security: Fix REDOS vulnerability

Problem:
As disclosed by email, the regex in this module was vulnerable to
catastrophic backtracking.
This could be used as a REDOS vector for Node.js servers that use
this module.

Solution:
Use a two-step validation process.
Split the vulnerable regex into three safe ones.

index.js

@@ -6,10 +6,15 @@
 module.exports = isUrl;
 
 /**
- * Matcher.
+ * RegExps.
+ * A URL must match #1 and then at least one of #2/#3.
+ * Use two levels of REs to avoid REDOS.
  */
 
-var matcher = /^(?:\w+:)?\/\/([^\s\.]+\.\S{2}|localhost[\:?\d]*)\S*$/;
+var protocolAndDomainRE = /^(?:\w+:)?\/\/(\S+)$/;
+
+var localhostDomainRE = /^localhost[\:?\d]*(?:[^\:?\d]\S*)?$/
+var nonLocalhostDomainRE = /^[^\s\.]+\.\S{2,}$/;
 
 /**
  * Loosely validate a URL `string`.
@@ -19,5 +24,20 @@ var matcher = /^(?:\w+:)?\/\/([^\s\.]+\.\S{2}|localhost[\:?\d]*)\S*$/;
  */
 
 function isUrl(string){
-  return matcher.test(string);
+  var match = string.match(protocolAndDomainRE);
+  if (!match) {
+    return false;
+  }
+
+  var everythingAfterProtocol = match[1];
+  if (!everythingAfterProtocol) {
+    return false;
+  }
+
+  if (localhostDomainRE.test(everythingAfterProtocol) ||
+      nonLocalhostDomainRE.test(everythingAfterProtocol)) {
+    return true;
+  }
+
+  return false;
 }

test/index.js

@@ -119,4 +119,15 @@ describe('is-url', function () {
       assert(!url('google.com'));
     });
   });
+
+  describe('redos', function () {
+    it('redos exploit', function () {
+      // Invalid. This should be discovered in under 1 second.
+      var attackString = 'a://localhost' + '9'.repeat(100000) + '\t';
+      var before = process.hrtime();
+      assert(!url(attackString), 'attackString was valid');
+      var elapsed = process.hrtime(before);
+      assert(elapsed[0] < 1, 'attackString took ' + elapsed[0] + ' > 1 seconds');
+    });
+  });
 });