I'm considering supporting both Textile and Markdown on a current project. I would prefer not forcing users to choose one or the other. Is there a way to auto-detect which the user is using? How would you go about this? I'd like to find / develop both a JavaScript and a PHP solution so I can provide live previews as well as process the user input on the server-side.
Consider that users might only use one specific syntax element in a posting, so you'd have to check for everything. Looking for "h1." obviously only works if the user uses exactly that element.
It's pretty easy with things like headers, but consider that markdown formats *this* as <em>this</em> and Textile will convert that to <strong>this</strong> instead. So you'd have ambiguous syntax constructs that would yield different results in each language.
I'd suggest going with a user choice. Try to find out what syntax is generally preferred by your users (or you), offer an "use x instead of y" checkbox for those who want the other choice.