A C# library for stemming and comparing URLs.
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URL Stemmer

This utility class attempts to normalize, stem, or simplify a URL based on provided settings, usually to prepare it for comparison or storage.

An online tester is located here: http://url-stemmer.azurewebsites.net/. (The source of this tester is contained in this repository under "Website" folder.)

Basic usage:

UrlStemmer.Settings.RemoveSubdomain = true;
UrlStemmer.Settings.ForceScheme = "http";
UrlStemmer.Settings.ReorderQuerystringArguments = true;
var stemmedUrl = UrlStemmer.Stem("https://www.gadgetopia.com/?c=d&a=b");
// Result: "http://gadgetopia.com/?a=b&c=d

If desired, the StemmedUrl class can be created directly:

var url = new StemmedUrl("http://gadgetopia.com");
var stem = url.ToString();

StemmedUrl overrides ToString which always provides the stemmed output of the URL passed into the constructor.

If the URL begins with a leading slash, it's assumed that it's local URL without a domain. The values of DefaultScheme and DefaultHost will be prepended. If the URL does not begin with a leading slash and does not contain "://", it's assumed that it's an absolute URL with a domain but not scheme. The value of DefaultScheme will be prepended. The default values of these settings are http and example.com.

// Result: http://example.com/my/path

// Result: http://gadgetopia.com/

UrlStemmer has a static UrlStemmingSettings object which is changed to affect how URLs are stemmed.

UrlStemmer.Settings.ForceHost = "gadgetopia.com";
UrlStemmer.Settings.RemoveBookmarks = true;

Stemming will always these static settings to ensure all URLs are stemmed identically. However it is possible to pass a custom settings object into the Stem and Compare methods, and into the constructor of StemmedUrl:

var customSettings = new UrlStemmingSettings() { RemoveBookmarks = true; };
var stemmedUrl = UrlStemmer.Stem("http://gadgetopia.com", customSettings);

All settings default to a "pass-through" state, meaning that if no settings are changed from the defaults, the output of Stem should be the same as the input (with a few built-in exceptions -- see "Built-in Stemming" below for some rules that the UrlBuilder class imposes on us).

Available settings on UrlStemmingSettings:

ForceHost (string)
If set, the host (domain) will be changed to this value

ForceLowerCase (bool)
The URL will be converted to lower-case

ForceScheme (string)
If set, the scheme (protocol) will be changed to this value

ForcePort (int)
If left at 0 (the default), this will leave the port unchanged. If set to -1, this will always remove the port. If set to any positive value, it will change the port to that value.

RemoveBookmarks (bool)
Any bookmarks at the end of the URL will be removed

RemoveSubdomain (bool)
Any (and all) subdomains will be removed

ReorderQuerystringArguments (bool)
Querystring arguments will be reordered alphabetically

ArgumentBlacklist (List<string>)
These querystring argument keys will be removed from the URL

ArgumentWhitelist (List<string>)
Anything other than these querystring argument keys will be removed from the URL

ClearQuerystring (bool)
Will remove all querystring arguments. (Setting this flag will make any ArgumentWhitelist, ArgumentBlacklist, and ReorderQuerystringArguments a moot point. They will simply be ignored.)

TrailingSlashes (enum)
Trailing slashes will be always added, always stripped, or ignored (meaning, slashes will be left as they were passed in). There is a limitation here (see "Built-in Stemming" below), in that you cannot strip the trailing slash if there is no folder path.

DefaultHost (string)
The default host prepended to the URL when no host is provided. Defaults to "example.com" as specified in RFC 2606.

DefaultScheme (string)
The default scheme prepended to the URL when no scheme is provided. Defaults to "http".

EncodingBehavior (enum: None, Decode, Encode)
Whether and how to encode or decode "%" encoded URL characters. Defaults to "None," which passes the URL through.

Built-in Stemming

UrlStemmer uses UrlBuilder under the hood, and there are three built-in stemming formats that it imposes which cannot be changed. However, in most cases, these are acceptable and desirable.

  • The URL will always have a path, even if that path is simply a forward slash to represent "root." It is not possible to generate a URL like http://gadgetopia.com (with no slash on the end).

  • Backslashes in a URL will always be replaced with forward slashes.

  • Double- and single dot notation ("parent pathing") will be removed and normalized. So, a double-dot segment (/../) meant to climb back up the path will be applied and the URL automatically corrected for it. (/foo/../bar/./baz becomes /bar/baz)


Calling Reset will replace all settings with a default UrlStemmingSettings object.

String stem;

UrlStemmer.Settings.RemoveSubdomain = true;
stem = UrlStemmer.Stem("http://www.gadgetopia.com/");
// Result: "http://gadgetopia.com"

UrlStemmer.Settings.ReorderQuerystringArgs = true;
stem = UrlStemmer.Stem("http://gadgetopia.com/?e=f&c=d&a=b
// Result: "http://gadgetopia.com/?c=d&e=f

UrlStemmer.Settings.RemoveBookmarks = true;
UrlStemmer.Settings.ForceScheme = "http";
UrlStemmer.Settings.TrailingSlashes = TrailingSlashes.AlwaysRemove;
stem = UrlStemmer.Stem("https://gadgetopia.com/foo/#chapter-1
// Result: "http://gadgetopia.com/foo

UrlStemmer.Settings.ForceLowerCase = true;
stem = UrlStemmer.Stem("http://gadgetopia.com/FOO/?e=f&c=d&a=b
// Result: "http://gadgetopia.com/foo/?c=d

To Do

  • We need to have a consistent handling of multiple querystring arguments. For example: ?a=b&c=d&a=f