Match patterns

Match patterns are a way to specify groups of URLs: a match pattern matches a specific set of URLs. They are for extensions using WebExtensions APIs in a few places, most notably to specify which documents to load content scripts into, and to specify which URLs to add webRequest listeners to.

APIs that use match patterns usually accept a list of match patterns, and will perform the appropriate action if the URL matches any of the patterns. See, for example, the content_scripts key in manifest.json.

All match patterns are specified as strings. Apart from the special "<all_urls>" pattern, match patterns consist of three parts: scheme, host, and path. The scheme and host are separated by "://".

<scheme>://<host><path>

The scheme component may take one of two forms:

The host component may take one of three forms:

host is optional only if the scheme is "file".

Note that the wildcard may only appear at the start.

The path component must begin with a "/".

After that, it may subsequently contain any combination of the "*" wildcard and any of the characters that are allowed in URL paths. Unlike host, the path component may contain the "*" wildcard in the middle or at the end, and the "*" wildcard may appear more than once.

The special value "<all_urls>" matches all URLs under any of the supported schemes: that is, "http", "https", "file", "ftp", "app".

When writing extensions, you don't generally work with match patterns directly: usually you pass a match pattern string into an API, and the API constructs a match pattern and uses it to test URLs. However, if you're trying to work out which match pattern to use, or debugging a problem with one, it can be useful to be able to create and test match patterns directly. This section explains how to do this.

Note that the code here will not work in an extension, and is only provided to help manually test match patterns using the console.

First, open the developer tool settings and check the setting marked "Enable browser chrome and add-on debugging toolboxes":

Next, open the "Browser Console":

This gives you a command line that you can use to execute privileged JavaScript in Firefox.

Now paste this code into the command line and press enter:

Cu.import("resource://gre/modules/MatchPattern.jsm");
Cu.import("resource://gre/modules/BrowserUtils.jsm");

This does two things:

• imports "MatchPattern.jsm": this is the system module that implements match patterns. Specifically, the module contains a constructor for MatchPattern objects. MatchPattern objects define a function called matches(), that takes a URI and returns true or false.
• imports "BrowserUtils.jsm": this includes a function makeURI(), that converts a string into an nsIURI object. nsIURI is the type that matches() expects to receive.

Now you can construct MatchPattern objects, construct URIs, and check whether the URIs match:

var match = new MatchPattern("*://mozilla.org/");

var uri = BrowserUtils.makeURI("https://mozilla.org/");
match.matches(uri); //        < true

uri = BrowserUtils.makeURI("https://mozilla.org/path");
match.matches(uri); //        < false

All match patterns can be represented by regular expressions. This code converts a match pattern to a regular expression:

/**
 * Transforms a valid match pattern into a regular expression
 * which matches all URLs included by that pattern.
 *
 * @param  {string}  pattern  The pattern to transform.
 * @return {RegExp}           The pattern's equivalent as a RegExp.
 * @throws {TypeError}        If the pattern is not a valid MatchPattern
 */
function matchPatternToRegExp(pattern) {
    if (pattern === '') {
        return (/^(?:http|https|file|ftp|app):\/\//);
    }

    const schemeSegment = '(\\*|http|https|file|ftp)';
    const hostSegment = '(\\*|(?:\\*\\.)?(?:[^/*]+))?';
    const pathSegment = '(.*)';
    const matchPatternRegExp = new RegExp(
        `^${schemeSegment}://${hostSegment}/${pathSegment}$`
    );

    let match = matchPatternRegExp.exec(pattern);
    if (!match) {
         throw new TypeError(`"${pattern}" is not a valid MatchPattern`);
    }

    let [, scheme, host, path] = match;
    if (!host) {
        throw new TypeError(`"${pattern}" does not have a valid host`);
    }

    let regex = '^';

    if (scheme === '*') {
        regex += '(http|https)';
    } else {
        regex += scheme;
    }

    regex += '://';

    if (host && host === '*') {
        regex += '[^/]+?';
    } else if (host) {
        if (host.match(/^\*\./)) {
            regex += '[^/]*?';
            host = host.substring(2);
        }
        regex += host.replace(/\./g, '\\.');
    }

    if (path) {
        if (path === '*') {
            regex += '(/.*)?';
        } else if (path.charAt(0) !== '/') {
            regex += '/';
            regex += path.replace(/\./g, '\\.').replace(/\*/g, '.*?');
            regex += '/?';
        }
    }

    regex += '$';
    return new RegExp(regex);
}