fancy-regex
A Rust regex library with hybrid NFA/backtracking engine for advanced features like look-around and backreferences.
What is fancy-regex?
fancy-regex is a Rust library for regular expressions that supports advanced features like look-around and backreferences through a hybrid implementation. It combines backtracking for these "fancy" features with delegation to a high-performance NFA engine (the Rust regex crate) for parts of expressions that don't require them, aiming to provide both rich functionality and good performance.
Rust developers needing advanced regex features (like look-around or backreferences) for tasks such as syntax highlighting, text processing, or compatibility with Oniguruma-based grammars, where purely NFA-based engines are insufficient.
Developers choose fancy-regex because it uniquely offers advanced regex features in Rust without the exponential worst-case performance of pure backtracking engines, by intelligently delegating to the efficient regex crate where possible, and providing Oniguruma syntax compatibility for editor grammars.
Overview
Rust library for regular expressions using "fancy" features like look-around and backreferences. Has a mode which aims to be compatible with Oniguruma syntax.
Use Cases
Best For
- Implementing syntax highlighting grammars that use Oniguruma-style regex features
- Matching patterns with look-ahead or look-behind assertions in Rust applications
- Handling regex patterns with backreferences for text processing tasks
- Ensuring compatibility with TextMate/Sublime Text/Atom editor syntax definitions
- Avoiding exponential blowup in regex matching while using advanced features
- Building text editors or IDEs with advanced regex-based parsing in Rust
Not Ideal For
- Security-critical applications where regex patterns are user-controlled and denial-of-service attacks are a concern
- Projects requiring recursive backreferences or other incomplete features listed in the README
- Use cases that only need basic regex matching without fancy features, where the standard regex crate is sufficient
- Environments demanding strict worst-case performance guarantees for all regex patterns, such as high-availability servers
Pros & Cons
Pros
Supports look-around and backreferences, enabling complex patterns like those used in TextMate syntax definitions, as shown in the README's example of capturing Rust raw strings with r(#*)".*?"\1.
Delegates to the Rust regex crate for NFA-based matching where possible, minimizing backtracking and aiming for linear-time performance, as detailed in the theory section on bottom-up and top-down analysis.
Offers a mode compatible with Oniguruma syntax, making it suitable for porting editor grammars from Sublime Text or Atom, as mentioned in the philosophy for syntax highlighting.
Provides a web-based playground for testing and exploring regexes with advanced features, linked in the README for easy experimentation and debugging.
Cons
The README explicitly states that backreferences at recursion levels and optimizations are currently missing, limiting functionality for advanced or edge-case regex patterns.
For regexes with fancy features, fancy-regex gives no guarantees against exponential blowup, posing a denial-of-service risk if patterns are attacker-controlled, as warned in the 'A warning about worst-case performance' section.
Still in development with ongoing changes, which might lead to breaking updates or incomplete documentation compared to more mature libraries like the regex crate.
Open Source Alternative To
fancy-regex is an open-source alternative to the following products:
Frequently Asked Questions
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