Question

Can I allow the domain matching for my extension to be user configurable?
I’d like to let my users choose when the extension runs.

Answer 1

To implement customizable “match patterns” for content scripts, the Content script need to be executed in by the background page using the chrome.tabs.executeScript method (after detecting a page load using the chrome.tabs.onUpdated event listener).

Because the match pattern check is not exposed in any API, you have to create the method yourself. It is implemented in url_pattern.cc, and the specification is available at match patterns.

Here’s an example of a parser:

/**
  * @param String input  A match pattern
  * @returns  null if input is invalid
  * @returns  String to be passed to the RegExp constructor */
function parse_match_pattern(input) {
    if (typeof input !== 'string') return null;
    var match_pattern = '(?:^'
      , regEscape = function(s) {return s.replace(/[[^$.|?*+(){}\\]/g, '\\$&');}
      , result = /^(\*|https?|file|ftp|chrome-extension):\/\//.exec(input);

    // Parse scheme
    if (!result) return null;
    input = input.substr(result[0].length);
    match_pattern += result[1] === '*' ? 'https?://' : result[1] + '://';

    // Parse host if scheme is not `file`
    if (result[1] !== 'file') {
        if (!(result = /^(?:\*|(\*\.)?([^\/*]+))(?=\/)/.exec(input))) return null;
        input = input.substr(result[0].length);
        if (result[0] === '*') {    // host is '*'
            match_pattern += '[^/]+';
        } else {
            if (result[1]) {         // Subdomain wildcard exists
                match_pattern += '(?:[^/]+\\.)?';
            }
            // Append host (escape special regex characters)
            match_pattern += regEscape(result[2]);
        }
    }
    // Add remainder (path)
    match_pattern += input.split('*').map(regEscape).join('.*');
    match_pattern += '$)';
    return match_pattern;
}

Example: Run content script on pages which match the pattern

In the example below, the array is hard-coded. In practice, you would store the match patterns in an array using localStorage or chrome.storage.

// Example: Parse a list of match patterns:
var patterns = ['*://*/*', '*exampleofinvalid*', 'file://*'];

// Parse list and filter(exclude) invalid match patterns
var parsed = patterns.map(parse_match_pattern)
                     .filter(function(pattern){return pattern !== null});
// Create pattern for validation:
var pattern = new RegExp(parsed.join('|'));

// Example of filtering:
chrome.tabs.onUpdated.addListener(function(tabId, changeInfo, tab) {
    if (changeInfo.status === 'complete') {
        var url = tab.url.split('#')[0]; // Exclude URL fragments
        if (pattern.test(url)) {
            chrome.tabs.executeScript(tabId, {
                file: 'contentscript.js'
                // or: code: '<JavaScript code here>'
                // Other valid options: allFrames, runAt
            });
        }
    }
});

To get this to work, you need to request the following permissions in the manifest file:

• "tabs" – To enable the necessary tabs API.
• "<all_urls>" – To be able to use chrome.tabs.executeScript to execute a content script in a specific page.

A fixed list of permissions

If the set of match patterns is fixed (ie. the user cannot define new ones, only toggle patterns), "<all_urls>" can be replaced with this set of permissions. You may even use optional permissions to reduce the initial number of requested permissions (clearly explained in the documentation of chrome.permissions).