Hi,

You can do something like this:

public static string RewriteUrlsToFriendly(this string input, PageData page)
{
    UrlBuilder internalUrl = new UrlBuilder(page.LinkURL);
    UrlBuilder externalUrl = new UrlBuilder(page.LinkURL);

    HtmlRewriteToExternal rewriter = EPiServer.Global.UrlRewriteProvider.GetHtmlRewriter();

    EPiServer.Global.UrlRewriteProvider.ConvertToExternal(externalUrl, page.PageLink, UTF8Encoding.UTF8);

    return rewriter.RewriteString(internalUrl, externalUrl, UTF8Encoding.UTF8, input);
}

You can also see this article http://world.episerver.com/Blogs/Deane-Barker/Dates/2010/9/Correcting-All-Raw-URLs-Inside-a-String-of-HTML/