node package manager


Event emitters with support for regular expressions


Event emitters with support for regular expressions. Inherits from Node's EventEmitter.

Build Status


Using npm, you can install pattern-emitter with npm install pattern-emitter. You can also require it as a dependency in your package.json file:

"dependencies": {
    "pattern-emitter": ">0.0.0"


The PatternEmitter class both extends and is backwards compatible with EventEmitter when dealing with string event types. However, when registering a listener to a RegExp, it has the added benefit of listening to all events matching the expression, rather than that particular object. In addition, it exposes a new set of methods on top of the existing API for requesting details on those patterns and their listeners. As a result, getting started with this library is as simple as replacing instances of:

var Emitter = require('events').EventEmitter; // Node 0.10.x 
var Emitter = require('events');              // Node 0.12.x 


var Emitter = require('pattern-emitter');

Afterwards, you're ready to start listening to patterns.

var emitter = new Emitter();
var result;
emitter.on(/^example/, function(arg1, arg2) {
  result = arg1 + ' ' + arg2;
emitter.emit('exampleEvent', "It's", 'that simple');
console.log(result); // "It's that simple" 


The use of PatternEmitter is backwards compatible with EventEmitter for all who haven't been registering listeners to instances of RegExp. I suspect that this covers a great majority of event use.

Class: PatternEmitter

As with EventEmitter, when a PatternEmitter experiences an error, it emits an error event. If no listeners exist for the event, then by default, a stack trace is printed and the program is closed.

Furthermore, all PatternEmitters emit the newListener event when new listeners are added, and removeListener when removed.

These string events are treated like any other, and may be caught by a listener registered to a matching pattern, e.g. /.*/.

Instance Methods

In the following examples, let emitter be an instance of PatternEmitter. Furthermore, let pattern refer to any instance of RegExp, and event all other values.

When a listener is invoked, it's given the event name as a property, which is accessible via this.event.

emitter.addListener(event | pattern, listener)

Given a RegExp event type, stores the regular expression and registers the listener to any events matching the pattern. Otherwise, it behaves exactly as EventEmitter. As with EventEmitter.prototype.addListener, it emits a newListener' event on success. Returns an instance of itself.

emitter.addListener('userCreated', function(object) {
  console.log('Listener:', this.event);
emitter.addListener(/user/, function(object) {
  console.log('Pattern Listener:', this.event);
emitter.emit('userCreated', {id: 10, username: 'Alice'});
// Listener: userCreated 
// Pattern Listener: userCreated 
emitter.emit('userUpdated', {id: 19, username: 'Bob'});
// Pattern Listener: userUpdated 

emitter.emit(event, [arg1], [arg2], [...])

Emits an event to all listeners for the specified type. In addition, if type is a string, emits the event to all listeners whose patterns match. Returns true if any listeners existed, false otherwise.

emitter.addListener(/^namespace:entry:1\d{4}$/, function() {
  // Handler for entries within the given range: 10000-19999 
emitter.emit('namespace:entry:12345'); // true 
emitter.emit('namespace:entry:20000'); // false 


Returns an array of listeners for the given event.

emitter.addListener('foo:1:newBar', function() {
  console.log('event listener');
emitter.addListener(/^foo:[0-9]+:newBar$/, function() {
  console.log('pattern listener');
var listeners = emitter.listeners('foo:1:newBar');
console.log(listeners.length); // 1 
listeners[0]();                // 'event listener' 


Returns an array of listeners for the supplied event type, and whose patterns match the event if given a string.

emitter.addListener('foo:1:newBar', function() {
  console.log('event listener');
emitter.addListener(/^foo:[0-9]+:newBar$/, function() {
  console.log('pattern listener');
var listeners = emitter.matchingListeners('foo:1:newBar');
console.log(listeners.length); // 2 
listeners[0]();                // 'event listener' 
listeners[1]();                // 'pattern listener' 

emitter.on(event | pattern, listener)

An alias for addListener.

emitter.on(/.*/, function() {
  // Listen to all events, including 'error', 'newListener', 
  // and 'removeListener' 

emitter.once(event | pattern, listener)

Adds a one time listener for an event or pattern. The listener is invoked only once after an event is fired, after which it is removed.

var counter = 0;
emitter.once(/foo:[0-9]+:updated$/, function() {
console.log(counter); // 1 


Returns an array of pattern listeners for the specified RegExp.

emitter.on(/.*/, function() {
  // Example pattern listener 
emitter.on('/.*/', function() {
 // Won't be included in the results below 
var count = emitter.patternListeners(/.*/).length; // 1 

emitter.removeAllListeners(event | pattern)

Removes all listeners for the specified event type. If given an instance of RegExp, it matches the RegExp object with the same expression. Emits a 'removeListener' event for each removed listener. Returns an instance of itself.

emitter.addListener(/foo:.*[0-9]+/, function() {});
emitter.addListener(/foo:.*[0-9]+/, function() {});
emitter.addListener('/foo:.*[0-9]+/', function() {});
var count = emitter.patternListeners(/foo:.*[0-9]+/).length; // 2 
count = emitter.patternListeners(/foo:.*[0-9]+/).length; // 0 

emitter.removeListener(event | pattern, listener)

Removes the listener from the specified event type. If given an instance of RegExp, it matches any RegExp object with the same expression. Emits a 'removeListener' event on success. Returns an instance of itself.

var listener = function() {
  // Example event listener 
emitter.addListener(/foo:.*[0-9]+/, listener);
emitter.addListener(/foo:.*[0-9]+/, function() {});
var count = emitter.patternListeners(/foo:.*[0-9]+/).length; // 2 
emitter.removeListener(/foo:.*[0-9]+/, listener);
count = emitter.patternListeners(/foo:.*[0-9]+/).length; // 1 


By default, PatternEmitters will print a warning once more than 10 listeners are added for a particular event. This may be used to modify that threshold. Setting to 0 will disable the threshold altogether.

emitter.on(/^foo.*/, function() {});
emitter.on(/^foo.*/, function() {});
// (node) warning: possible EventEmitter memory leak detected. 2 listeners 
// added. Use emitter.setMaxListeners() to increase limit. 

Class Methods

In the outline below, let pattern refer to any instance of RegExp, and event all other values.

PatternEmitter.listenerCount(emitter, event)

Returns the number of listeners for a given event. An alias for EventEmitter.listenerCount.

emitter.on('foo', function() {});
emitter.on(/foo/, function() {});
PatternEmitter.listenerCount(emitter, 'foo'); // 1 

PatternEmitter.matchingListenerCount(emitter, event)

Returns the number of listeners and pattern listeners registered to the emitter for the event type or a matching pattern.

emitter.on('foo', function() {});
emitter.on(/foo/, function() {});
PatternEmitter.matchingListenerCount(emitter, 'foo'); // 2 

PatternEmitter.patternListenerCount(emitter, pattern)

Returns the number of listeners registered to the emitter for the specified pattern.

emitter.on('foo', function() {});
emitter.on(/foo/, function() {});
PatternEmitter.patternListenerCount(emitter, /foo/); // 1 


Like EventEmitter, instances of PatternEmitter emit events both when listeners are added and removed. The events can be matched by pattern listeners, and as such will include the name/type as a property, accessible via this.event.


The event is emitted any time a new listener is added to the emitter. The event is emitted just prior to the listener being added, to prevent recursion.

  • * event | pattern
  • function listener
emitter.on(/Listener/, function(pattern, listener) {
  // this.event will equal 'newListener' when a new listener is added 


The event is emitted any time an existing listener is removed from the emitter. The event is emitted just prior to the listener being removed, to prevent recursion.

  • * event | pattern
  • function listener
emitter.on('removeListener', function(event, listener) {
  // invoked when a listener is removed 


TL;DR: Likely not an issue unless you're invoking in excess of 1,000,000 listeners per second

Despite the ease of replacing EventEmitter throughout your application, performance should be considered for any event-heavy code. This implementation was done in an attempt to avoid copying a majority of the source in node/lib/events.js, which resulted in a couple otherwise unnecessary assignments and function calls when dealing with EventEmitter's default behaviour.

To illustrate, consider the performance difference between both modules when only registering to string events, no regular expressions. Running benchmarks/eventEmitting.js:

$ node benchmarks/eventEmitting.js
EventEmitter x 208,424 ops/sec ±0.28% (102 runs sampled)
PatternEmitter x 164,004 ops/sec ±0.45% (101 runs sampled)

Each operation in the above benchmark is invoking 100 listeners: 10 for each of 10 different events. That is, 20,842,400 vs 16,400,400 invocations a second on my Macbook Air. So while a performance drop, it may not be a problem for your average node instance.

For testing PatternEmitter with its pattern matching behaviour, a second naive benchmark currently exists. With 100 pattern listeners, 10 for each of 10 different patterns, benchmarks/patternEmitting.js can be used:

$ node benchmarks/patternEmitting.js
PatternEmitter x 33,179 ops/sec ±0.26% (101 runs sampled)

Still, 3,317,900 invocations a second for simple, small regular expressions. Of course, your own numbers will vary depending on the complexity of the patterns.