Ragel works fine. You just need to be careful about what you're matching. Your question uses both [[tag]] and {{tag}}, but your example uses [[tag]], so I figure that's what you're trying to treat as special. What you want to do is eat text until you hit an open-bracket. If that bracket is followed by another bracket, then it's time to start eating lowercase characters till you hit a close-bracket
![How to parse template languages in Ragel?](https://cdn-ak-scissors.b.st-hatena.com/image/square/98d6f053a97a87156775f60757c60865d0f2c47d/height=288;version=1;width=512/https%3A%2F%2Fcdn.sstatic.net%2FSites%2Fstackoverflow%2FImg%2Fapple-touch-icon%402.png%3Fv%3D73d79a89bded)