pub enum MatchKind {
    // some variants omitted
Expand description

A knob for controlling the match semantics of an Aho-Corasick automaton.

There are two generally different ways that Aho-Corasick automatons can report matches. The first way is the “standard” approach that results from implementing most textbook explanations of Aho-Corasick. The second way is to report only the leftmost non-overlapping matches. The leftmost approach is in turn split into two different ways of resolving ambiguous matches: leftmost-first and leftmost-longest.

The Standard match kind is the default and is the only one that supports overlapping matches and stream searching. (Trying to find overlapping or streaming matches using leftmost match semantics will result in a panic.) The Standard match kind will report matches as they are seen. When searching for overlapping matches, then all possible matches are reported. When searching for non-overlapping matches, the first match seen is reported. For example, for non-overlapping matches, given the patterns abcd and b and the subject string abcdef, only a match for b is reported since it is detected first. The abcd match is never reported since it overlaps with the b match.

In contrast, the leftmost match kind always prefers the leftmost match among all possible matches. Given the same example as above with abcd and b as patterns and abcdef as the subject string, the leftmost match is abcd since it begins before the b match, even though the b match is detected before the abcd match. In this case, the b match is not reported at all since it overlaps with the abcd match.

The difference between leftmost-first and leftmost-longest is in how they resolve ambiguous matches when there are multiple leftmost matches to choose from. Leftmost-first always chooses the pattern that was provided earliest, where as leftmost-longest always chooses the longest matching pattern. For example, given the patterns a and ab and the subject string ab, the leftmost-first match is a but the leftmost-longest match is ab. Conversely, if the patterns were given in reverse order, i.e., ab and a, then both the leftmost-first and leftmost-longest matches would be ab. Stated differently, the leftmost-first match depends on the order in which the patterns were given to the Aho-Corasick automaton. Because of that, when leftmost-first matching is used, if a pattern A that appears before a pattern B is a prefix of B, then it is impossible to ever observe a match of B.

If you’re not sure which match kind to pick, then stick with the standard kind, which is the default. In particular, if you need overlapping or streaming matches, then you must use the standard kind. The leftmost kinds are useful in specific circumstances. For example, leftmost-first can be very useful as a way to implement match priority based on the order of patterns given and leftmost-longest can be useful for dictionary searching such that only the longest matching words are reported.

Relationship with regular expression alternations

Understanding match semantics can be a little tricky, and one easy way to conceptualize non-overlapping matches from an Aho-Corasick automaton is to think about them as a simple alternation of literals in a regular expression. For example, let’s say we wanted to match the strings Sam and Samwise, which would turn into the regex Sam|Samwise. It turns out that regular expression engines have two different ways of matching this alternation. The first way, leftmost-longest, is commonly found in POSIX compatible implementations of regular expressions (such as grep). The second way, leftmost-first, is commonly found in backtracking implementations such as Perl. (Some regex engines, such as RE2 and Rust’s regex engine do not use backtracking, but still implement leftmost-first semantics in an effort to match the behavior of dominant backtracking regex engines such as those found in Perl, Ruby, Python, Javascript and PHP.)

That is, when matching Sam|Samwise against Samwise, a POSIX regex will match Samwise because it is the longest possible match, but a Perl-like regex will match Sam since it appears earlier in the alternation. Indeed, the regex Sam|Samwise in a Perl-like regex engine will never match Samwise since Sam will always have higher priority. Conversely, matching the regex Samwise|Sam against Samwise will lead to a match of Samwise in both POSIX and Perl-like regexes since Samwise is still longest match, but it also appears earlier than Sam.

The “standard” match semantics of Aho-Corasick generally don’t correspond to the match semantics of any large group of regex implementations, so there’s no direct analogy that can be made here. Standard match semantics are generally useful for overlapping matches, or if you just want to see matches as they are detected.

The main conclusion to draw from this section is that the match semantics can be tweaked to precisely match either Perl-like regex alternations or POSIX regex alternations.



Use standard match semantics, which support overlapping matches. When used with non-overlapping matches, matches are reported as they are seen.


Use leftmost-first match semantics, which reports leftmost matches. When there are multiple possible leftmost matches, the match corresponding to the pattern that appeared earlier when constructing the automaton is reported.

This does not support overlapping matches or stream searching. If this match kind is used, attempting to find overlapping matches or stream matches will panic.


Use leftmost-longest match semantics, which reports leftmost matches. When there are multiple possible leftmost matches, the longest match is chosen.

This does not support overlapping matches or stream searching. If this match kind is used, attempting to find overlapping matches or stream matches will panic.

Trait Implementations

Returns a copy of the value. Read more
Performs copy-assignment from source. Read more
Formats the value using the given formatter. Read more

The default match kind is MatchKind::Standard.

Returns the “default value” for a type. Read more
This method tests for self and other values to be equal, and is used by ==. Read more
This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more

Auto Trait Implementations

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Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning. Read more
Uses borrowed data to replace owned data, usually by cloning. Read more
The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.