diff --git a/README.md b/README.md
index 9be8aa9..54c448c 100644
--- a/README.md
+++ b/README.md
@@ -37,6 +37,20 @@ expect(expanded[3][0]).to.equal('2.4.01');
 - ```even```
 - ```odd```
 
+## Custom matchers
+
+It is possible to add a custom *matcher callback*, a function which gets executed for each possible position within the range.
+
+The stack of matchers will continue to execute until a position has been accepted or until the stack ends.
+
+```js
+expr.addMatcher(function (position, fragments) {
+  if (position === '1.1.45') return true;
+  if (fragments[0] === 1 && fragments[2] === 96) return true;
+  return false;
+});
+```
+
 ## Develop
 
 - ```make test```
@@ -51,6 +65,7 @@ expect(expanded[3][0]).to.equal('2.4.01');
   - CHANGED: *events* are now called *notes* [dilla/8](https://github.com/adamrenklint/dilla/issues/8)
 - **1.1.0**
   - CHANGED: expects options object instead of barsPerLoop and beatsPerBar separately [#4](https://github.com/adamrenklint/dilla-expressions/issues/4)
+  - NEW: possible to add custom matcher callback [#3](https://github.com/adamrenklint/dilla-expressions/issues/3)
 
 ## License
 
diff --git a/index.js b/index.js
index 740bfe0..aea8411 100644
--- a/index.js
+++ b/index.js
@@ -26,6 +26,13 @@ function getFragments (position) {
   });
 }
 
+var matchers = [];
+
+function addMatcher (matcher) {
+  if (!matcher || typeof matcher !== 'function') throw new Error('Invalid argument: matcher is not a function');
+  matchers.push(matcher);
+}
+
 function makeExpressionFunction (expression) {
   var exprFragments = getFragments(expression);
   return function expressionFn (position) {
@@ -36,13 +43,24 @@ function makeExpressionFunction (expression) {
       if (exprFragment === 'even' && positionFragments[index] % 2 === 0) return;
       if (exprFragment === 'odd' && positionFragments[index] % 2 === 1) return;
       if (exprFragment === '*') return;
+
       // if (typeof exprFragment === 'string' && exprFragment.indexOf('%') >= 0) {
         // console.log('deal with modulus', exprFragment, positionFragments[index])
       // }
       // position is invalid, break out early
+      // console.log('>', position, positionFragments);
       valid = false;
       return true;
     });
+
+    var _matchers = matchers.slice();
+    var matcher;
+    while (!valid && _matchers.length) {
+      matcher = _matchers.shift();
+      if (matcher(position, positionFragments)) {
+        valid = true;
+      }
+    }
     return valid;
   };
 }
@@ -75,4 +93,5 @@ function expressions (notes, options) {
   });
 }
 
+expressions.addMatcher = addMatcher;
 module.exports = expressions;
diff --git a/test/index.test.js b/test/index.test.js
index 1df7124..0de1b42 100644
--- a/test/index.test.js
+++ b/test/index.test.js
@@ -368,3 +368,114 @@ describe('when using mixed expression', function () {
   //   expect(result[7][0]).to.equal('1.2.91');
   // });
 });
+
+describe('addMatcher (matcher)', function () {
+  describe('when matcher is not a function', function () {
+    it('should throw an error', function () {
+      expect(function () {
+        expr.addMatcher();
+      }).to.throw(Error);
+      expect(function () {
+        expr.addMatcher('foo');
+      }).to.throw(Error);
+      expect(function () {
+        expr.addMatcher(1323);
+      }).to.throw(Error);
+      expect(function () {
+        expr.addMatcher({'pos': '1.1.01' });
+      }).to.throw(Error);
+      expect(function () {
+        expr.addMatcher(['1.1.1.1.1']);
+      }).to.throw(Error);
+    });
+  });
+
+  describe('when matcher is a function', function () {
+    it('should execute the matcher for each possible note', function () {
+      var count = 0;
+      var last = null;
+      expr.addMatcher(function (position, fragments) {
+        count++;
+        last = position;
+      });
+      expr([
+        ['1.1.*']
+      ], standardOptions);
+      expect(count).to.equal(672);
+      expect(last).to.equal('2.4.96');
+    });
+    describe('when matcher returns false', function () {
+      describe('when another rule matches', function () {
+        it('should include the note', function () {
+          expr.addMatcher(function (position, fragments) {
+            if (position === '1.1.03') return false;
+          });
+          var result = expr([
+            ['1.1.odd']
+          ], standardOptions);
+          var found = result.filter(function (res) {
+            return res[0] === '1.1.03';
+          });
+          expect(found.length).to.equal(1);
+        });
+      });
+      describe('when no other rule matches', function () {
+        it('should not include the note', function () {
+          expr.addMatcher(function (position, fragments) {
+            if (position === '1.1.02') return false;
+          });
+          var result = expr([
+            ['1.1.odd']
+          ], standardOptions);
+          var found = result.filter(function (res) {
+            return res[0] === '1.1.02';
+          });
+          expect(found.length).to.equal(0);
+        });
+      });
+    });
+    describe('when matcher returns true', function () {
+      describe('when a previous rule matches', function () {
+        it('should include the note', function () {
+          expr.addMatcher(function (position, fragments) {
+            if (position === '1.1.05') return true;
+          });
+          var result = expr([
+            ['1.1.odd']
+          ], standardOptions);
+          var found = result.filter(function (res) {
+            return res[0] === '1.1.05';
+          });
+          expect(found.length).to.equal(1);
+        });
+        it('should not execute matcher', function () {
+          var count = 0;
+          expr.addMatcher(function (position, fragments) {
+            return true;
+          });
+          expr.addMatcher(function (position, fragments) {
+            count++;
+          });
+          var result = expr([
+            ['1.1.*']
+          ], standardOptions);
+          expect(count).to.equal(0);
+        });
+      });
+      describe('when no other rule matches', function () {
+        it('should include the note', function () {
+          expr.addMatcher(function (position, fragments) {
+            if (position === '1.1.04') return true;
+          });
+          var result = expr([
+            ['1.1.odd']
+          ], standardOptions);
+          var found = result.filter(function (res) {
+            return res[0] === '1.1.04';
+          });
+          expect(found.length).to.equal(1);
+        });
+      });
+    });
+  });
+});