How do you convert Html to plain text?

If you are talking about tag stripping, it is relatively straight forward if you don't have to worry about things like <script> tags. If all you need to do is display the text without the tags you can accomplish that with a regular expression:

<[^>]*>

If you do have to worry about <script> tags and the like then you'll need something a bit more powerful then regular expressions because you need to track state, omething more like a Context Free Grammar (CFG). Althought you might be able to accomplish it with 'Left To Right' or non-greedy matching.

If you can use regular expressions there are many web pages out there with good info:

• http://weblogs.asp.net/rosherove/archive/2003/05/13/6963.aspx
• http://www.google.com/search?hl=en&q=html+tag+stripping+&btnG=Search

If you need the more complex behaviour of a CFG I would suggest using a third party tool, unfortunately I don't know of a good one to recommend.

The MIT licensed HtmlAgilityPack has in one of its samples a method that converts from HTML to plain text.

var plainText = HtmlUtilities.ConvertToPlainText(string html);

Feed it an HTML string like

<b>hello, <i>world!</i></b>

And you'll get a plain text result like:

hello world!

I could not use HtmlAgilityPack, so I wrote a second best solution for myself

private static string HtmlToPlainText(string html)
{
    const string tagWhiteSpace = @"(>|$)(\W|\n|\r)+<";//matches one or more (white space or line breaks) between '>' and '<'
    const string stripFormatting = @"<[^>]*(>|$)";//match any character between '<' and '>', even when end tag is missing
    const string lineBreak = @"<(br|BR)\s{0,1}\/{0,1}>";//matches: <br>,<br/>,<br />,<BR>,<BR/>,<BR />
    var lineBreakRegex = new Regex(lineBreak, RegexOptions.Multiline);
    var stripFormattingRegex = new Regex(stripFormatting, RegexOptions.Multiline);
    var tagWhiteSpaceRegex = new Regex(tagWhiteSpace, RegexOptions.Multiline);

    var text = html;
    //Decode html specific characters
    text = System.Net.WebUtility.HtmlDecode(text); 
    //Remove tag whitespace/line breaks
    text = tagWhiteSpaceRegex.Replace(text, "><");
    //Replace <br /> with line breaks
    text = lineBreakRegex.Replace(text, Environment.NewLine);
    //Strip formatting
    text = stripFormattingRegex.Replace(text, string.Empty);

    return text;
}