Crate regex_syntax

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This crate provides a robust regular expression parser.

This crate defines two primary types:

  • Ast is the abstract syntax of a regular expression. An abstract syntax corresponds to a structured representation of the concrete syntax of a regular expression, where the concrete syntax is the pattern string itself (e.g., foo(bar)+). Given some abstract syntax, it can be converted back to the original concrete syntax (modulo some details, like whitespace). To a first approximation, the abstract syntax is complex and difficult to analyze.
  • Hir is the high-level intermediate representation (“HIR” or “high-level IR” for short) of regular expression. It corresponds to an intermediate state of a regular expression that sits between the abstract syntax and the low level compiled opcodes that are eventually responsible for executing a regular expression search. Given some high-level IR, it is not possible to produce the original concrete syntax (although it is possible to produce an equivalent concrete syntax, but it will likely scarcely resemble the original pattern). To a first approximation, the high-level IR is simple and easy to analyze.

These two types come with conversion routines:

As a convenience, the above two conversion routines are combined into one via the top-level Parser type. This Parser will first convert your pattern to an Ast and then convert the Ast to an 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![

Concrete syntax supported

The concrete syntax is documented as part of the public API of the regex crate.

Input safety

A key feature of this library is that it is safe to use with end user facing input. This plays a significant role in the internal implementation. In particular:

  1. Parsers provide a nest_limit option that permits callers to control how deeply nested a regular expression is allowed to be. This makes it possible to do case analysis over an Ast or an Hir using recursion without worrying about stack overflow.
  2. Since relying on a particular stack size is brittle, this crate goes to great lengths to ensure that all interactions with both the Ast and the Hir do not use recursion. Namely, they use constant stack space and heap space proportional to the size of the original pattern string (in bytes). This includes the type’s corresponding destructors. (One exception to this is literal extraction, but this will eventually get fixed.)

Error reporting

The Display implementations on all Error types exposed in this library provide nice human readable errors that are suitable for showing to end users in a monospace font.

Literal extraction

This crate provides limited support for literal extraction from Hir values. Be warned that literal extraction currently uses recursion, and therefore, stack size proportional to the size of the Hir.

The purpose of literal extraction is to speed up searches. That is, if you know a regular expression must match a prefix or suffix literal, then it is often quicker to search for instances of that literal, and then confirm or deny the match using the full regular expression engine. These optimizations are done automatically in the regex crate.

Crate features

An important feature provided by this crate is its Unicode support. This includes things like case folding, boolean properties, general categories, scripts and Unicode-aware support for the Perl classes \w, \s and \d. However, a downside of this support is that it requires bundling several Unicode data tables that are substantial in size.

A fair number of use cases do not require full Unicode support. For this reason, this crate exposes a number of features to control which Unicode data is available.

If a regular expression attempts to use a Unicode feature that is not available because the corresponding crate feature was disabled, then translating that regular expression to an Hir will return an error. (It is still possible construct an Ast for such a regular expression, since Unicode data is not used until translation to an Hir.) Stated differently, enabling or disabling any of the features below can only add or subtract from the total set of valid regular expressions. Enabling or disabling a feature will never modify the match semantics of a regular expression.

The following features are available:

  • unicode - Enables all Unicode features. This feature is enabled by default, and will always cover all Unicode features, even if more are added in the future.
  • unicode-age - Provide the data for the Unicode Age property. This makes it possible to use classes like \p{Age:6.0} to refer to all codepoints first introduced in Unicode 6.0
  • unicode-bool - Provide the data for numerous Unicode boolean properties. The full list is not included here, but contains properties like Alphabetic, Emoji, Lowercase, Math, Uppercase and White_Space.
  • unicode-case - Provide the data for case insensitive matching using Unicode’s “simple loose matches” specification.
  • unicode-gencat - Provide the data for Uncode general categories. This includes, but is not limited to, Decimal_Number, Letter, Math_Symbol, Number and Punctuation.
  • unicode-perl - Provide the data for supporting the Unicode-aware Perl character classes, corresponding to \w, \s and \d. This is also necessary for using Unicode-aware word boundary assertions. Note that if this feature is disabled, the \s and \d character classes are still available if the unicode-bool and unicode-gencat features are enabled, respectively.
  • unicode-script - Provide the data for Unicode scripts and script extensions. This includes, but is not limited to, Arabic, Cyrillic, Hebrew, Latin and Thai.
  • unicode-segment - Provide the data necessary to provide the properties used to implement the Unicode text segmentation algorithms. This enables using classes like \p{gcb=Extend}, \p{wb=Katakana} and \p{sb=ATerm}.


  • Defines an abstract syntax for regular expressions.
  • Defines a high-level intermediate representation for regular expressions.
  • Converts ranges of Unicode scalar values to equivalent ranges of UTF-8 bytes.


  • A convenience parser for regular expressions.
  • A builder for a regular expression parser.
  • An error that occurs when the Unicode-aware \w class is unavailable.


  • This error type encompasses any error that can be returned by this crate.


  • Escapes all regular expression meta characters in text.
  • Escapes all meta characters in text and writes the result into buf.
  • Returns true if the given character has significance in a regex.
  • Returns true if and only if the given character is an ASCII word character.
  • Returns true if and only if the given character is a Unicode word character.
  • Returns true if and only if the given character is a Unicode word character.

Type Definitions

  • A type alias for dealing with errors returned by this crate.