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CKEditor Samples » Data Filtering and Features Activation

This sample is not maintained anymore. Check out its brand new version in CKEditor SDK.

This sample page demonstrates the idea of Advanced Content Filter (ACF), a sophisticated tool that takes control over what kind of data is accepted by the editor and what kind of output is produced.

When and what is being filtered?

ACF controls every single source of data that comes to the editor. It process both HTML that is inserted manually (i.e. pasted by the user) and programmatically like:

editor.setData( '<p>Hello world!</p>' );

ACF discards invalid, useless HTML tags and attributes so the editor remains "clean" during runtime. ACF behaviour can be configured and adjusted for a particular case to prevent the output HTML (i.e. in CMS systems) from being polluted. This kind of filtering is a first, client-side line of defense against "tag soups", the tool that precisely restricts which tags, attributes and styles are allowed (desired). When properly configured, ACF is an easy and fast way to produce a high-quality, intentionally filtered HTML.

How to configure or disable ACF?

Advanced Content Filter is enabled by default, working in "automatic mode", yet it provides a set of easy rules that allow adjusting filtering rules and disabling the entire feature when necessary. The config property responsible for this feature is config.allowedContent.

By "automatic mode" is meant that loaded plugins decide which kind of content is enabled and which is not. For example, if the link plugin is loaded it implies that <a> tag is automatically allowed. Each plugin is given a set of predefined ACF rules that control the editor until config.allowedContent is defined manually.

Let's assume our intention is to restrict the editor to accept (produce) paragraphs only: no attributes, no styles, no other tags. With ACF this is very simple. Basically set config.allowedContent to 'p':

var editor = CKEDITOR.replace( textarea_id, {
	allowedContent: 'p'
} );

Now try to play with allowed content:

// Trying to insert disallowed tag and attribute.
editor.setData( '<p style="color: red">Hello <em>world</em>!</p>' );
alert( editor.getData() );

// Filtered data is returned.
"<p>Hello world!</p>"

What happened? Since config.allowedContent: 'p' is set the editor assumes that only plain <p> are accepted. Nothing more. This is why style attribute and <em> tag are gone. The same filtering would happen if we pasted disallowed HTML into this editor.

This is just a small sample of what ACF can do. To know more, please refer to the sample section below and the official Advanced Content Filter guide.

You may, of course, want CKEditor to avoid filtering of any kind. To get rid of ACF, basically set config.allowedContent to true like this:

CKEDITOR.replace( textarea_id, {
	allowedContent: true
} );

Beyond data flow: Features activation

ACF is far more than I/O control: the entire UI of the editor is adjusted to what filters restrict. For example: if <a> tag is disallowed by ACF, then accordingly link command, toolbar button and link dialog are also disabled. Editor is smart: it knows which features must be removed from the interface to match filtering rules.

CKEditor can be far more specific. If <a> tag is allowed by filtering rules to be used but it is restricted to have only one attribute (href) config.allowedContent = 'a[!href]', then "Target" tab of the link dialog is automatically disabled as target attribute isn't included in ACF rules for <a>. This behaviour applies to dialog fields, context menus and toolbar buttons.

Sample configurations

There are several editor instances below that present different ACF setups. All of them, except the inline instance, share the same HTML content to visualize how different filtering rules affect the same input data.

This editor is using default configuration ("automatic mode"). It means that config.allowedContent is defined by loaded plugins. Each plugin extends filtering rules to make it's own associated content available for the user.

This editor is using a custom configuration for ACF:

CKEDITOR.replace( 'editor2', {
		'h1 h2 h3 p blockquote strong em;' +
		'a[!href];' +
		'img(left,right)[!src,alt,width,height];' +
		'table tr th td caption;' +
		'span{!font-family};' +'
		'span{!color};' +
		'span(!marker);' +
		'del ins'
} );

The following rules may require additional explanation:

  • h1 h2 h3 p blockquote strong em - These tags are accepted by the editor. Any tag attributes will be discarded.
  • a[!href] - href attribute is obligatory for <a> tag. Tags without this attribute are disarded. No other attribute will be accepted.
  • img(left,right)[!src,alt,width,height] - src attribute is obligatory for <img> tag. alt, width, height and class attributes are accepted but class must be either class="left" or class="right"
  • table tr th td caption - These tags are accepted by the editor. Any tag attributes will be discarded.
  • span{!font-family}, span{!color}, span(!marker) - <span> tags will be accepted if either font-family or color style is set or class="marker" is present.
  • del ins - These tags are accepted by the editor. Any tag attributes will be discarded.

Please note that UI of the editor is different. It's a response to what happened to the filters. Since text-align isn't allowed, the align toolbar is gone. The same thing happened to subscript/superscript, strike, underline (<u>, <sub>, <sup> are disallowed by config.allowedContent) and many other buttons.

This editor is using a custom configuration for ACF. Note that filters can be configured as an object literal as an alternative to a string-based definition.

CKEDITOR.replace( 'editor3', {
	allowedContent: {
		'b i ul ol big small': true,
		'h1 h2 h3 p blockquote li': {
			styles: 'text-align'
		a: { attributes: '!href,target' },
		img: {
			attributes: '!src,alt',
			styles: 'width,height',
			classes: 'left,right'
} );

This editor is using a custom set of plugins and buttons.

CKEDITOR.replace( 'editor4', {
	removePlugins: 'bidi,font,forms,flash,horizontalrule,iframe,justify,table,tabletools,smiley',
	removeButtons: 'Anchor,Underline,Strike,Subscript,Superscript,Image',
	format_tags: 'p;h1;h2;h3;pre;address'
} );

As you can see, removing plugins and buttons implies filtering. Several tags are not allowed in the editor because there's no plugin/button that is responsible for creating and editing this kind of content (for example: the image is missing because of removeButtons: 'Image'). The conclusion is that ACF works "backwards" as well: modifying UI elements is changing allowed content rules.

This editor is built on editable <h1> element. ACF takes care of what can be included in <h1>. Note that there are no block styles in Styles combo. Also why lists, indentation, blockquote, div, form and other buttons are missing.

ACF makes sure that no disallowed tags will come to <h1> so the final markup is valid. If the user tried to paste some invalid HTML into this editor (let's say a list), it would be automatically converted into plain text.

Apollo 11 was the spaceflight that landed the first humans, Americans Neil Armstrong and Buzz Aldrin, on the Moon on July 20, 1969, at 20:18 UTC.

This editor is using a custom configuration for ACF. It's using the Disallowed Content property of the filter to eliminate all title attributes.

CKEDITOR.replace( 'editor6', {
	allowedContent: {
		'b i ul ol big small': true,
		'h1 h2 h3 p blockquote li': {
			styles: 'text-align'
		a: {attributes: '!href,target'},
		img: {
			attributes: '!src,alt',
			styles: 'width,height',
			classes: 'left,right'
	disallowedContent: '*{title*}'
} );

This editor is using a custom configuration for ACF. It's using the Disallowed Content property of the filter to eliminate all a and img tags, while allowing all other tags.

CKEDITOR.replace( 'editor7', {
	allowedContent: {
		// Allow all content.
		$1: {
			elements: CKEDITOR.dtd,
			attributes: true,
			styles: true,
			classes: true
	disallowedContent: 'img a'
} );

Airly Forest