Question

How would you write a regular expression to convert mark down into HTML? For example, you would type in the following:

This would be *italicized* text and this would be **bold** text 

This would then need to be converted to:

This would be <em>italicized</em> text and this would be <strong>bold</strong> text 

Very similar to the mark down edit control used by stackoverflow.

Clarification

For what it is worth, I am using C#. Also, these are the only real tags/markdown that I want to allow. The amount of text being converted would be less than 300 characters or so.

Answer 1

The best way is to find a version of the Markdown library ported to whatever language you are using (you did not specify in your question).

---

Now that you have clarified that you only want STRONG and EM to be processed, and that you are using C#, I recommend you take a look at Markdown.NET to see how those tags are implemented. As you can see, it is in fact two expressions. Here is the code:

private string DoItalicsAndBold (string text) {     // <strong> must go first:     text = Regex.Replace (text, @'(\*\*|__) (?=\S) (.+?[*_]*) (?<=\S) \1',                            new MatchEvaluator (BoldEvaluator),                           RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline);      // Then <em>:     text = Regex.Replace (text, @'(\*|_) (?=\S) (.+?) (?<=\S) \1',                           new MatchEvaluator (ItalicsEvaluator),                           RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline);     return text; }  private string ItalicsEvaluator (Match match) {     return string.Format ('<em>{0}</em>', match.Groups[2].Value); }  private string BoldEvaluator (Match match) {     return string.Format ('<strong>{0}</strong>', match.Groups[2].Value); }