Issue 4593 - Support regular expressions for CSS property matching

lib/filterClasses.js

Index: lib/filterClasses.js
===================================================================
--- a/lib/filterClasses.js
+++ b/lib/filterClasses.js
@@ -1037,13 +1037,17 @@
   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 = Filter.toRegExp(this.regexpSource);
+    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.

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