressionString.charAt(closingParenIndex) === ')') {
+          if (numOpenBeforeSelf){
+            numOpenBeforeSelf--;
+          } else {
+            break;
+          }
+        }
+      }
+      if (expressionString.charAt(closingParenIndex) !== ')'){
+        throw 'Syntax Error: Expression <' + expressionString.substr(index) + '> is never terminated. Did you forget to add a closing ")"?';
+      }
 
-    // detect [moduleName] syntax for individual modules not trees
-    var singleModule = moduleName.substr(0, 1) == '[' && moduleName.substr(moduleName.length - 1, 1) == ']';
-    if (singleModule)
-      moduleName = moduleName.substr(1, moduleName.length - 2);
+      var wholeExpression = expressionString.substring(index + 1, closingParenIndex);
+      index = closingParenIndex + 1;
+      return { bulkOperation: wholeExpression };
+    }
 
-    var canonicalized = moduleName.substr(0, 1) == '`' && moduleName.substr(moduleName.length - 1, 1) == '`';
-    if (canonicalized)
-      moduleName = moduleName.substr(1, moduleName.length - 2);
+    var result = "";
+    //scan the identifier
+    for (; index < expressionString.length; index++) {
+      var currentChar = expressionString.charAt(index);
+      //can have spaces in file names - so we need whitespace, operator, whitespace.
+      if (/^\s+[\+\-\&]\s+/.test(expressionString.substr(index))) {
+        return result;
+      } else {
+        result += currentChar;
+      }
+    }
+    return result.replace(/\s+$/, ''); //it appears as though trailing whitespace is trimmed downstream, but I'm snipping here to be safe
+  }
 
-    operations.push({
-      operator: operator,
-      moduleName: moduleName,
-      singleModule: singleModule,
-      canonicalized: canonicalized
-    });
+  function getNextOperator() {
+    eatWhitespace();
+    if (index === expressionString.length) return null;
+
+    var candidateResult = expressionString.charAt(index++); //all operators are single characters at the moment
+
+    if (!operatorRegex.test(candidateResult)){
+      throw 'Syntax Error: An operator was expected after <' + expressionString.slice(errorMessagesFromIndex).substr(0, index - 1 - errorMessagesFromIndex) + '> but found <' + expressionString.substring(index - 1) + '> instead';
+    }
+
+    return candidateResult;
+  }
+
+  function eatWhitespace() {
+    //wind past whitespace
+    for (; index < expressionString.length; index++) {
+      if (/\S/.test(expressionString.charAt(index))) {
+        break;
+      }
+    }
+  }
+
+  var operator;
+  while (index < expressionString.length && (operator = getNextOperator())) {
+    var moduleNameOrSubExpression = getNextIdentifier();
+
+    if (typeof moduleNameOrSubExpression === 'object'){
+      operations.push({
+        operator: operator,
+        bulkOperation: moduleNameOrSubExpression.bulkOperation
+      });
+    } else {
+      // detect [moduleName] syntax for individual modules not trees
+      var singleModule = moduleNameOrSubExpression.substr(0, 1) == '[' && moduleNameOrSubExpression.substr(moduleNameOrSubExpression.length - 1, 1) == ']';
+      if (singleModule) {
+        moduleNameOrSubExpression = moduleNameOrSubExpression.substr(1, moduleNameOrSubExpression.length - 2);
+      }
+
+      var canonicalized = moduleNameOrSubExpression.substr(0, 1) == '`' && moduleNameOrSubExpression.substr(moduleNameOrSubExpression.length - 1, 1) == '`';
+      if (canonicalized) {
+        moduleNameOrSubExpression = moduleNameOrSubExpression.substr(1, moduleNameOrSubExpression.length - 2);
+      }
+
+      operations.push({
+        operator: operator,
+        moduleName: moduleNameOrSubExpression,
+        singleModule: singleModule,
+        canonicalized: canonicalized
+      });
+    }
   }
 
   return operations;
@@ -149,42 +220,61 @@ function expandGlobAndCanonicalize(builder, operation) {
   });
 }
 
-exports.traceExpression = function(builder, expression, traceOpts) {
-  if (!expression)
-    throw new Error('A module expression must be provided to trace.');
+  exports.traceExpression = function(builder, expression, traceOpts) {
+    if (!expression)
+      throw new Error('A module expression must be provided to trace.');
+
+    return Promise
+      .resolve(expandAndCanonicalizeExpression(builder, expression))
+      .then(function processExpandedOperations(expandedOperations) {
+        // chain the operations, applying them with the trace of the next module
+        return expandedOperations.reduce(function(p, op) {
+          return p.then(function(curTree) {
+            // tree . module
+            if (op.singleModule)
+              return getTreeModuleOperation(builder, op.operator)(curTree, op.moduleName);
+
+            if (op.operationsTree){
+              return processExpandedOperations(op.operationsTree).then(function(expandedTree){
+                return getTreeOperation(op.operator)(curTree, expandedTree);
+              });
+            }
+            // tree . tree
+            return builder.tracer.traceCanonical(op.moduleName, traceOpts)
+            .then(function(nextTrace) {
+              return getTreeOperation(op.operator)(curTree, nextTrace.tree);
+            });
+          });
+        }, Promise.resolve({}));
+    });
+  };
 
+function expandAndCanonicalizeExpression(builder, expression){
   var operations = parseExpression(expression);
+  var expandPromise = Promise.resolve(1);
   var expandedOperations = [];
 
-  // expand any globbing operations in the expression
-  var expandPromise = Promise.resolve();
-  operations.forEach(function(operation) {
-    expandPromise = expandPromise.then(function() {
-      return Promise.resolve(expandGlobAndCanonicalize(builder, operation))
-      .then(function(expanded) {
-        expandedOperations = expandedOperations.concat(expanded);
+  operations.forEach(function(operation){
+    if (operation.bulkOperation) {
+      var expandedTreePromise = expandAndCanonicalizeExpression(builder, operation.bulkOperation);
+      expandPromise = expandPromise.then(function() {
+        return Promise.resolve(expandedTreePromise)
+              .then(function(expressionsOperations){
+                expandedOperations = expandedOperations.concat({ operator: operation.operator, operationsTree: expressionsOperations });
+              });
       });
-    });
-  });
-
-  return expandPromise.then(function() {
-    // chain the operations, applying them with the trace of the next module
-    return expandedOperations.reduce(function(p, op) {
-      return p.then(function(curTree) {
-        // tree . module
-        if (op.singleModule)
-          return getTreeModuleOperation(builder, op.operator)(curTree, op.moduleName);
-        
-        // tree . tree
-        return builder.tracer.traceCanonical(op.moduleName, traceOpts)
-        .then(function(nextTrace) {
-          return getTreeOperation(op.operator)(curTree, nextTrace.tree);
-        });
-      });
-    }, Promise.resolve({}));
+    } else {
+      expandPromise = expandPromise.then(function() {
+        return Promise.resolve(expandGlobAndCanonicalize(builder, operation))
+               .then(function (expanded) {
+                 expandedOperations = expandedOperations.concat(expanded);
+               });
+      })
+    }
   });
-};
 
+  return Promise.resolve(expandPromise).then(function(){ return expandedOperations; });
+}
 
 // returns a new tree containing tree1 n tree2
 exports.intersectTrees = intersectTrees;
@@ -210,7 +300,7 @@ function intersectTrees(tree1, tree2) {
   }
 
   return intersectTree;
-};
+}
 
 // returns a new tree containing tree1 + tree2
 exports.addTrees = addTrees;
@@ -249,7 +339,7 @@ function subtractTrees(tree1, tree2) {
   }
 
   return subtractTree;
-};
+}
 
 // pre-order tree traversal with a visitor and stop condition
 exports.traverseTree = traverseTree;
diff --git a/test/arithmetic.js b/test/arithmetic.js
index ade36d5..97fa960 100644
--- a/test/arithmetic.js
+++ b/test/arithmetic.js
@@ -36,7 +36,8 @@ suite('Bundle Expressions', function() {
     builder.trace('*.js - [amd-*] - [sfx-format-*]')
     .then(function(tree) {
       assert.deepEqual(Object.keys(tree).sort(), [
-          'Buffer.js', 'amd.js', 'babel', 'cjs-globals.js', 'cjs-resolve.js', 'cjs.js', 'component.jsx!jsx.js', 'file.json', 'first.js', 
+          'Buffer.js', 'amd.js', 'babel', 'cjs space.js', 'cjs-1.js', 'cjs-2.js', 'cjs-3.js', 'cjs-4.js', 'cjs-5.js', 'cjs-globals.js', 'cjs-in-12.js', 'cjs-in-13.js',
+          'cjs-resolve.js', 'cjs.js', 'component.jsx!jsx.js', 'file.json', 'first.js',
           'global-inner.js', 'global-outer.js', 'global.js', 'jquery-cdn', 'jquery.js', 'json-plugin.js', 'jsx.js', 'plugin.js', 'runtime.js', 
           'second.js', 'some.js!plugin.js', 'text-plugin.js', 'text.txt!text-plugin.js', 'third.js', 'umd.js']);
     })
@@ -50,4 +51,231 @@ suite('Bundle Expressions', function() {
     })
     .then(done, done);
   });
-});
\ No newline at end of file
+
+  test('cjs bundles added', function(done){
+    builder.trace('cjs-1.js + cjs-2.js + cjs-3.js')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), [
+          'cjs-1.js', 'cjs-2.js', 'cjs-3.js', 'cjs-in-12.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles added with parens and extra spaces', function(done){
+    builder.trace('  (   cjs-1.js    &     cjs-2.js          )       + ( cjs-1.js & cjs-3.js)')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-12.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles added with single-value parameters', function(done){
+    builder.trace('(cjs-1.js) + (cjs-2.js) + (cjs-3.js)')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), [
+        'cjs-1.js', 'cjs-2.js', 'cjs-3.js', 'cjs-in-12.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles added with parens', function(done){
+    builder.trace('(cjs-1.js & cjs-2.js) + (cjs-1.js & cjs-3.js)')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-12.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles added with parens 2', function(done){
+    builder.trace('(cjs-1.js & cjs-2.js)')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-12.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles added with parens 3', function(done){
+    builder.trace('(cjs-1.js & cjs-2.js) + cjs-in-13.js - cjs-in-13.js')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-12.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles added with parens 4', function(done){
+    builder.trace('(cjs-1.js & cjs-2.js) + cjs-in-13.js - cjs-in-13.js')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-12.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles added with parens 5', function(done){
+    builder.trace('cjs-in-13.js + (cjs-1.js & cjs-2.js) - cjs-in-13.js')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-12.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles added with multiple parens', function(done){
+    builder.trace('cjs-in-13.js + (cjs-1.js & cjs-2.js) - (cjs-in-13.js)')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-12.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles added with multiple parens 2', function(done){
+    builder.trace('(cjs-1.js & cjs-2.js) + (cjs-1.js & cjs-3.js) - cjs-in-12.js')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles with nested parens', function(done){
+    builder.trace('(cjs-1.js + cjs-2.js - ([cjs-1.js] + [cjs-2.js])) - (cjs-in-12.js)')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles with nested parens 2', function(done){
+    builder.trace('(cjs-1.js + cjs-2.js - ([cjs-1.js] + [cjs-2.js])) - (cjs-in-12.js) + (cjs-4.js + cjs-5.js)')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-4.js', 'cjs-5.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles with nested parens 3', function(done){
+    builder.trace('((cjs-1.js + cjs-2.js - ([cjs-1.js] + [cjs-2.js])) - (cjs-in-12.js) + (cjs-4.js + cjs-5.js)) - ([cjs-4.js] + [cjs-5.js])')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles with nested parens 4', function(done){
+    builder.trace('((cjs-1.js + cjs-2.js - ([cjs-1.js] + [cjs-2.js] + ([cjs-4.js] + [cjs-5.js]))) - (cjs-4.js + cjs-5.js))')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-12.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+
+  test('ibid with single module subtracted', function(done){
+    builder.trace('(cjs-1.js + cjs-2.js - ([cjs-1.js] + [cjs-2.js])) - ([cjs-in-12.js])')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles added with nested parens', function(done){
+    builder.trace('(cjs-1.js + cjs-2.js - (cjs-1.js & cjs-2.js))')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-1.js', 'cjs-2.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('cjs bundles with parens and single modules', function(done){
+    builder.trace('(cjs-1.js + cjs-2.js) - ([cjs-1.js] + [cjs-2.js])')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), ['cjs-in-12.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('file with space', function(done){
+    builder.trace('cjs-1.js + cjs space.js')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), [
+        'cjs space.js', 'cjs-1.js', 'cjs-in-12.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('file with space 2', function(done){
+    builder.trace('cjs-1.js + cjs space.js   ')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), [
+        'cjs space.js', 'cjs-1.js', 'cjs-in-12.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('file with space 3', function(done){
+    builder.trace('cjs-1.js + cjs space.js   + cjs-2.js')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), [
+        'cjs space.js', 'cjs-1.js', 'cjs-2.js', 'cjs-in-12.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+  test('file with space 4', function(done){
+    builder.trace('cjs-1.js + cjs space.js   + cjs-2.js   ')
+    .then(function(tree) {
+      assert.deepEqual(Object.keys(tree).sort(), [
+        'cjs space.js', 'cjs-1.js', 'cjs-2.js', 'cjs-in-12.js', 'cjs-in-13.js']);
+    })
+    .then(done, done);
+  });
+
+});
+
+suite('Bundle Expression Validation', function() {
+  test('missing identifier 1', function(){
+    return validateInvalidExpression('cjs-1.js + +');
+  });
+
+  test('missing identifier 2', function(){
+    return validateInvalidExpression('cjs-1.js + -');
+  });
+
+  test('missing identifier 3', function(){
+    return validateInvalidExpression('cjs-1.js + &');
+  });
+
+  test('unclosed parens 1', function(){
+    return validateInvalidExpression('(cjs-1.js + cjs-2.js');
+  });
+
+  test('unclosed parens 2', function(){
+    return validateInvalidExpression('(cjs-1.js + (cjs-2.js + cjs-3.js');
+  });
+
+  test('unclosed parens 3', function(){
+    return validateInvalidExpression('(cjs-1.js + (cjs-2.js + cjs-3.js)');
+  });
+
+  test('unclosed parens 4', function(){
+    return validateInvalidExpression('(cjs-2.js + cjs-3.js) + (cjs-1.js + (cjs-2.js + cjs-3.js)');
+  });
+
+  function validateInvalidExpression(expression){
+    return Promise
+           .resolve()
+           .then(function(){ return builder.trace(expression) })
+           .then(
+             function(){ return Promise.reject('Invalid expression <' + expression + '> was parsed without error'); }, //it worked but shouldn't have
+             function(err){
+               //uncomment this line to view the Syntax Errors' wordings in the test console
+               //console.log(err);
+               if (typeof err !== 'string' || !/^Syntax Error/i.test(err)){
+                 return Promise.reject('Syntax error was expected, but not generated')
+               } else {
+                 return Promise.resolve(1);
+               }
+             }
+           )
+  }
+
+});
+
+
diff --git a/test/fixtures/test-tree/cjs space.js b/test/fixtures/test-tree/cjs space.js
new file mode 100644
index 0000000..66db397
--- /dev/null
+++ b/test/fixtures/test-tree/cjs space.js	
@@ -0,0 +1,2 @@
+module.exports = { name: 'cjs space' };
+
diff --git a/test/fixtures/test-tree/cjs-1.js b/test/fixtures/test-tree/cjs-1.js
new file mode 100644
index 0000000..5ac73c8
--- /dev/null
+++ b/test/fixtures/test-tree/cjs-1.js
@@ -0,0 +1,4 @@
+var shared1 = require('./cjs-in-12.js');
+var shared2 = require('./cjs-in-13.js');
+
+module.exports = { name: 'cjs1' };
diff --git a/test/fixtures/test-tree/cjs-2.js b/test/fixtures/test-tree/cjs-2.js
new file mode 100644
index 0000000..7e6f547
--- /dev/null
+++ b/test/fixtures/test-tree/cjs-2.js
@@ -0,0 +1,3 @@
+var shared1 = require('./cjs-in-12.js');
+
+module.exports = { name: 'cjs2' };
diff --git a/test/fixtures/test-tree/cjs-3.js b/test/fixtures/test-tree/cjs-3.js
new file mode 100644
index 0000000..10b5d9d
--- /dev/null
+++ b/test/fixtures/test-tree/cjs-3.js
@@ -0,0 +1,4 @@
+var shared1 = require('./cjs-in-13.js');
+
+module.exports = { name: 'cjs3' };
+
diff --git a/test/fixtures/test-tree/cjs-4.js b/test/fixtures/test-tree/cjs-4.js
new file mode 100644
index 0000000..1d0bef1
--- /dev/null
+++ b/test/fixtures/test-tree/cjs-4.js
@@ -0,0 +1 @@
+module.exports = { name: 'cjs4' };
diff --git a/test/fixtures/test-tree/cjs-5.js b/test/fixtures/test-tree/cjs-5.js
new file mode 100644
index 0000000..1f27aef
--- /dev/null
+++ b/test/fixtures/test-tree/cjs-5.js
@@ -0,0 +1 @@
+module.exports = { name: 'cjs5' };
diff --git a/test/fixtures/test-tree/cjs-in-12.js b/test/fixtures/test-tree/cjs-in-12.js
new file mode 100644
index 0000000..2f81c6d
--- /dev/null
+++ b/test/fixtures/test-tree/cjs-in-12.js
@@ -0,0 +1 @@
+module.exports = { name: 'cjs-in-12' };
diff --git a/test/fixtures/test-tree/cjs-in-13.js b/test/fixtures/test-tree/cjs-in-13.js
new file mode 100644
index 0000000..a81e3ba
--- /dev/null
+++ b/test/fixtures/test-tree/cjs-in-13.js
@@ -0,0 +1 @@
+module.exports = { name: 'cjs-in-13' };