Crate aho_corasick

A library for finding occurrences of many patterns at once. This library provides multiple pattern search principally through an implementation of the Aho-Corasick algorithm, which builds a fast finite state machine for executing searches in linear time.

Additionally, this library provides a number of configuration options for building the automaton that permit controlling the space versus time trade off. Other features include simple ASCII case insensitive matching, finding overlapping matches, replacements, searching streams and even searching and replacing text in streams.

Finally, unlike most other Aho-Corasick implementations, this one supports enabling leftmost-first or leftmost-longest match semantics, using a (seemingly) novel alternative construction algorithm. For more details on what match semantics means, see the MatchKind type.

§Overview

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

• AhoCorasick is the primary type and represents an Aho-Corasick automaton. This is the type you use to execute searches.
• AhoCorasickBuilder can be used to build an Aho-Corasick automaton, and supports configuring a number of options.
• Match represents a single match reported by an Aho-Corasick automaton. Each match has two pieces of information: the pattern that matched and the start and end byte offsets corresponding to the position in the haystack at which it matched.

§Example: basic searching

This example shows how to search for occurrences of multiple patterns simultaneously. Each match includes the pattern that matched along with the byte offsets of the match.

use aho_corasick::{AhoCorasick, PatternID};

let patterns = &["apple", "maple", "Snapple"];
let haystack = "Nobody likes maple in their apple flavored Snapple.";

let ac = AhoCorasick::new(patterns).unwrap();
let mut matches = vec![];
for mat in ac.find_iter(haystack) {
    matches.push((mat.pattern(), mat.start(), mat.end()));
}
assert_eq!(matches, vec![
    (PatternID::must(1), 13, 18),
    (PatternID::must(0), 28, 33),
    (PatternID::must(2), 43, 50),
]);

§Example: case insensitivity

This is like the previous example, but matches Snapple case insensitively using AhoCorasickBuilder:

use aho_corasick::{AhoCorasick, PatternID};

let patterns = &["apple", "maple", "snapple"];
let haystack = "Nobody likes maple in their apple flavored Snapple.";

let ac = AhoCorasick::builder()
    .ascii_case_insensitive(true)
    .build(patterns)
    .unwrap();
let mut matches = vec![];
for mat in ac.find_iter(haystack) {
    matches.push((mat.pattern(), mat.start(), mat.end()));
}
assert_eq!(matches, vec![
    (PatternID::must(1), 13, 18),
    (PatternID::must(0), 28, 33),
    (PatternID::must(2), 43, 50),
]);

§Example: replacing matches in a stream

This example shows how to execute a search and replace on a stream without loading the entire stream into memory first.

use aho_corasick::AhoCorasick;

let patterns = &["fox", "brown", "quick"];
let replace_with = &["sloth", "grey", "slow"];

// In a real example, these might be `std::fs::File`s instead. All you need to
// do is supply a pair of `std::io::Read` and `std::io::Write` implementations.
let rdr = "The quick brown fox.";
let mut wtr = vec![];

let ac = AhoCorasick::new(patterns).unwrap();
ac.try_stream_replace_all(rdr.as_bytes(), &mut wtr, replace_with)?;
assert_eq!(b"The slow grey sloth.".to_vec(), wtr);

§Example: finding the leftmost first match

In the textbook description of Aho-Corasick, its formulation is typically structured such that it reports all possible matches, even when they overlap with another. In many cases, overlapping matches may not be desired, such as the case of finding all successive non-overlapping matches like you might with a standard regular expression.

Unfortunately the “obvious” way to modify the Aho-Corasick algorithm to do this doesn’t always work in the expected way, since it will report matches as soon as they are seen. For example, consider matching the regex Samwise|Sam against the text Samwise. Most regex engines (that are Perl-like, or non-POSIX) will report Samwise as a match, but the standard Aho-Corasick algorithm modified for reporting non-overlapping matches will report Sam.

A novel contribution of this library is the ability to change the match semantics of Aho-Corasick (without additional search time overhead) such that Samwise is reported instead. For example, here’s the standard approach:

use aho_corasick::AhoCorasick;

let patterns = &["Samwise", "Sam"];
let haystack = "Samwise";

let ac = AhoCorasick::new(patterns).unwrap();
let mat = ac.find(haystack).expect("should have a match");
assert_eq!("Sam", &haystack[mat.start()..mat.end()]);

And now here’s the leftmost-first version, which matches how a Perl-like regex will work:

use aho_corasick::{AhoCorasick, MatchKind};

let patterns = &["Samwise", "Sam"];
let haystack = "Samwise";

let ac = AhoCorasick::builder()
    .match_kind(MatchKind::LeftmostFirst)
    .build(patterns)
    .unwrap();
let mat = ac.find(haystack).expect("should have a match");
assert_eq!("Samwise", &haystack[mat.start()..mat.end()]);

In addition to leftmost-first semantics, this library also supports leftmost-longest semantics, which match the POSIX behavior of a regular expression alternation. See MatchKind for more details.

§Prefilters

While an Aho-Corasick automaton can perform admirably when compared to more naive solutions, it is generally slower than more specialized algorithms that are accelerated using vector instructions such as SIMD.

For that reason, this library will internally use a “prefilter” to attempt to accelerate searches when possible. Currently, this library has several different algorithms it might use depending on the patterns provided. Once the number of patterns gets too big, prefilters are no longer used.

While a prefilter is generally good to have on by default since it works well in the common case, it can lead to less predictable or even sub-optimal performance in some cases. For that reason, prefilters can be explicitly disabled via AhoCorasickBuilder::prefilter.

§Lower level APIs

This crate also provides several sub-modules that collectively expose many of the implementation details of the main AhoCorasick type. Most users of this library can completely ignore the submodules and their contents, but if you needed finer grained control, some parts of them may be useful to you. Here is a brief overview of each and why you might want to use them:

• The packed sub-module contains a lower level API for using fast vectorized routines for finding a small number of patterns in a haystack. You might want to use this API when you want to completely side-step using Aho-Corasick automata. Otherwise, the fast vectorized routines are used automatically as prefilters for AhoCorasick searches whenever possible.
• The automaton sub-module provides a lower level finite state machine interface that the various Aho-Corasick implementations in this crate implement. This sub-module’s main contribution is the Automaton trait, which permits manually walking the state transitions of an Aho-Corasick automaton.
• The dfa and nfa sub-modules provide DFA and NFA implementations of the aforementioned Automaton trait. The main reason one might want to use these sub-modules is to get access to a type that implements the Automaton trait. (The top-level AhoCorasick type does not implement the Automaton trait.)

As mentioned above, if you aren’t sure whether you need these sub-modules, you should be able to safely ignore them and just focus on the AhoCorasick type.

§Crate features

This crate exposes a few features for controlling dependency usage and whether this crate can be used without the standard library.

• std - Enables support for the standard library. This feature is enabled by default. When disabled, only core and alloc are used. At an API level, enabling std enables std::error::Error trait impls for the various error types, and higher level stream search routines such as AhoCorasick::try_stream_find_iter. But the std feature is also required to enable vectorized prefilters. Prefilters can greatly accelerate searches, but generally only apply when the number of patterns is small (less than ~100).
• perf-literal - Enables support for literal prefilters that use vectorized routines from external crates. This feature is enabled by default. If you’re only using Aho-Corasick for large numbers of patterns or otherwise can abide lower throughput when searching with a small number of patterns, then it is reasonable to disable this feature.
• logging - Enables a dependency on the log crate and emits messages to aide in diagnostics. This feature is disabled by default.