An interactive tool that builds an LR(0) parser step by step, so students can see how a context-free grammar becomes a DFA and a parsing table, and watch a sentence being parsed on the stack. Live at lr0parser.com.

Most parser generators just print the answer. This one walks through the construction the way a compilers course teaches it, with a plain-language explanation at every phase.

• Six guided phases: grammar, augment & number, LR(0) item sets, DFA, parsing table, and parse a sentence. Jump to any phase from the sticky rail, or scroll straight through.
• Closure & GOTO, made visible. Each item set shows its kernel vs. closure items and where every symbol transitions next.
• Interactive DFA. The item sets are rendered as a pannable automaton with zoom controls; click a state to inspect its item set.
• Colour-coded ACTION/GOTO table so shift, reduce, accept, and go-to each read at a glance.
• Conflict detection. When a grammar isn't LR(0), the clashing cells are flagged and explained instead of being silently overwritten.
• Step-through parsing. Run a string and step, play, or skip through the stack trace.
• Quick by default, step-by-step on demand. The default view shows the full result at once; one button reveals a guided walkthrough with explanations at every phase.
• Light & dark themes, responsive, keyboard-accessible.

• The first line is the start non-terminal on its own.
• One production per line, e.g. A->(AB).
• Every character is its own symbol. Non-terminals are A to Z, everything else is a terminal. Don't separate symbols with spaces.
• Don't add the end-of-input marker $ or the augmented rule; the tool adds them for you.

S
S->A
A->(AB)
A->()
B->(A)
B->()

React (Create React App). The parsing engine lives in src/lib as pure, framework-free functions (grammar, itemsets, table, parse) covered by unit tests; the UI in src/components renders that data as real components. Theming is driven by CSS custom properties. The DFA is drawn with react-graph-vis.

npm install
npm start      # http://localhost:3000
npm test       # run the parsing-engine tests
npm run build  # production build

Built by Mohamed Abdelmaksoud. If it helped you, you can buy me a coffee.