PEGTL (Parsing Expression Grammar Template Library)
PEGTL is a header-only C++ library for parsing based on Parsing Expression Grammar (PEG). It provides a lightweight, efficient, and flexible solution for building parsers for expressions, DSLs, and other structured input.
Repository
PEGTL is actively maintained on GitHub:
https://github.com/taocpp/PEGTL
The repository includes extensive examples in the examples directory, covering expression parsing, DSLs, and more.
These examples provide practical guidance for integrating PEGTL into your C++ projects.
Why PEGTL?
PEGTL is designed for C++ ecosystems and is suitable when:
- You need to parse small to medium-sized expressions or DSLs.
- You want control over parser behavior and memory usage.
- Integration with existing C++ codebase is a priority.
- Performance is important, but parsing load is moderate, not extreme online QPS.
It is not intended for full SQL engines or complex grammars that require advanced optimizations or backtracking-heavy parsing.
Integration Complexity
- Header-only: no library linking required.
- Easy to integrate into existing C++ projects.
- Minimal dependencies, purely C++17-compatible.
Performance Characteristics
| Metric | Typical Range / Notes |
|---|---|
| Parsing Speed | Very fast for small expressions, moderate for large nested grammars |
| Memory Usage | Low, can be tightly controlled |
| Complexity Handling | Handles moderate recursion and backtracking efficiently |
| Scalability | Suitable for offline compilers or internal DSL parsing |
For extremely complex grammars or very high online request rates (thousands of QPS parsing large inputs), PEGTL may not be sufficient.
Use Cases
- Expression Evaluation: Parsing
a + b * cor user-defined functions. - Internal DSLs: Small domain-specific languages for configuration, query fragments, or scripting.
- Offline Code Generation: Intermediate parser for compiler-like workflows.
Comparison with Other IR Parsing Tools
- Bison / Flex: Better for full SQL parsing or production-grade compilers. More complex integration, but necessary for high-performance or large-scale grammars.
- PEGTL: Preferable for small, controllable grammars, offline tooling, or expression evaluation. Easier to integrate and maintain in C++ projects.
Summary
PEGTL offers a lightweight, header-only, C++-friendly parsing solution. It is best for offline compilation, DSL processing, and expression evaluation, where grammar size is manageable, performance is important, and full compiler complexity is unnecessary. The examples directory in the repository provides practical templates to accelerate integration and learning.