The CssSelector Component

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The CssSelector component converts CSS selectors to XPath expressions.

Installation

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$ composer require symfony/css-selector

Note

If you install this component outside of a Symfony application, you must require the vendor/autoload.php file in your code to enable the class autoloading mechanism provided by Composer. Read this article for more details.

Usage

See also

This article explains how to use the CssSelector features as an independent component in any PHP application. Read the Symfony Functional Tests article to learn about how to use it when creating Symfony tests.

Why Use CSS selectors?

When you're parsing an HTML or an XML document, by far the most powerful method is XPath.

XPath expressions are incredibly flexible, so there is almost always an XPath expression that will find the element you need. Unfortunately, they can also become very complicated, and the learning curve is steep. Even common operations (such as finding an element with a particular class) can require long and unwieldy expressions.

Many developers -- particularly web developers -- are more comfortable using CSS selectors to find elements. As well as working in stylesheets, CSS selectors are used in JavaScript with the querySelectorAll() function and in popular JavaScript libraries such as jQuery.

CSS selectors are less powerful than XPath, but far easier to write, read and understand. Since they are less powerful, almost all CSS selectors can be converted to an XPath equivalent. This XPath expression can then be used with other functions and classes that use XPath to find elements in a document.

The CssSelector Component

The component's only goal is to convert CSS selectors to their XPath equivalents, using toXPath():

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use Symfony\Component\CssSelector\CssSelectorConverter;

$converter = new CssSelectorConverter();
var_dump($converter->toXPath('div.item > h4 > a'));

This gives the following output:

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descendant-or-self::div[@class and contains(concat(' ',normalize-space(@class), ' '), ' item ')]/h4/a

You can use this expression with, for instance, DOMXPath or SimpleXMLElement to find elements in a document.

Tip

The Crawler::filter() method uses the CssSelector component to find elements based on a CSS selector string. See the The DomCrawler Component for more details.

Limitations of the CssSelector Component

Not all CSS selectors can be converted to XPath equivalents.

There are several CSS selectors that only make sense in the context of a web-browser.

  • link-state selectors: :link, :visited, :target
  • selectors based on user action: :hover, :focus, :active
  • UI-state selectors: :invalid, :indeterminate (however, :enabled, :disabled, :checked and :unchecked are available)

Pseudo-elements (:before, :after, :first-line, :first-letter) are not supported because they select portions of text rather than elements.

Pseudo-classes are partially supported:

  • Not supported: *:first-of-type, *:last-of-type, *:nth-of-type and *:nth-last-of-type (all these work with an element name (e.g. li:first-of-type) but not with the * selector).
  • Supported: *:only-of-type.

Learn more

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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