Issue 4593 - Support regular expressions for CSS property matching

lib/filterClasses.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 1025 matching lines @@
    */
   get regexpString()
   {
     // Despite this property being cached, the getter is called
     // several times on Safari, due to WebKit bug 132872
     let prop = Object.getOwnPropertyDescriptor(this, "regexpString");
     if (prop)
       return prop.value;
 
     let regexp;
-    if (this.regexpSource.startsWith("/") && this.regexpSource.endsWith("/"))

[Wladimir Palant, 2016/11/08 09:56:41]

This is a potentially performance-sensitive code path. We shouldn't be using
startsWith() and endsWith() unnecessarily, these are slow compared to the
`source.length >= 2 && source[0] == "/" && source[source.length - 1] == "/"`
check we use in RegExpFilter. For reference,
https://jsperf.com/startswith-vs-direct-access gives me 46% slower in Firefox
and 39% slower in Chrome.

Besides, this check will currently consider "/" to be a regular expression - it
shouldn't.

[Felix Dahlke, 2016/11/08 15:46:36]

I'm not so sure about the performance impact of this being very relevant - this
is being called from the CSS property filters content script, and only for
filters that match on the current domain.

But anyway, you're definitely right about the semantic difference, "/" should
not be considered to be a regexp. So one way to fix that with my code would be
`this.regexpSource.length >= 2 && this.regexpSource.startsWith("/") and
this.regexpSource.endsWidth("/")`, from which it isn't far to your suggestion
which is also faster. So I'll change that.

[Felix Dahlke, 2016/11/08 17:28:39]

Done.

+    if (this.regexpSource.length >= 2 && this.regexpSource[0] == "/" &&
+        this.regexpSource[this.regexpSource.length - 1] == "/")

[Wladimir Palant, 2016/11/08 17:41:34]

Well, accessing this.regexpSource four times is obviously not perfect either but
whatever - I guess that by the time this code path becomes relevant
performance-wise we'll have it replaced by C++ anyway.

       regexp = this.regexpSource.slice(1, -1);
     else
       regexp = Filter.toRegExp(this.regexpSource);
     Object.defineProperty(this, "regexpString", {value: regexp});
     return regexp;
   }
 });