Module regex_automata::hybrid

A module for building and searching with lazy deterministic finite automata (DFAs).

Like other modules in this crate, lazy DFAs support a rich regex syntax with Unicode features. The key feature of a lazy DFA is that it builds itself incrementally during search, and never uses more than a configured capacity of memory. Thus, when searching with a lazy DFA, one must supply a mutable “cache” in which the actual DFA’s transition table is stored.

If you’re looking for fully compiled DFAs, then please see the top-level dfa module.

Overview

This section gives a brief overview of the primary types in this module:

• A regex::Regex provides a way to search for matches of a regular expression using lazy DFAs. This includes iterating over matches with both the start and end positions of each match.
• A dfa::DFA provides direct low level access to a lazy DFA.

Example: basic regex searching

This example shows how to compile a regex using the default configuration and then use it to find matches in a byte string:

use regex_automata::{hybrid::regex::Regex, Match};

let re = Regex::new(r"[0-9]{4}-[0-9]{2}-[0-9]{2}")?;
let mut cache = re.create_cache();

let haystack = "2018-12-24 2016-10-08";
let matches: Vec<Match> = re.find_iter(&mut cache, haystack).collect();
assert_eq!(matches, vec![
    Match::must(0, 0..10),
    Match::must(0, 11..21),
]);

Example: searching with multiple regexes

The lazy DFAs in this module all fully support searching with multiple regexes simultaneously. You can use this support with standard leftmost-first style searching to find non-overlapping matches:

use regex_automata::{hybrid::regex::Regex, Match};

let re = Regex::new_many(&[r"\w+", r"\S+"])?;
let mut cache = re.create_cache();

let haystack = "@foo bar";
let matches: Vec<Match> = re.find_iter(&mut cache, haystack).collect();
assert_eq!(matches, vec![
    Match::must(1, 0..4),
    Match::must(0, 5..8),
]);

When should I use this?

Generally speaking, if you can abide the use of mutable state during search, and you don’t need things like capturing groups or Unicode word boundary support in non-ASCII text, then a lazy DFA is likely a robust choice with respect to both search speed and memory usage. Note however that its speed may be worse than a general purpose regex engine if you don’t select a good prefilter.

If you know ahead of time that your pattern would result in a very large DFA if it was fully compiled, it may be better to use an NFA simulation instead of a lazy DFA. Either that, or increase the cache capacity of your lazy DFA to something that is big enough to hold the state machine (likely through experimentation). The issue here is that if the cache is too small, then it could wind up being reset too frequently and this might decrease searching speed significantly.

Differences with fully compiled DFAs

A hybrid::regex::Regex and a dfa::regex::Regex both have the same capabilities (and similarly for their underlying DFAs), but they achieve them through different means. The main difference is that a hybrid or “lazy” regex builds its DFA lazily during search, where as a fully compiled regex will build its DFA at construction time. While building a DFA at search time might sound like it’s slow, it tends to work out where most bytes seen during a search will reuse pre-built parts of the DFA and thus can be almost as fast as a fully compiled DFA. The main downside is that searching requires mutable space to store the DFA, and, in the worst case, a search can result in a new state being created for each byte seen, which would make searching quite a bit slower.

A fully compiled DFA never has to worry about searches being slower once it’s built. (Aside from, say, the transition table being so large that it is subject to harsh CPU cache effects.) However, of course, building a full DFA can be quite time consuming and memory hungry. Particularly when large Unicode character classes are used, which tend to translate into very large DFAs.

A lazy DFA strikes a nice balance in practice, particularly in the presence of Unicode mode, by only building what is needed. It avoids the worst case exponential time complexity of DFA compilation by guaranteeing that it will only build at most one state per byte searched. While the worst case here can lead to a very high constant, it will never be exponential.

Syntax

This module supports the same syntax as the regex crate, since they share the same parser. You can find an exhaustive list of supported syntax in the documentation for the regex crate.

There are two things that are not supported by the lazy DFAs in this module:

• Capturing groups. The DFAs (and Regexes built on top of them) can only find the offsets of an entire match, but cannot resolve the offsets of each capturing group. This is because DFAs do not have the expressive power necessary. Note that it is okay to build a lazy DFA from an NFA that contains capture groups. The capture groups will simply be ignored.
• Unicode word boundaries. These present particularly difficult challenges for DFA construction and would result in an explosion in the number of states. One can enable dfa::Config::unicode_word_boundary though, which provides heuristic support for Unicode word boundaries that only works on ASCII text. Otherwise, one can use (?-u:\b) for an ASCII word boundary, which will work on any input.

There are no plans to lift either of these limitations.

Note that these restrictions are identical to the restrictions on fully compiled DFAs.

Modules

• dfa
  Types and routines specific to lazy DFAs.
• regex
  A lazy DFA backed Regex.

Structs

• BuildError
  An error that occurs when initial construction of a lazy DFA fails.
• CacheError
  An error that occurs when cache usage has become inefficient.
• LazyStateID
  A state identifier specifically tailored for lazy DFAs.

Enums

• StartError
  An error that can occur when computing the start state for a search.