regex-syntax 0.6.29

A regular expression parser.


This crate provides a robust regular expression parser.

Build status Rust



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.


This example shows how to parse a pattern string into its HIR:

use regex_syntax::Parser;
use regex_syntax::hir::{self, Hir};

let hir = Parser::new().parse("a|b").unwrap();
assert_eq!(hir, Hir::alternation(vec![


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.


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.


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.