I think you will find www.debuggex.com very valuable. Some advantages of debuggex are:
- Feedback is in real-time, so you can understand what's happening faster
- The ability to answer the question "Why does/doesn't it match here?" for any position in your text.
- Very heavy on the visuals, it was a core design decision behind it's creation
At the time of writing, it only supports Javascript. However, more languages are planned for the future.
