Markingbird is a Markdown processor written in Swift for OS X and iOS. It is a translation/port of the MarkdownSharp processor used by the Stack Overflow website.
(If you have no idea what "Markdown" and "Swift" are, you can just stop reading now.)
Repository: https://github.com/kristopherjohnson/Markingbird
See the README for details on how to use it, and some of the details of how the C# code was translated to Swift.
I can't claim much credit for this project. It is really just a line-by-line translation of Jeff Atwood's C# code into Swift. The result is not very pretty, as the C# code is itself based upon translations of Perl and PHP code. I don't recommend this as an example of how to write Swift code, but it does seem to work, and so I hope it is valuable to somebody someday.
This project was useful to me in that it let me write a lot of Swift code without thinking too much about it. It helped me develop Swift-editing muscle memory, quick scanning and correction of Swift syntax issues, and interpretation of the Swift compiler's not-always-helpful error messages. I thought about giving up a few times, as manual translation of 1,800 lines of C# and regular expressions wasn't a great way to spend nights and weekends, but I just couldn't leave it partially finished.
I did the translation by just copying and pasting the C# code into Xcode, and then fixing whatever was wrong. This was a real workout for Xcode 6 beta 5, as the SourceKitService or Xcode itself would often crash or lock up when it encountered C# code.
The most annoying and fiddly part of the process was translating the C# regular-expression strings into Swift. It would be nice if Swift had a "raw" string literal like other languages do, but it doesn't, so I manually transformed each regular expression into a Swift/NSRegularExpression-compatible form. Sublime Text's multiple-selection feature was really helpful for this. I probably could have written some sort of a script to automate the translations, but actually looking at every line was valuable in finding edge cases.
I also discovered a few differences between .NET regular expressions and Cocoa regular expressions. For example, in .NET you can use [ ] (a single space between square brackets) to mean a single space, but NSRegularExpression won't accept this, so I had to use \\p{Z} instead. Also, if a regular expression pattern ends with a | character, .NET apparently ignores it, but NSRegularExpression just lets it match empty strings.
Some of the messiest code that I wrote myself (rather than simply translating) is code that manipulates individual characters and substrings. Swift's String type can make this difficult, due to its extensive Unicode support, so I found myself going back and forth between using String and NSString for string manipulation. The automatic bridging between those two types provided by Swift makes it pretty easy, but it would be nice if somebody made a library of utility functions for dealing with String. (BTW, I strongly recommend that all Swift programmers read Ole Begemann's Strings in Swift article to find out what's going on with String.)
As of now, the SimpleTests test suite from MarkdownSharp works. I plan to port the remaining unit tests from MarkdownSharp, and add a few tests of my own. There are also a few TODOs left in the code that need to be addressed.
I make no promises about the robustness or performance of Markingbird. If you find problems, please submit issues and pull requests via GitHub.
I know some people will want to tell me that using regular expressions for this is bad, and this should be a real parser written in pure Swift. I agree completely: let me know when you have implemented that, and I'll post links to it.