When writing a grammar, it is difficult to imagine (especially in the big grammars) all the connections and internal states that will be generated by the parser generator.
Tunnel Grammar Studio (TGS) is a parser generator which provides a visualization of your grammars as automata. For debugging purposes, in real-time, with the help of the graphical unit interface, the developer can step by step, forward and backward, observe the creation of the syntax tree (generic, abstract or concrete) for the supplied input. Additionally to the syntax tree, the developer can see the stages of the input bytes decoded as a Unicode characters, their formation in tokens by the lexer grammar and the current parsing stack. This helps the developer to better understand the language defined by the grammars and to see quickly, without a compilation and any external tools, how the actual parsing is going to be done for this particular input.
In the TGS Online Laboratory, you can view the automata for your grammars and generate syntax trees for a your input string, in real-time, directly in your browser.

For more information: Tunnel Grammar Studio