# Struct fst::Regex
[−]
[src]

pub struct Regex { /* fields omitted */ }

A regular expression for searching FSTs with Unicode support.

Regular expressions are compiled down to a deterministic finite automaton that can efficiently search any finite state transducer. Notably, most regular expressions only need to explore a small portion of a finite state transducer without loading all of it into memory.

# Syntax

`Regex`

supports fully featured regular expressions. Namely, it supports
all of the same constructs as the standard `regex`

crate except for the
following things:

- Lazy quantifiers, since a regular expression automaton only reports
whether a key matches at all, and not its location. Namely, lazy
quantifiers such as
`+?`

only modify the location of a match, but never change a non-match into a match or a match into a non-match. - Word boundaries (i.e.,
`\b`

). Because such things are hard to do in a deterministic finite automaton, but not impossible. As such, these may be allowed some day. - Other zero width assertions like
`^`

and`$`

. These are easier to support than word boundaries, but are still tricky and usually aren't as useful when searching dictionaries.

Otherwise, the full syntax of the `regex`

crate
is supported. This includes all Unicode support and relevant flags.
(The `U`

and `m`

flags are no-ops because of (1) and (3) above,
respectively.)

# Matching semantics

A regular expression matches a key in a finite state transducer if and only
if it matches from the start of a key all the way to end. Stated
differently, every regular expression `(re)`

is matched as if it were
`^(re)$`

. This means that if you want to do a substring match, then you
must use `.*substring.*`

.

**Caution**: Starting a regular expression with `.*`

means that it could
potentially match *any* key in a finite state transducer. This implies that
all keys could be visited, which could be slow. It is possible that this
crate will grow facilities for detecting regular expressions that will
scan a large portion of a transducer and optionally disallow them.

# Example

This example shows how to run a regular expression on a `Set`

.

use fst::{IntoStreamer, Streamer, Regex, Set}; let set = Set::from_iter(&["foo", "foo1", "foo2", "foo3", "foobar"]) .unwrap(); let re = Regex::new("f[a-z]+3?").unwrap(); let mut stream = set.search(&re).into_stream(); let mut keys = vec![]; while let Some(key) = stream.next() { keys.push(key.to_vec()); } assert_eq!(keys, vec![ "foo".as_bytes(), "foo3".as_bytes(), "foobar".as_bytes(), ]);

# Warning: experimental

While executing a regular expression against a finite state transducer will
be very fast, *construction* of a regular expression automaton may not be.
Namely, this implementation is a proof of concept. In particular, one of
its major deficiencies is that it can use enormous amounts of memory.
Note though, that the construction phase will return an error if the
underlying automata grows too big (tens of MB).

This is important functionality, so one should count on this implementation being vastly improved in the future.

## Methods

`impl Regex`

[src]

`fn new(re: &str) -> Result<Regex>`

Create a new regular expression query.

The query finds all terms matching the regular expression.

If the regular expression is malformed or if it results in an automaton that is too big, then an error is returned.

A `Regex`

value satisfies the `Automaton`

trait, which means it can be
used with the `search`

method of any finite state transducer.

## Trait Implementations

`impl Automaton for Regex`

[src]

`type State = Option<usize>`

The type of the state used in the automaton.

`fn start(&self) -> Option<usize>`

Returns a single start state for this automaton. Read more

`fn is_match(&self, state: &Option<usize>) -> bool`

Returns true if and only if `state`

is a match state.

`fn can_match(&self, state: &Option<usize>) -> bool`

Returns true if and only if `state`

can lead to a match in zero or more steps. Read more

`fn accept(&self, state: &Option<usize>, byte: u8) -> Option<usize>`

Return the next state given `state`

and an input.

`fn will_always_match(&self, _state: &Self::State) -> bool`

Returns true if and only if `state`

matches and must match no matter what steps are taken. Read more

`fn starts_with(self) -> StartsWith<Self> where Self: Sized`

Returns an automaton that matches the strings that start with something this automaton matches. Read more

`fn union<Rhs: Automaton>(self, rhs: Rhs) -> Union<Self, Rhs> where Self: Sized`

Returns an automaton that matches the strings matched by either this or the other automaton. Read more

`fn intersection<Rhs: Automaton>(self, rhs: Rhs) -> Intersection<Self, Rhs> where Self: Sized`

Returns an automaton that matches the strings matched by both this and the other automaton. Read more

`fn complement(self) -> Complement<Self> where Self: Sized`

Returns an automaton that matches the strings not matched by this automaton.