Richard Johnson » 2010

Archive for November, 2010

Converting relative URLs to absolute ones.

Tuesday, November 30th, 2010

On a number of occasions I have found that I need to convert all URLs in a webpage from relative ones to absolute ones. This is usually done when you are displaying a page sourced from one domain from a different domain.

Note that this doesn’t really provide full emulation or proxying of a site, you need to take into account cookies, etc. to do that correctly.

Here is how I most recently cracked it (in C# .NET). This isn’t fool proof; however it should work with most HTML:

public static string RelativeToAbsoluteURLs(string input, string uriPrefix, string path)
{
	string[] attributesToPrefix = {"href", "src", "action"};

foreach (string attributeToPrefix in attributesToPrefix) {

string regex = "<([^>]*?)" + attributeToPrefix + "=([\"'])(.*?)([\"'])([^>]*?)>";

Regex reg = new Regex(regex, RegexOptions.IgnoreCase | RegexOptions.Singleline);
		MatchCollection matches = reg.Matches(input);
		foreach (Match match in matches)
		{
			string add = match.Groups[3].Value;
			if (add.Length > 1)
			{
				if (add.IndexOf("//") >= 0 || add.IndexOf(":") >= 0)
				{
					// we are already absolute at the protocol level, nothing to do with this match
				}
				else if (add.Substring(0, 1) == "/")
				{
					// we are starting with a /, just throw the uri prefix in there
					string src = match.Groups[0].Value;
					string dest = "<" + match.Groups[1].Value + attributeToPrefix + "=" + match.Groups[2].Value + uriPrefix + match.Groups[3].Value + match.Groups[4].Value + match.Groups[5].Value + ">";

input = input.Replace(src, dest);
				}
				else
				{
					// normal, relative url!
					string src = match.Groups[0].Value;
					string dest = "<" + match.Groups[1].Value + attributeToPrefix + "=" + match.Groups[2].Value + uriPrefix + path + match.Groups[3].Value + match.Groups[4].Value + match.Groups[5].Value + ">";

input = input.Replace(src, dest);
				}
			}
		}
	}
	return input;
}

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Archive for November, 2010

Converting relative URLs to absolute ones.