regex-syntax ============ This crate provides a robust regular expression parser. [![Build status](https://github.com/rust-lang/regex/workflows/ci/badge.svg)](https://github.com/rust-lang/regex/actions) [![Crates.io](https://img.shields.io/crates/v/regex-syntax.svg)](https://crates.io/crates/regex-syntax) [![Rust](https://img.shields.io/badge/rust-1.28.0%2B-blue.svg?maxAge=3600)](https://github.com/rust-lang/regex) ### Documentation https://docs.rs/regex-syntax ### Overview There are two primary types exported by this crate: `Ast` and `Hir`. The former is a faithful abstract syntax of a regular expression, and can convert regular expressions back to their concrete syntax while mostly preserving its original form. The latter type is a high level intermediate representation of a regular expression that is amenable to analysis and compilation into byte codes or automata. An `Hir` achieves this by drastically simplifying the syntactic structure of the regular expression. While an `Hir` can be converted back to its equivalent concrete syntax, the result is unlikely to resemble the original concrete syntax that produced the `Hir`. ### Example This example shows how to parse a pattern string into its HIR: ```rust use regex_syntax::Parser; use regex_syntax::hir::{self, Hir}; let hir = Parser::new().parse("a|b").unwrap(); assert_eq!(hir, Hir::alternation(vec![ Hir::literal(hir::Literal::Unicode('a')), Hir::literal(hir::Literal::Unicode('b')), ])); ``` ### Safety This crate has no `unsafe` code and sets `forbid(unsafe_code)`. While it's possible this crate could use `unsafe` code in the future, the standard for doing so is extremely high. In general, most code in this crate is not performance critical, since it tends to be dwarfed by the time it takes to compile a regular expression into an automaton. Therefore, there is little need for extreme optimization, and therefore, use of `unsafe`. The standard for using `unsafe` in this crate is extremely high because this crate is intended to be reasonably safe to use with user supplied regular expressions. Therefore, while there may be bugs in the regex parser itself, they should _never_ result in memory unsafety unless there is either a bug in the compiler or the standard library. (Since `regex-syntax` has zero dependencies.) ### Crate features By default, this crate bundles a fairly large amount of Unicode data tables (a source size of ~750KB). Because of their large size, one can disable some or all of these data tables. If a regular expression attempts to use Unicode data that is not available, then an error will occur when translating the `Ast` to the `Hir`. The full set of features one can disable are [in the "Crate features" section of the documentation](https://docs.rs/regex-syntax/*/#crate-features). ### Testing Simply running `cargo test` will give you very good coverage. However, because of the large number of features exposed by this crate, a `test` script is included in this directory which will test several feature combinations. This is the same script that is run in CI. ### Motivation The primary purpose of this crate is to provide the parser used by `regex`. Specifically, this crate is treated as an implementation detail of the `regex`, and is primarily developed for the needs of `regex`. Since this crate is an implementation detail of `regex`, it may experience breaking change releases at a different cadence from `regex`. This is only possible because this crate is _not_ a public dependency of `regex`. Another consequence of this de-coupling is that there is no direct way to compile a `regex::Regex` from a `regex_syntax::hir::Hir`. Instead, one must first convert the `Hir` to a string (via its `std::fmt::Display`) and then compile that via `Regex::new`. While this does repeat some work, compilation typically takes much longer than parsing. Stated differently, the coupling between `regex` and `regex-syntax` exists only at the level of the concrete syntax.