lark-parser/lark

★ 5,876PythonAudience · developerComplexity · 2/5Setup · easy

Mindmap

mindmap
  root((repo))
    What It Does
      Parses text via grammar
      Builds parse trees
      Handles ambiguity
    Parsing Strategies
      Earley parser
      LALR1 parser
      Standalone export
    Features
      EBNF grammar syntax
      Line column tracking
      Interactive mode
      Unicode support
    Use Cases
      Custom languages
      Config file parsers
      Language tooling
    Tech Stack
      Python
      No dependencies

mindmap root((repo)) What It Does Parses text via grammar Builds parse trees Handles ambiguity Parsing Strategies Earley parser LALR1 parser Standalone export Features EBNF grammar syntax Line column tracking Interactive mode Unicode support Use Cases Custom languages Config file parsers Language tooling Tech Stack Python No dependencies

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Things people build with this

USE CASE 1

Build a custom config file parser that validates syntax and reports line and column errors to users

USE CASE 2

Create a domain-specific mini-language for your app and transform it into executable instructions

USE CASE 3

Parse programming language source code into a tree for analysis or transformation

USE CASE 4

Generate a standalone parser file that runs without Lark installed, for distribution in other projects

Tech stack

Python

Getting it running

Difficulty · easy Time to first run · 30min

Pure Python with no external dependencies, install with a single pip command.

In plain English

Lark is a Python library for parsing text that follows a formal grammar. A parser is a program that reads structured input, such as a programming language, a configuration format, or a custom mini-language, and turns it into a structured tree your code can work with. Lark handles a wide class of grammars called context-free grammars, which covers virtually every programming language and many other structured text formats. You define the rules of your language using a notation called EBNF, a standard way to describe grammars with patterns, repetitions, and alternatives. Once you provide a grammar, Lark reads it and builds a parser automatically. You do not need to write the tree-building code yourself: Lark produces an annotated parse tree from your input, ready to traverse or transform. Lark offers two main parsing strategies. Earley can handle any valid grammar, including ambiguous ones where a sentence can be interpreted multiple ways, and Lark will return all possible interpretations. LALR(1) is faster and more memory-efficient, suited for grammars that have no ambiguity, it can also generate a stand-alone parser file that does not depend on Lark at runtime. You pick the strategy that fits your speed and flexibility needs. The library is pure Python with no external dependencies, so installation is a single pip command. It tracks line and column numbers automatically, which helps when you need to report errors to users. An interactive parser mode lets you step through parsing incrementally, useful for debugging or building editors with real-time feedback. Unicode is fully supported throughout. Lark is used in several well-known Python projects, including Poetry for package management, Vyper for smart contract development, and the Hypothesis testing library. It supports syntax highlighting extensions for several popular editors, has a companion JavaScript port for LALR(1) grammars, and comes with a tutorial showing how to build a JSON parser from scratch.

Copy-paste prompts

Prompt 1

Using Lark, write a Python grammar for simple arithmetic expressions handling +, -, *, / and parentheses, then parse the string (3 + 4) * 2 and print the parse tree.

Prompt 2

Write a Lark EBNF grammar and Python parser that reads an ini-style config file with sections and key=value pairs, returning a Python dict.

Prompt 3

Use Lark's LALR(1) strategy to build a parser for a mini-language and export it as a standalone file that works without the Lark library installed.

Prompt 4

Build a Lark grammar for JSON from scratch following the Lark tutorial, then parse a sample JSON string and traverse the resulting tree.

Prompt 5

Using Lark's Earley parser, write a grammar for an ambiguous expression language and print all possible parse trees for the input a + b + c.

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