Merging upstream version 1.74.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2124 insertions, 0 deletions

diff --git a/vendor/regex-automata-0.2.0/src/hybrid/regex.rs b/vendor/regex-automata-0.2.0/src/hybrid/regex.rs
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+++ b/vendor/regex-automata-0.2.0/src/hybrid/regex.rs
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+/*!
+A lazy DFA backed `Regex`.
+
+This module provides [`Regex`] using lazy DFA. A `Regex` implements convenience
+routines you might have come to expect, such as finding a match and iterating
+over all non-overlapping matches. This `Regex` type is limited in its
+capabilities to what a lazy DFA can provide. Therefore, APIs involving
+capturing groups, for example, are not provided.
+
+Internally, a `Regex` is composed of two DFAs. One is a "forward" DFA that
+finds the end offset of a match, where as the other is a "reverse" DFA that
+find the start offset of a match.
+
+See the [parent module](crate::hybrid) for examples.
+*/
+
+use core::borrow::Borrow;
+
+use alloc::boxed::Box;
+
+use crate::{
+    hybrid::{
+        dfa::{self, DFA},
+        error::BuildError,
+        OverlappingState,
+    },
+    nfa::thompson,
+    util::{
+        matchtypes::{MatchError, MatchKind, MultiMatch},
+        prefilter::{self, Prefilter},
+    },
+};
+
+/// A regular expression that uses hybrid NFA/DFAs (also called "lazy DFAs")
+/// for searching.
+///
+/// A regular expression is comprised of two lazy DFAs, a "forward" DFA and a
+/// "reverse" DFA. The forward DFA is responsible for detecting the end of
+/// a match while the reverse DFA is responsible for detecting the start
+/// of a match. Thus, in order to find the bounds of any given match, a
+/// forward search must first be run followed by a reverse search. A match
+/// found by the forward DFA guarantees that the reverse DFA will also find
+/// a match.
+///
+/// A `Regex` can also have a prefilter set via the
+/// [`set_prefilter`](Regex::set_prefilter) method. By default, no prefilter is
+/// enabled.
+///
+/// # Earliest vs Leftmost vs Overlapping
+///
+/// The search routines exposed on a `Regex` reflect three different ways
+/// of searching:
+///
+/// * "earliest" means to stop as soon as a match has been detected.
+/// * "leftmost" means to continue matching until the underlying
+///   automaton cannot advance. This reflects "standard" searching you
+///   might be used to in other regex engines. e.g., This permits
+///   non-greedy and greedy searching to work as you would expect.
+/// * "overlapping" means to find all possible matches, even if they
+///   overlap.
+///
+/// Generally speaking, when doing an overlapping search, you'll want to
+/// build your regex lazy DFAs with [`MatchKind::All`] semantics. Using
+/// [`MatchKind::LeftmostFirst`] semantics with overlapping searches is
+/// likely to lead to odd behavior since `LeftmostFirst` specifically omits
+/// some matches that can never be reported due to its semantics.
+///
+/// The following example shows the differences between how these different
+/// types of searches impact looking for matches of `[a-z]+` in the
+/// haystack `abc`.
+///
+/// ```
+/// use regex_automata::{hybrid::{dfa, regex}, MatchKind, MultiMatch};
+///
+/// let pattern = r"[a-z]+";
+/// let haystack = "abc".as_bytes();
+///
+/// // With leftmost-first semantics, we test "earliest" and "leftmost".
+/// let re = regex::Builder::new()
+///     .dfa(dfa::Config::new().match_kind(MatchKind::LeftmostFirst))
+///     .build(pattern)?;
+/// let mut cache = re.create_cache();
+///
+/// // "earliest" searching isn't impacted by greediness
+/// let mut it = re.find_earliest_iter(&mut cache, haystack);
+/// assert_eq!(Some(MultiMatch::must(0, 0, 1)), it.next());
+/// assert_eq!(Some(MultiMatch::must(0, 1, 2)), it.next());
+/// assert_eq!(Some(MultiMatch::must(0, 2, 3)), it.next());
+/// assert_eq!(None, it.next());
+///
+/// // "leftmost" searching supports greediness (and non-greediness)
+/// let mut it = re.find_leftmost_iter(&mut cache, haystack);
+/// assert_eq!(Some(MultiMatch::must(0, 0, 3)), it.next());
+/// assert_eq!(None, it.next());
+///
+/// // For overlapping, we want "all" match kind semantics.
+/// let re = regex::Builder::new()
+///     .dfa(dfa::Config::new().match_kind(MatchKind::All))
+///     .build(pattern)?;
+/// let mut cache = re.create_cache();
+///
+/// // In the overlapping search, we find all three possible matches
+/// // starting at the beginning of the haystack.
+/// let mut it = re.find_overlapping_iter(&mut cache, haystack);
+/// assert_eq!(Some(MultiMatch::must(0, 0, 1)), it.next());
+/// assert_eq!(Some(MultiMatch::must(0, 0, 2)), it.next());
+/// assert_eq!(Some(MultiMatch::must(0, 0, 3)), it.next());
+/// assert_eq!(None, it.next());
+///
+/// # Ok::<(), Box<dyn std::error::Error>>(())
+/// ```
+///
+/// # Fallibility
+///
+/// In non-default configurations, the lazy DFAs generated in this module may
+/// return an error during a search. (Currently, the only way this happens is
+/// if quit bytes are added, Unicode word boundaries are heuristically enabled,
+/// or if the cache is configured to "give up" on a search if it has been
+/// cleared too many times. All of these are turned off by default, which means
+/// a search can never fail in the default configuration.) For convenience,
+/// the main search routines, like [`find_leftmost`](Regex::find_leftmost),
+/// will panic if an error occurs. However, if you need to use DFAs which may
+/// produce an error at search time, then there are fallible equivalents of
+/// all search routines. For example, for `find_leftmost`, its fallible analog
+/// is [`try_find_leftmost`](Regex::try_find_leftmost). The routines prefixed
+/// with `try_` return `Result<Option<MultiMatch>, MatchError>`, where as the
+/// infallible routines simply return `Option<MultiMatch>`.
+///
+/// # Example
+///
+/// This example shows how to cause a search to terminate if it sees a
+/// `\n` byte, and handle the error returned. This could be useful if, for
+/// example, you wanted to prevent a user supplied pattern from matching
+/// across a line boundary.
+///
+/// ```
+/// use regex_automata::{hybrid::{dfa, regex::Regex}, MatchError};
+///
+/// let re = Regex::builder()
+///     .dfa(dfa::Config::new().quit(b'\n', true))
+///     .build(r"foo\p{any}+bar")?;
+/// let mut cache = re.create_cache();
+///
+/// let haystack = "foo\nbar".as_bytes();
+/// // Normally this would produce a match, since \p{any} contains '\n'.
+/// // But since we instructed the automaton to enter a quit state if a
+/// // '\n' is observed, this produces a match error instead.
+/// let expected = MatchError::Quit { byte: 0x0A, offset: 3 };
+/// let got = re.try_find_leftmost(&mut cache, haystack).unwrap_err();
+/// assert_eq!(expected, got);
+///
+/// # Ok::<(), Box<dyn std::error::Error>>(())
+/// ```
+#[derive(Debug)]
+pub struct Regex {
+    /// An optional prefilter that is passed down to the lazy DFA search
+    /// routines when present. By default, no prefilter is set.
+    pre: Option<Box<dyn Prefilter>>,
+    /// The forward lazy DFA. This can only find the end of a match.
+    forward: DFA,
+    /// The reverse lazy DFA. This can only find the start of a match.
+    ///
+    /// This is built with 'all' match semantics (instead of leftmost-first)
+    /// so that it always finds the longest possible match (which corresponds
+    /// to the leftmost starting position). It is also compiled as an anchored
+    /// matcher and has 'starts_for_each_pattern' enabled. Including starting
+    /// states for each pattern is necessary to ensure that we only look for
+    /// matches of a pattern that matched in the forward direction. Otherwise,
+    /// we might wind up finding the "leftmost" starting position of a totally
+    /// different pattern!
+    reverse: DFA,
+    /// Whether iterators on this type should advance by one codepoint or one
+    /// byte when an empty match is seen.
+    utf8: bool,
+}
+
+/// Convenience routines for regex and cache construction.
+impl Regex {
+    /// Parse the given regular expression using the default configuration and
+    /// return the corresponding regex.
+    ///
+    /// If you want a non-default configuration, then use the [`Builder`] to
+    /// set your own configuration.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use regex_automata::{MultiMatch, hybrid::regex::Regex};
+    ///
+    /// let re = Regex::new("foo[0-9]+bar")?;
+    /// let mut cache = re.create_cache();
+    /// assert_eq!(
+    ///     Some(MultiMatch::must(0, 3, 14)),
+    ///     re.find_leftmost(&mut cache, b"zzzfoo12345barzzz"),
+    /// );
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn new(pattern: &str) -> Result<Regex, BuildError> {
+        Regex::builder().build(pattern)
+    }
+
+    /// Like `new`, but parses multiple patterns into a single "regex set."
+    /// This similarly uses the default regex configuration.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use regex_automata::{MultiMatch, hybrid::regex::Regex};
+    ///
+    /// let re = Regex::new_many(&["[a-z]+", "[0-9]+"])?;
+    /// let mut cache = re.create_cache();
+    ///
+    /// let mut it = re.find_leftmost_iter(
+    ///     &mut cache,
+    ///     b"abc 1 foo 4567 0 quux",
+    /// );
+    /// assert_eq!(Some(MultiMatch::must(0, 0, 3)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(1, 4, 5)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 6, 9)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(1, 10, 14)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(1, 15, 16)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 17, 21)), it.next());
+    /// assert_eq!(None, it.next());
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn new_many<P: AsRef<str>>(
+        patterns: &[P],
+    ) -> Result<Regex, BuildError> {
+        Regex::builder().build_many(patterns)
+    }
+
+    /// Return a default configuration for a `Regex`.
+    ///
+    /// This is a convenience routine to avoid needing to import the `Config`
+    /// type when customizing the construction of a regex.
+    ///
+    /// # Example
+    ///
+    /// This example shows how to disable UTF-8 mode for `Regex` iteration.
+    /// When UTF-8 mode is disabled, the position immediately following an
+    /// empty match is where the next search begins, instead of the next
+    /// position of a UTF-8 encoded codepoint.
+    ///
+    /// ```
+    /// use regex_automata::{hybrid::regex::Regex, MultiMatch};
+    ///
+    /// let re = Regex::builder()
+    ///     .configure(Regex::config().utf8(false))
+    ///     .build(r"")?;
+    /// let mut cache = re.create_cache();
+    ///
+    /// let haystack = "a☃z".as_bytes();
+    /// let mut it = re.find_leftmost_iter(&mut cache, haystack);
+    /// assert_eq!(Some(MultiMatch::must(0, 0, 0)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 1, 1)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 2, 2)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 3, 3)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 4, 4)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 5, 5)), it.next());
+    /// assert_eq!(None, it.next());
+    ///
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn config() -> Config {
+        Config::new()
+    }
+
+    /// Return a builder for configuring the construction of a `Regex`.
+    ///
+    /// This is a convenience routine to avoid needing to import the
+    /// [`Builder`] type in common cases.
+    ///
+    /// # Example
+    ///
+    /// This example shows how to use the builder to disable UTF-8 mode
+    /// everywhere.
+    ///
+    /// ```
+    /// use regex_automata::{
+    ///     hybrid::regex::Regex,
+    ///     nfa::thompson,
+    ///     MultiMatch, SyntaxConfig,
+    /// };
+    ///
+    /// let re = Regex::builder()
+    ///     .configure(Regex::config().utf8(false))
+    ///     .syntax(SyntaxConfig::new().utf8(false))
+    ///     .thompson(thompson::Config::new().utf8(false))
+    ///     .build(r"foo(?-u:[^b])ar.*")?;
+    /// let mut cache = re.create_cache();
+    ///
+    /// let haystack = b"\xFEfoo\xFFarzz\xE2\x98\xFF\n";
+    /// let expected = Some(MultiMatch::must(0, 1, 9));
+    /// let got = re.find_leftmost(&mut cache, haystack);
+    /// assert_eq!(expected, got);
+    ///
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn builder() -> Builder {
+        Builder::new()
+    }
+
+    /// Create a new cache for this `Regex`.
+    ///
+    /// The cache returned should only be used for searches for this
+    /// `Regex`. If you want to reuse the cache for another `Regex`, then
+    /// you must call [`Cache::reset`] with that `Regex` (or, equivalently,
+    /// [`Regex::reset_cache`]).
+    pub fn create_cache(&self) -> Cache {
+        Cache::new(self)
+    }
+
+    /// Reset the given cache such that it can be used for searching with the
+    /// this `Regex` (and only this `Regex`).
+    ///
+    /// A cache reset permits reusing memory already allocated in this cache
+    /// with a different `Regex`.
+    ///
+    /// Resetting a cache sets its "clear count" to 0. This is relevant if the
+    /// `Regex` has been configured to "give up" after it has cleared the cache
+    /// a certain number of times.
+    ///
+    /// # Example
+    ///
+    /// This shows how to re-purpose a cache for use with a different `Regex`.
+    ///
+    /// ```
+    /// use regex_automata::{hybrid::regex::Regex, MultiMatch};
+    ///
+    /// let re1 = Regex::new(r"\w")?;
+    /// let re2 = Regex::new(r"\W")?;
+    ///
+    /// let mut cache = re1.create_cache();
+    /// assert_eq!(
+    ///     Some(MultiMatch::must(0, 0, 2)),
+    ///     re1.find_leftmost(&mut cache, "Δ".as_bytes()),
+    /// );
+    ///
+    /// // Using 'cache' with re2 is not allowed. It may result in panics or
+    /// // incorrect results. In order to re-purpose the cache, we must reset
+    /// // it with the Regex we'd like to use it with.
+    /// //
+    /// // Similarly, after this reset, using the cache with 're1' is also not
+    /// // allowed.
+    /// re2.reset_cache(&mut cache);
+    /// assert_eq!(
+    ///     Some(MultiMatch::must(0, 0, 3)),
+    ///     re2.find_leftmost(&mut cache, "☃".as_bytes()),
+    /// );
+    ///
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn reset_cache(&self, cache: &mut Cache) {
+        self.forward().reset_cache(&mut cache.forward);
+        self.reverse().reset_cache(&mut cache.reverse);
+    }
+}
+
+/// Standard infallible search routines for finding and iterating over matches.
+impl Regex {
+    /// Returns true if and only if this regex matches the given haystack.
+    ///
+    /// This routine may short circuit if it knows that scanning future input
+    /// will never lead to a different result. In particular, if the underlying
+    /// DFA enters a match state or a dead state, then this routine will return
+    /// `true` or `false`, respectively, without inspecting any future input.
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_is_match`](Regex::try_is_match).
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use regex_automata::hybrid::regex::Regex;
+    ///
+    /// let re = Regex::new("foo[0-9]+bar")?;
+    /// let mut cache = re.create_cache();
+    ///
+    /// assert_eq!(true, re.is_match(&mut cache, b"foo12345bar"));
+    /// assert_eq!(false, re.is_match(&mut cache, b"foobar"));
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn is_match(&self, cache: &mut Cache, haystack: &[u8]) -> bool {
+        self.try_is_match(cache, haystack).unwrap()
+    }
+
+    /// Returns the first position at which a match is found.
+    ///
+    /// This routine stops scanning input in precisely the same circumstances
+    /// as `is_match`. The key difference is that this routine returns the
+    /// position at which it stopped scanning input if and only if a match
+    /// was found. If no match is found, then `None` is returned.
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_find_earliest`](Regex::try_find_earliest).
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use regex_automata::{MultiMatch, hybrid::regex::Regex};
+    ///
+    /// // Normally, the leftmost first match would greedily consume as many
+    /// // decimal digits as it could. But a match is detected as soon as one
+    /// // digit is seen.
+    /// let re = Regex::new("foo[0-9]+")?;
+    /// let mut cache = re.create_cache();
+    /// assert_eq!(
+    ///     Some(MultiMatch::must(0, 0, 4)),
+    ///     re.find_earliest(&mut cache, b"foo12345"),
+    /// );
+    ///
+    /// // Normally, the end of the leftmost first match here would be 3,
+    /// // but the "earliest" match semantics detect a match earlier.
+    /// let re = Regex::new("abc|a")?;
+    /// let mut cache = re.create_cache();
+    /// assert_eq!(
+    ///     Some(MultiMatch::must(0, 0, 1)),
+    ///     re.find_earliest(&mut cache, b"abc"),
+    /// );
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn find_earliest(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+    ) -> Option<MultiMatch> {
+        self.try_find_earliest(cache, haystack).unwrap()
+    }
+
+    /// Returns the start and end offset of the leftmost match. If no match
+    /// exists, then `None` is returned.
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_find_leftmost`](Regex::try_find_leftmost).
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use regex_automata::{MultiMatch, hybrid::regex::Regex};
+    ///
+    /// // Greediness is applied appropriately when compared to find_earliest.
+    /// let re = Regex::new("foo[0-9]+")?;
+    /// let mut cache = re.create_cache();
+    /// assert_eq!(
+    ///     Some(MultiMatch::must(0, 3, 11)),
+    ///     re.find_leftmost(&mut cache, b"zzzfoo12345zzz"),
+    /// );
+    ///
+    /// // Even though a match is found after reading the first byte (`a`),
+    /// // the default leftmost-first match semantics demand that we find the
+    /// // earliest match that prefers earlier parts of the pattern over latter
+    /// // parts.
+    /// let re = Regex::new("abc|a")?;
+    /// let mut cache = re.create_cache();
+    /// assert_eq!(
+    ///     Some(MultiMatch::must(0, 0, 3)),
+    ///     re.find_leftmost(&mut cache, b"abc"),
+    /// );
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn find_leftmost(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+    ) -> Option<MultiMatch> {
+        self.try_find_leftmost(cache, haystack).unwrap()
+    }
+
+    /// Search for the first overlapping match in `haystack`.
+    ///
+    /// This routine is principally useful when searching for multiple patterns
+    /// on inputs where multiple patterns may match the same regions of text.
+    /// In particular, callers must preserve the automaton's search state from
+    /// prior calls so that the implementation knows where the last match
+    /// occurred and which pattern was reported.
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_find_overlapping`](Regex::try_find_overlapping).
+    ///
+    /// # Example
+    ///
+    /// This example shows how to run an overlapping search with multiple
+    /// regexes.
+    ///
+    /// ```
+    /// use regex_automata::{
+    ///     hybrid::{dfa::DFA, regex::Regex, OverlappingState},
+    ///     MatchKind,
+    ///     MultiMatch,
+    /// };
+    ///
+    /// let re = Regex::builder()
+    ///     .dfa(DFA::config().match_kind(MatchKind::All))
+    ///     .build_many(&[r"\w+$", r"\S+$"])?;
+    /// let mut cache = re.create_cache();
+    ///
+    /// let haystack = "@foo".as_bytes();
+    /// let mut state = OverlappingState::start();
+    ///
+    /// let expected = Some(MultiMatch::must(1, 0, 4));
+    /// let got = re.find_overlapping(&mut cache, haystack, &mut state);
+    /// assert_eq!(expected, got);
+    ///
+    /// // The first pattern also matches at the same position, so re-running
+    /// // the search will yield another match. Notice also that the first
+    /// // pattern is returned after the second. This is because the second
+    /// // pattern begins its match before the first, is therefore an earlier
+    /// // match and is thus reported first.
+    /// let expected = Some(MultiMatch::must(0, 1, 4));
+    /// let got = re.find_overlapping(&mut cache, haystack, &mut state);
+    /// assert_eq!(expected, got);
+    ///
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn find_overlapping(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+        state: &mut OverlappingState,
+    ) -> Option<MultiMatch> {
+        self.try_find_overlapping(cache, haystack, state).unwrap()
+    }
+
+    /// Returns an iterator over all non-overlapping "earliest" matches.
+    ///
+    /// Match positions are reported as soon as a match is known to occur, even
+    /// if the standard leftmost match would be longer.
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_find_earliest_iter`](Regex::try_find_earliest_iter).
+    ///
+    /// # Example
+    ///
+    /// This example shows how to run an "earliest" iterator.
+    ///
+    /// ```
+    /// use regex_automata::{hybrid::regex::Regex, MultiMatch};
+    ///
+    /// let re = Regex::new("[0-9]+")?;
+    /// let mut cache = re.create_cache();
+    /// let haystack = "123".as_bytes();
+    ///
+    /// // Normally, a standard leftmost iterator would return a single
+    /// // match, but since "earliest" detects matches earlier, we get
+    /// // three matches.
+    /// let mut it = re.find_earliest_iter(&mut cache, haystack);
+    /// assert_eq!(Some(MultiMatch::must(0, 0, 1)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 1, 2)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 2, 3)), it.next());
+    /// assert_eq!(None, it.next());
+    ///
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn find_earliest_iter<'r, 'c, 't>(
+        &'r self,
+        cache: &'c mut Cache,
+        haystack: &'t [u8],
+    ) -> FindEarliestMatches<'r, 'c, 't> {
+        FindEarliestMatches::new(self, cache, haystack)
+    }
+
+    /// Returns an iterator over all non-overlapping leftmost matches in the
+    /// given bytes. If no match exists, then the iterator yields no elements.
+    ///
+    /// This corresponds to the "standard" regex search iterator.
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_find_leftmost_iter`](Regex::try_find_leftmost_iter).
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use regex_automata::{MultiMatch, hybrid::regex::Regex};
+    ///
+    /// let re = Regex::new("foo[0-9]+")?;
+    /// let mut cache = re.create_cache();
+    ///
+    /// let text = b"foo1 foo12 foo123";
+    /// let matches: Vec<MultiMatch> = re
+    ///     .find_leftmost_iter(&mut cache, text)
+    ///     .collect();
+    /// assert_eq!(matches, vec![
+    ///     MultiMatch::must(0, 0, 4),
+    ///     MultiMatch::must(0, 5, 10),
+    ///     MultiMatch::must(0, 11, 17),
+    /// ]);
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn find_leftmost_iter<'r, 'c, 't>(
+        &'r self,
+        cache: &'c mut Cache,
+        haystack: &'t [u8],
+    ) -> FindLeftmostMatches<'r, 'c, 't> {
+        FindLeftmostMatches::new(self, cache, haystack)
+    }
+
+    /// Returns an iterator over all overlapping matches in the given haystack.
+    ///
+    /// This routine is principally useful when searching for multiple patterns
+    /// on inputs where multiple patterns may match the same regions of text.
+    /// The iterator takes care of handling the overlapping state that must be
+    /// threaded through every search.
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_find_overlapping_iter`](Regex::try_find_overlapping_iter).
+    ///
+    /// # Example
+    ///
+    /// This example shows how to run an overlapping search with multiple
+    /// regexes.
+    ///
+    /// ```
+    /// use regex_automata::{
+    ///     hybrid::{dfa::DFA, regex::Regex},
+    ///     MatchKind,
+    ///     MultiMatch,
+    /// };
+    ///
+    /// let re = Regex::builder()
+    ///     .dfa(DFA::config().match_kind(MatchKind::All))
+    ///     .build_many(&[r"\w+$", r"\S+$"])?;
+    /// let mut cache = re.create_cache();
+    /// let haystack = "@foo".as_bytes();
+    ///
+    /// let mut it = re.find_overlapping_iter(&mut cache, haystack);
+    /// assert_eq!(Some(MultiMatch::must(1, 0, 4)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 1, 4)), it.next());
+    /// assert_eq!(None, it.next());
+    ///
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn find_overlapping_iter<'r, 'c, 't>(
+        &'r self,
+        cache: &'c mut Cache,
+        haystack: &'t [u8],
+    ) -> FindOverlappingMatches<'r, 'c, 't> {
+        FindOverlappingMatches::new(self, cache, haystack)
+    }
+}
+
+/// Lower level infallible search routines that permit controlling where
+/// the search starts and ends in a particular sequence. This is useful for
+/// executing searches that need to take surrounding context into account. This
+/// is required for correctly implementing iteration because of look-around
+/// operators (`^`, `$`, `\b`).
+impl Regex {
+    /// Returns true if and only if this regex matches the given haystack.
+    ///
+    /// This routine may short circuit if it knows that scanning future input
+    /// will never lead to a different result. In particular, if the underlying
+    /// DFA enters a match state or a dead state, then this routine will return
+    /// `true` or `false`, respectively, without inspecting any future input.
+    ///
+    /// # Searching a substring of the haystack
+    ///
+    /// Being an "at" search routine, this permits callers to search a
+    /// substring of `haystack` by specifying a range in `haystack`.
+    /// Why expose this as an API instead of just asking callers to use
+    /// `&input[start..end]`? The reason is that regex matching often wants
+    /// to take the surrounding context into account in order to handle
+    /// look-around (`^`, `$` and `\b`).
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_is_match_at`](Regex::try_is_match_at).
+    pub fn is_match_at(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+    ) -> bool {
+        self.try_is_match_at(cache, haystack, start, end).unwrap()
+    }
+
+    /// Returns the first position at which a match is found.
+    ///
+    /// This routine stops scanning input in precisely the same circumstances
+    /// as `is_match`. The key difference is that this routine returns the
+    /// position at which it stopped scanning input if and only if a match
+    /// was found. If no match is found, then `None` is returned.
+    ///
+    /// # Searching a substring of the haystack
+    ///
+    /// Being an "at" search routine, this permits callers to search a
+    /// substring of `haystack` by specifying a range in `haystack`.
+    /// Why expose this as an API instead of just asking callers to use
+    /// `&input[start..end]`? The reason is that regex matching often wants
+    /// to take the surrounding context into account in order to handle
+    /// look-around (`^`, `$` and `\b`).
+    ///
+    /// This is useful when implementing an iterator over matches
+    /// within the same haystack, which cannot be done correctly by simply
+    /// providing a subslice of `haystack`.
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_find_earliest_at`](Regex::try_find_earliest_at).
+    pub fn find_earliest_at(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+    ) -> Option<MultiMatch> {
+        self.try_find_earliest_at(cache, haystack, start, end).unwrap()
+    }
+
+    /// Returns the same as `find_leftmost`, but starts the search at the given
+    /// offset.
+    ///
+    /// # Searching a substring of the haystack
+    ///
+    /// Being an "at" search routine, this permits callers to search a
+    /// substring of `haystack` by specifying a range in `haystack`.
+    /// Why expose this as an API instead of just asking callers to use
+    /// `&input[start..end]`? The reason is that regex matching often wants
+    /// to take the surrounding context into account in order to handle
+    /// look-around (`^`, `$` and `\b`).
+    ///
+    /// This is useful when implementing an iterator over matches within the
+    /// same haystack, which cannot be done correctly by simply providing a
+    /// subslice of `haystack`.
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_find_leftmost_at`](Regex::try_find_leftmost_at).
+    pub fn find_leftmost_at(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+    ) -> Option<MultiMatch> {
+        self.try_find_leftmost_at(cache, haystack, start, end).unwrap()
+    }
+
+    /// Search for the first overlapping match within a given range of
+    /// `haystack`.
+    ///
+    /// This routine is principally useful when searching for multiple patterns
+    /// on inputs where multiple patterns may match the same regions of text.
+    /// In particular, callers must preserve the automaton's search state from
+    /// prior calls so that the implementation knows where the last match
+    /// occurred and which pattern was reported.
+    ///
+    /// # Searching a substring of the haystack
+    ///
+    /// Being an "at" search routine, this permits callers to search a
+    /// substring of `haystack` by specifying a range in `haystack`.
+    /// Why expose this as an API instead of just asking callers to use
+    /// `&input[start..end]`? The reason is that regex matching often wants
+    /// to take the surrounding context into account in order to handle
+    /// look-around (`^`, `$` and `\b`).
+    ///
+    /// This is useful when implementing an iterator over matches
+    /// within the same haystack, which cannot be done correctly by simply
+    /// providing a subslice of `haystack`.
+    ///
+    /// # Panics
+    ///
+    /// If the underlying lazy DFAs return an error, then this routine panics.
+    /// This only occurs in non-default configurations where quit bytes are
+    /// used, Unicode word boundaries are heuristically enabled or limits are
+    /// set on the number of times the lazy DFA's cache may be cleared.
+    ///
+    /// The fallible version of this routine is
+    /// [`try_find_overlapping_at`](Regex::try_find_overlapping_at).
+    pub fn find_overlapping_at(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+        state: &mut OverlappingState,
+    ) -> Option<MultiMatch> {
+        self.try_find_overlapping_at(cache, haystack, start, end, state)
+            .unwrap()
+    }
+}
+
+/// Fallible search routines. These may return an error when the underlying
+/// lazy DFAs have been configured in a way that permits them to fail during a
+/// search.
+///
+/// Errors during search only occur when the lazy DFA has been explicitly
+/// configured to do so, usually by specifying one or more "quit" bytes or by
+/// heuristically enabling Unicode word boundaries.
+///
+/// Errors will never be returned using the default configuration. So these
+/// fallible routines are only needed for particular configurations.
+impl Regex {
+    /// Returns true if and only if this regex matches the given haystack.
+    ///
+    /// This routine may short circuit if it knows that scanning future input
+    /// will never lead to a different result. In particular, if the underlying
+    /// DFA enters a match state or a dead state, then this routine will return
+    /// `true` or `false`, respectively, without inspecting any future input.
+    ///
+    /// # Errors
+    ///
+    /// This routine only errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`is_match`](Regex::is_match).
+    pub fn try_is_match(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+    ) -> Result<bool, MatchError> {
+        self.try_is_match_at(cache, haystack, 0, haystack.len())
+    }
+
+    /// Returns the first position at which a match is found.
+    ///
+    /// This routine stops scanning input in precisely the same circumstances
+    /// as `is_match`. The key difference is that this routine returns the
+    /// position at which it stopped scanning input if and only if a match
+    /// was found. If no match is found, then `None` is returned.
+    ///
+    /// # Errors
+    ///
+    /// This routine only errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`find_earliest`](Regex::find_earliest).
+    pub fn try_find_earliest(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+    ) -> Result<Option<MultiMatch>, MatchError> {
+        self.try_find_earliest_at(cache, haystack, 0, haystack.len())
+    }
+
+    /// Returns the start and end offset of the leftmost match. If no match
+    /// exists, then `None` is returned.
+    ///
+    /// # Errors
+    ///
+    /// This routine only errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`find_leftmost`](Regex::find_leftmost).
+    pub fn try_find_leftmost(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+    ) -> Result<Option<MultiMatch>, MatchError> {
+        self.try_find_leftmost_at(cache, haystack, 0, haystack.len())
+    }
+
+    /// Search for the first overlapping match in `haystack`.
+    ///
+    /// This routine is principally useful when searching for multiple patterns
+    /// on inputs where multiple patterns may match the same regions of text.
+    /// In particular, callers must preserve the automaton's search state from
+    /// prior calls so that the implementation knows where the last match
+    /// occurred and which pattern was reported.
+    ///
+    /// # Errors
+    ///
+    /// This routine only errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`find_overlapping`](Regex::find_overlapping).
+    pub fn try_find_overlapping(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+        state: &mut OverlappingState,
+    ) -> Result<Option<MultiMatch>, MatchError> {
+        self.try_find_overlapping_at(cache, haystack, 0, haystack.len(), state)
+    }
+
+    /// Returns an iterator over all non-overlapping "earliest" matches.
+    ///
+    /// Match positions are reported as soon as a match is known to occur, even
+    /// if the standard leftmost match would be longer.
+    ///
+    /// # Errors
+    ///
+    /// This iterator only yields errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`find_earliest_iter`](Regex::find_earliest_iter).
+    pub fn try_find_earliest_iter<'r, 'c, 't>(
+        &'r self,
+        cache: &'c mut Cache,
+        haystack: &'t [u8],
+    ) -> TryFindEarliestMatches<'r, 'c, 't> {
+        TryFindEarliestMatches::new(self, cache, haystack)
+    }
+
+    /// Returns an iterator over all non-overlapping leftmost matches in the
+    /// given bytes. If no match exists, then the iterator yields no elements.
+    ///
+    /// This corresponds to the "standard" regex search iterator.
+    ///
+    /// # Errors
+    ///
+    /// This iterator only yields errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`find_leftmost_iter`](Regex::find_leftmost_iter).
+    pub fn try_find_leftmost_iter<'r, 'c, 't>(
+        &'r self,
+        cache: &'c mut Cache,
+        haystack: &'t [u8],
+    ) -> TryFindLeftmostMatches<'r, 'c, 't> {
+        TryFindLeftmostMatches::new(self, cache, haystack)
+    }
+
+    /// Returns an iterator over all overlapping matches in the given haystack.
+    ///
+    /// This routine is principally useful when searching for multiple patterns
+    /// on inputs where multiple patterns may match the same regions of text.
+    /// The iterator takes care of handling the overlapping state that must be
+    /// threaded through every search.
+    ///
+    /// # Errors
+    ///
+    /// This iterator only yields errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`find_overlapping_iter`](Regex::find_overlapping_iter).
+    pub fn try_find_overlapping_iter<'r, 'c, 't>(
+        &'r self,
+        cache: &'c mut Cache,
+        haystack: &'t [u8],
+    ) -> TryFindOverlappingMatches<'r, 'c, 't> {
+        TryFindOverlappingMatches::new(self, cache, haystack)
+    }
+}
+
+/// Lower level fallible search routines that permit controlling where the
+/// search starts and ends in a particular sequence.
+impl Regex {
+    /// Returns true if and only if this regex matches the given haystack.
+    ///
+    /// This routine may short circuit if it knows that scanning future input
+    /// will never lead to a different result. In particular, if the underlying
+    /// DFA enters a match state or a dead state, then this routine will return
+    /// `true` or `false`, respectively, without inspecting any future input.
+    ///
+    /// # Searching a substring of the haystack
+    ///
+    /// Being an "at" search routine, this permits callers to search a
+    /// substring of `haystack` by specifying a range in `haystack`.
+    /// Why expose this as an API instead of just asking callers to use
+    /// `&input[start..end]`? The reason is that regex matching often wants
+    /// to take the surrounding context into account in order to handle
+    /// look-around (`^`, `$` and `\b`).
+    ///
+    /// # Errors
+    ///
+    /// This routine only errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`is_match_at`](Regex::is_match_at).
+    pub fn try_is_match_at(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+    ) -> Result<bool, MatchError> {
+        self.forward()
+            .find_leftmost_fwd_at(
+                &mut cache.forward,
+                self.scanner().as_mut(),
+                None,
+                haystack,
+                start,
+                end,
+            )
+            .map(|x| x.is_some())
+    }
+
+    /// Returns the first position at which a match is found.
+    ///
+    /// This routine stops scanning input in precisely the same circumstances
+    /// as `is_match`. The key difference is that this routine returns the
+    /// position at which it stopped scanning input if and only if a match
+    /// was found. If no match is found, then `None` is returned.
+    ///
+    /// # Searching a substring of the haystack
+    ///
+    /// Being an "at" search routine, this permits callers to search a
+    /// substring of `haystack` by specifying a range in `haystack`.
+    /// Why expose this as an API instead of just asking callers to use
+    /// `&input[start..end]`? The reason is that regex matching often wants
+    /// to take the surrounding context into account in order to handle
+    /// look-around (`^`, `$` and `\b`).
+    ///
+    /// This is useful when implementing an iterator over matches
+    /// within the same haystack, which cannot be done correctly by simply
+    /// providing a subslice of `haystack`.
+    ///
+    /// # Errors
+    ///
+    /// This routine only errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`find_earliest_at`](Regex::find_earliest_at).
+    pub fn try_find_earliest_at(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+    ) -> Result<Option<MultiMatch>, MatchError> {
+        self.try_find_earliest_at_imp(
+            self.scanner().as_mut(),
+            cache,
+            haystack,
+            start,
+            end,
+        )
+    }
+
+    /// Returns the start and end offset of the leftmost match. If no match
+    /// exists, then `None` is returned.
+    ///
+    /// # Searching a substring of the haystack
+    ///
+    /// Being an "at" search routine, this permits callers to search a
+    /// substring of `haystack` by specifying a range in `haystack`.
+    /// Why expose this as an API instead of just asking callers to use
+    /// `&input[start..end]`? The reason is that regex matching often wants
+    /// to take the surrounding context into account in order to handle
+    /// look-around (`^`, `$` and `\b`).
+    ///
+    /// This is useful when implementing an iterator over matches
+    /// within the same haystack, which cannot be done correctly by simply
+    /// providing a subslice of `haystack`.
+    ///
+    /// # Errors
+    ///
+    /// This routine only errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`find_leftmost_at`](Regex::find_leftmost_at).
+    pub fn try_find_leftmost_at(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+    ) -> Result<Option<MultiMatch>, MatchError> {
+        self.try_find_leftmost_at_imp(
+            self.scanner().as_mut(),
+            cache,
+            haystack,
+            start,
+            end,
+        )
+    }
+
+    /// Search for the first overlapping match within a given range of
+    /// `haystack`.
+    ///
+    /// This routine is principally useful when searching for multiple patterns
+    /// on inputs where multiple patterns may match the same regions of text.
+    /// In particular, callers must preserve the automaton's search state from
+    /// prior calls so that the implementation knows where the last match
+    /// occurred and which pattern was reported.
+    ///
+    /// # Searching a substring of the haystack
+    ///
+    /// Being an "at" search routine, this permits callers to search a
+    /// substring of `haystack` by specifying a range in `haystack`.
+    /// Why expose this as an API instead of just asking callers to use
+    /// `&input[start..end]`? The reason is that regex matching often wants
+    /// to take the surrounding context into account in order to handle
+    /// look-around (`^`, `$` and `\b`).
+    ///
+    /// This is useful when implementing an iterator over matches
+    /// within the same haystack, which cannot be done correctly by simply
+    /// providing a subslice of `haystack`.
+    ///
+    /// # Errors
+    ///
+    /// This routine only errors if the search could not complete. For
+    /// DFA-based regexes, this only occurs in a non-default configuration
+    /// where quit bytes are used, Unicode word boundaries are heuristically
+    /// enabled or limits are set on the number of times the lazy DFA's cache
+    /// may be cleared.
+    ///
+    /// When a search cannot complete, callers cannot know whether a match
+    /// exists or not.
+    ///
+    /// The infallible (panics on error) version of this routine is
+    /// [`find_overlapping_at`](Regex::find_overlapping_at).
+    pub fn try_find_overlapping_at(
+        &self,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+        state: &mut OverlappingState,
+    ) -> Result<Option<MultiMatch>, MatchError> {
+        self.try_find_overlapping_at_imp(
+            self.scanner().as_mut(),
+            cache,
+            haystack,
+            start,
+            end,
+            state,
+        )
+    }
+}
+
+impl Regex {
+    #[inline(always)]
+    fn try_find_earliest_at_imp(
+        &self,
+        pre: Option<&mut prefilter::Scanner>,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+    ) -> Result<Option<MultiMatch>, MatchError> {
+        let (fdfa, rdfa) = (self.forward(), self.reverse());
+        let (fcache, rcache) = (&mut cache.forward, &mut cache.reverse);
+        let end = match fdfa
+            .find_earliest_fwd_at(fcache, pre, None, haystack, start, end)?
+        {
+            None => return Ok(None),
+            Some(end) => end,
+        };
+        // N.B. The only time we need to tell the reverse searcher the pattern
+        // to match is in the overlapping case, since it's ambiguous. In the
+        // earliest case, I have tentatively convinced myself that it isn't
+        // necessary and the reverse search will always find the same pattern
+        // to match as the forward search. But I lack a rigorous proof. Why not
+        // just provide the pattern anyway? Well, if it is needed, then leaving
+        // it out gives us a chance to find a witness.
+        let start = rdfa
+            .find_earliest_rev_at(rcache, None, haystack, start, end.offset())?
+            .expect("reverse search must match if forward search does");
+        assert_eq!(
+            start.pattern(),
+            end.pattern(),
+            "forward and reverse search must match same pattern",
+        );
+        assert!(start.offset() <= end.offset());
+        Ok(Some(MultiMatch::new(end.pattern(), start.offset(), end.offset())))
+    }
+
+    #[inline(always)]
+    fn try_find_leftmost_at_imp(
+        &self,
+        pre: Option<&mut prefilter::Scanner>,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+    ) -> Result<Option<MultiMatch>, MatchError> {
+        let (fdfa, rdfa) = (self.forward(), self.reverse());
+        let (fcache, rcache) = (&mut cache.forward, &mut cache.reverse);
+        let end = match fdfa
+            .find_leftmost_fwd_at(fcache, pre, None, haystack, start, end)?
+        {
+            None => return Ok(None),
+            Some(end) => end,
+        };
+        // N.B. The only time we need to tell the reverse searcher the pattern
+        // to match is in the overlapping case, since it's ambiguous. In the
+        // leftmost case, I have tentatively convinced myself that it isn't
+        // necessary and the reverse search will always find the same pattern
+        // to match as the forward search. But I lack a rigorous proof. Why not
+        // just provide the pattern anyway? Well, if it is needed, then leaving
+        // it out gives us a chance to find a witness.
+        let start = rdfa
+            .find_leftmost_rev_at(rcache, None, haystack, start, end.offset())?
+            .expect("reverse search must match if forward search does");
+        assert_eq!(
+            start.pattern(),
+            end.pattern(),
+            "forward and reverse search must match same pattern",
+        );
+        assert!(start.offset() <= end.offset());
+        Ok(Some(MultiMatch::new(end.pattern(), start.offset(), end.offset())))
+    }
+
+    #[inline(always)]
+    fn try_find_overlapping_at_imp(
+        &self,
+        pre: Option<&mut prefilter::Scanner>,
+        cache: &mut Cache,
+        haystack: &[u8],
+        start: usize,
+        end: usize,
+        state: &mut OverlappingState,
+    ) -> Result<Option<MultiMatch>, MatchError> {
+        let (fdfa, rdfa) = (self.forward(), self.reverse());
+        let (fcache, rcache) = (&mut cache.forward, &mut cache.reverse);
+        let end = match fdfa.find_overlapping_fwd_at(
+            fcache, pre, None, haystack, start, end, state,
+        )? {
+            None => return Ok(None),
+            Some(end) => end,
+        };
+        // Unlike the leftmost cases, the reverse overlapping search may match
+        // a different pattern than the forward search. See test failures when
+        // using `None` instead of `Some(end.pattern())` below. Thus, we must
+        // run our reverse search using the pattern that matched in the forward
+        // direction.
+        let start = rdfa
+            .find_leftmost_rev_at(
+                rcache,
+                Some(end.pattern()),
+                haystack,
+                0,
+                end.offset(),
+            )?
+            .expect("reverse search must match if forward search does");
+        assert_eq!(
+            start.pattern(),
+            end.pattern(),
+            "forward and reverse search must match same pattern",
+        );
+        assert!(start.offset() <= end.offset());
+        Ok(Some(MultiMatch::new(end.pattern(), start.offset(), end.offset())))
+    }
+}
+
+/// Non-search APIs for querying information about the regex and setting a
+/// prefilter.
+impl Regex {
+    /// Return the underlying lazy DFA responsible for forward matching.
+    ///
+    /// This is useful for accessing the underlying lazy DFA and using it
+    /// directly if the situation calls for it.
+    pub fn forward(&self) -> &DFA {
+        &self.forward
+    }
+
+    /// Return the underlying lazy DFA responsible for reverse matching.
+    ///
+    /// This is useful for accessing the underlying lazy DFA and using it
+    /// directly if the situation calls for it.
+    pub fn reverse(&self) -> &DFA {
+        &self.reverse
+    }
+
+    /// Returns the total number of patterns matched by this regex.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use regex_automata::{MultiMatch, hybrid::regex::Regex};
+    ///
+    /// let re = Regex::new_many(&[r"[a-z]+", r"[0-9]+", r"\w+"])?;
+    /// assert_eq!(3, re.pattern_count());
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn pattern_count(&self) -> usize {
+        assert_eq!(
+            self.forward().pattern_count(),
+            self.reverse().pattern_count()
+        );
+        self.forward().pattern_count()
+    }
+
+    /// Convenience function for returning this regex's prefilter as a trait
+    /// object.
+    ///
+    /// If this regex doesn't have a prefilter, then `None` is returned.
+    pub fn prefilter(&self) -> Option<&dyn Prefilter> {
+        self.pre.as_ref().map(|x| &**x)
+    }
+
+    /// Attach the given prefilter to this regex.
+    pub fn set_prefilter(&mut self, pre: Option<Box<dyn Prefilter>>) {
+        self.pre = pre;
+    }
+
+    /// Convenience function for returning a prefilter scanner.
+    fn scanner(&self) -> Option<prefilter::Scanner> {
+        self.prefilter().map(prefilter::Scanner::new)
+    }
+}
+
+/// An iterator over all non-overlapping earliest matches for a particular
+/// infallible search.
+///
+/// The iterator yields a [`MultiMatch`] value until no more matches could be
+/// found. If the underlying search returns an error, then this panics.
+///
+/// The lifetime variables are as follows:
+///
+/// * `'r` is the lifetime of the regular expression itself.
+/// * `'c` is the lifetime of the mutable cache used during search.
+/// * `'t` is the lifetime of the text being searched.
+#[derive(Debug)]
+pub struct FindEarliestMatches<'r, 'c, 't>(TryFindEarliestMatches<'r, 'c, 't>);
+
+impl<'r, 'c, 't> FindEarliestMatches<'r, 'c, 't> {
+    fn new(
+        re: &'r Regex,
+        cache: &'c mut Cache,
+        text: &'t [u8],
+    ) -> FindEarliestMatches<'r, 'c, 't> {
+        FindEarliestMatches(TryFindEarliestMatches::new(re, cache, text))
+    }
+}
+
+impl<'r, 'c, 't> Iterator for FindEarliestMatches<'r, 'c, 't> {
+    type Item = MultiMatch;
+
+    fn next(&mut self) -> Option<MultiMatch> {
+        next_unwrap(self.0.next())
+    }
+}
+
+/// An iterator over all non-overlapping leftmost matches for a particular
+/// infallible search.
+///
+/// The iterator yields a [`MultiMatch`] value until no more matches could be
+/// found. If the underlying search returns an error, then this panics.
+///
+/// The lifetime variables are as follows:
+///
+/// * `'r` is the lifetime of the regular expression itself.
+/// * `'c` is the lifetime of the mutable cache used during search.
+/// * `'t` is the lifetime of the text being searched.
+#[derive(Debug)]
+pub struct FindLeftmostMatches<'r, 'c, 't>(TryFindLeftmostMatches<'r, 'c, 't>);
+
+impl<'r, 'c, 't> FindLeftmostMatches<'r, 'c, 't> {
+    fn new(
+        re: &'r Regex,
+        cache: &'c mut Cache,
+        text: &'t [u8],
+    ) -> FindLeftmostMatches<'r, 'c, 't> {
+        FindLeftmostMatches(TryFindLeftmostMatches::new(re, cache, text))
+    }
+}
+
+impl<'r, 'c, 't> Iterator for FindLeftmostMatches<'r, 'c, 't> {
+    type Item = MultiMatch;
+
+    fn next(&mut self) -> Option<MultiMatch> {
+        next_unwrap(self.0.next())
+    }
+}
+
+/// An iterator over all overlapping matches for a particular infallible
+/// search.
+///
+/// The iterator yields a [`MultiMatch`] value until no more matches could be
+/// found. If the underlying search returns an error, then this panics.
+///
+/// The lifetime variables are as follows:
+///
+/// * `'r` is the lifetime of the regular expression itself.
+/// * `'c` is the lifetime of the mutable cache used during search.
+/// * `'t` is the lifetime of the text being searched.
+#[derive(Debug)]
+pub struct FindOverlappingMatches<'r, 'c, 't>(
+    TryFindOverlappingMatches<'r, 'c, 't>,
+);
+
+impl<'r, 'c, 't> FindOverlappingMatches<'r, 'c, 't> {
+    fn new(
+        re: &'r Regex,
+        cache: &'c mut Cache,
+        text: &'t [u8],
+    ) -> FindOverlappingMatches<'r, 'c, 't> {
+        FindOverlappingMatches(TryFindOverlappingMatches::new(re, cache, text))
+    }
+}
+
+impl<'r, 'c, 't> Iterator for FindOverlappingMatches<'r, 'c, 't> {
+    type Item = MultiMatch;
+
+    fn next(&mut self) -> Option<MultiMatch> {
+        next_unwrap(self.0.next())
+    }
+}
+
+/// An iterator over all non-overlapping earliest matches for a particular
+/// fallible search.
+///
+/// The iterator yields a [`MultiMatch`] value until no more matches could be
+/// found.
+///
+/// The lifetime variables are as follows:
+///
+/// * `'r` is the lifetime of the regular expression itself.
+/// * `'c` is the lifetime of the mutable cache used during search.
+/// * `'t` is the lifetime of the text being searched.
+#[derive(Debug)]
+pub struct TryFindEarliestMatches<'r, 'c, 't> {
+    re: &'r Regex,
+    cache: &'c mut Cache,
+    scanner: Option<prefilter::Scanner<'r>>,
+    text: &'t [u8],
+    last_end: usize,
+    last_match: Option<usize>,
+}
+
+impl<'r, 'c, 't> TryFindEarliestMatches<'r, 'c, 't> {
+    fn new(
+        re: &'r Regex,
+        cache: &'c mut Cache,
+        text: &'t [u8],
+    ) -> TryFindEarliestMatches<'r, 'c, 't> {
+        let scanner = re.scanner();
+        TryFindEarliestMatches {
+            re,
+            cache,
+            scanner,
+            text,
+            last_end: 0,
+            last_match: None,
+        }
+    }
+}
+
+impl<'r, 'c, 't> Iterator for TryFindEarliestMatches<'r, 'c, 't> {
+    type Item = Result<MultiMatch, MatchError>;
+
+    fn next(&mut self) -> Option<Result<MultiMatch, MatchError>> {
+        if self.last_end > self.text.len() {
+            return None;
+        }
+        let result = self.re.try_find_earliest_at_imp(
+            self.scanner.as_mut(),
+            self.cache,
+            self.text,
+            self.last_end,
+            self.text.len(),
+        );
+        let m = match result {
+            Err(err) => return Some(Err(err)),
+            Ok(None) => return None,
+            Ok(Some(m)) => m,
+        };
+        if m.is_empty() {
+            // This is an empty match. To ensure we make progress, start
+            // the next search at the smallest possible starting position
+            // of the next match following this one.
+            self.last_end = if self.re.utf8 {
+                crate::util::next_utf8(self.text, m.end())
+            } else {
+                m.end() + 1
+            };
+            // Don't accept empty matches immediately following a match.
+            // Just move on to the next match.
+            if Some(m.end()) == self.last_match {
+                return self.next();
+            }
+        } else {
+            self.last_end = m.end();
+        }
+        self.last_match = Some(m.end());
+        Some(Ok(m))
+    }
+}
+
+/// An iterator over all non-overlapping leftmost matches for a particular
+/// fallible search.
+///
+/// The iterator yields a [`MultiMatch`] value until no more matches could be
+/// found.
+///
+/// The lifetime variables are as follows:
+///
+/// * `'r` is the lifetime of the regular expression itself.
+/// * `'c` is the lifetime of the mutable cache used during search.
+/// * `'t` is the lifetime of the text being searched.
+#[derive(Debug)]
+pub struct TryFindLeftmostMatches<'r, 'c, 't> {
+    re: &'r Regex,
+    cache: &'c mut Cache,
+    scanner: Option<prefilter::Scanner<'r>>,
+    text: &'t [u8],
+    last_end: usize,
+    last_match: Option<usize>,
+}
+
+impl<'r, 'c, 't> TryFindLeftmostMatches<'r, 'c, 't> {
+    fn new(
+        re: &'r Regex,
+        cache: &'c mut Cache,
+        text: &'t [u8],
+    ) -> TryFindLeftmostMatches<'r, 'c, 't> {
+        let scanner = re.scanner();
+        TryFindLeftmostMatches {
+            re,
+            cache,
+            scanner,
+            text,
+            last_end: 0,
+            last_match: None,
+        }
+    }
+}
+
+impl<'r, 'c, 't> Iterator for TryFindLeftmostMatches<'r, 'c, 't> {
+    type Item = Result<MultiMatch, MatchError>;
+
+    fn next(&mut self) -> Option<Result<MultiMatch, MatchError>> {
+        if self.last_end > self.text.len() {
+            return None;
+        }
+        let result = self.re.try_find_leftmost_at_imp(
+            self.scanner.as_mut(),
+            self.cache,
+            self.text,
+            self.last_end,
+            self.text.len(),
+        );
+        let m = match result {
+            Err(err) => return Some(Err(err)),
+            Ok(None) => return None,
+            Ok(Some(m)) => m,
+        };
+        if m.is_empty() {
+            // This is an empty match. To ensure we make progress, start
+            // the next search at the smallest possible starting position
+            // of the next match following this one.
+            self.last_end = if self.re.utf8 {
+                crate::util::next_utf8(self.text, m.end())
+            } else {
+                m.end() + 1
+            };
+            // Don't accept empty matches immediately following a match.
+            // Just move on to the next match.
+            if Some(m.end()) == self.last_match {
+                return self.next();
+            }
+        } else {
+            self.last_end = m.end();
+        }
+        self.last_match = Some(m.end());
+        Some(Ok(m))
+    }
+}
+
+/// An iterator over all overlapping matches for a particular fallible search.
+///
+/// The iterator yields a [`MultiMatch`] value until no more matches could be
+/// found.
+///
+/// The lifetime variables are as follows:
+///
+/// * `'r` is the lifetime of the regular expression itself.
+/// * `'c` is the lifetime of the mutable cache used during search.
+/// * `'t` is the lifetime of the text being searched.
+#[derive(Debug)]
+pub struct TryFindOverlappingMatches<'r, 'c, 't> {
+    re: &'r Regex,
+    cache: &'c mut Cache,
+    scanner: Option<prefilter::Scanner<'r>>,
+    text: &'t [u8],
+    last_end: usize,
+    state: OverlappingState,
+}
+
+impl<'r, 'c, 't> TryFindOverlappingMatches<'r, 'c, 't> {
+    fn new(
+        re: &'r Regex,
+        cache: &'c mut Cache,
+        text: &'t [u8],
+    ) -> TryFindOverlappingMatches<'r, 'c, 't> {
+        let scanner = re.scanner();
+        TryFindOverlappingMatches {
+            re,
+            cache,
+            scanner,
+            text,
+            last_end: 0,
+            state: OverlappingState::start(),
+        }
+    }
+}
+
+impl<'r, 'c, 't> Iterator for TryFindOverlappingMatches<'r, 'c, 't> {
+    type Item = Result<MultiMatch, MatchError>;
+
+    fn next(&mut self) -> Option<Result<MultiMatch, MatchError>> {
+        if self.last_end > self.text.len() {
+            return None;
+        }
+        let result = self.re.try_find_overlapping_at_imp(
+            self.scanner.as_mut(),
+            self.cache,
+            self.text,
+            self.last_end,
+            self.text.len(),
+            &mut self.state,
+        );
+        let m = match result {
+            Err(err) => return Some(Err(err)),
+            Ok(None) => return None,
+            Ok(Some(m)) => m,
+        };
+        // Unlike the non-overlapping case, we're OK with empty matches at this
+        // level. In particular, the overlapping search algorithm is itself
+        // responsible for ensuring that progress is always made.
+        self.last_end = m.end();
+        Some(Ok(m))
+    }
+}
+
+/// A cache represents a partially computed forward and reverse DFA.
+///
+/// A cache is the key component that differentiates a classical DFA and a
+/// hybrid NFA/DFA (also called a "lazy DFA"). Where a classical DFA builds a
+/// complete transition table that can handle all possible inputs, a hybrid
+/// NFA/DFA starts with an empty transition table and builds only the parts
+/// required during search. The parts that are built are stored in a cache. For
+/// this reason, a cache is a required parameter for nearly every operation on
+/// a [`Regex`].
+///
+/// Caches can be created from their corresponding `Regex` via
+/// [`Regex::create_cache`]. A cache can only be used with either the `Regex`
+/// that created it, or the `Regex` that was most recently used to reset it
+/// with [`Cache::reset`]. Using a cache with any other `Regex` may result in
+/// panics or incorrect results.
+#[derive(Debug, Clone)]
+pub struct Cache {
+    forward: dfa::Cache,
+    reverse: dfa::Cache,
+}
+
+impl Cache {
+    /// Create a new cache for the given `Regex`.
+    ///
+    /// The cache returned should only be used for searches for the given
+    /// `Regex`. If you want to reuse the cache for another `Regex`, then you
+    /// must call [`Cache::reset`] with that `Regex`.
+    pub fn new(re: &Regex) -> Cache {
+        let forward = dfa::Cache::new(re.forward());
+        let reverse = dfa::Cache::new(re.reverse());
+        Cache { forward, reverse }
+    }
+
+    /// Reset this cache such that it can be used for searching with the given
+    /// `Regex` (and only that `Regex`).
+    ///
+    /// A cache reset permits reusing memory already allocated in this cache
+    /// with a different `Regex`.
+    ///
+    /// Resetting a cache sets its "clear count" to 0. This is relevant if the
+    /// `Regex` has been configured to "give up" after it has cleared the cache
+    /// a certain number of times.
+    ///
+    /// # Example
+    ///
+    /// This shows how to re-purpose a cache for use with a different `Regex`.
+    ///
+    /// ```
+    /// use regex_automata::{hybrid::regex::Regex, MultiMatch};
+    ///
+    /// let re1 = Regex::new(r"\w")?;
+    /// let re2 = Regex::new(r"\W")?;
+    ///
+    /// let mut cache = re1.create_cache();
+    /// assert_eq!(
+    ///     Some(MultiMatch::must(0, 0, 2)),
+    ///     re1.find_leftmost(&mut cache, "Δ".as_bytes()),
+    /// );
+    ///
+    /// // Using 'cache' with re2 is not allowed. It may result in panics or
+    /// // incorrect results. In order to re-purpose the cache, we must reset
+    /// // it with the Regex we'd like to use it with.
+    /// //
+    /// // Similarly, after this reset, using the cache with 're1' is also not
+    /// // allowed.
+    /// cache.reset(&re2);
+    /// assert_eq!(
+    ///     Some(MultiMatch::must(0, 0, 3)),
+    ///     re2.find_leftmost(&mut cache, "☃".as_bytes()),
+    /// );
+    ///
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn reset(&mut self, re: &Regex) {
+        self.forward.reset(re.forward());
+        self.reverse.reset(re.reverse());
+    }
+
+    /// Returns the heap memory usage, in bytes, as a sum of the forward and
+    /// reverse lazy DFA caches.
+    ///
+    /// This does **not** include the stack size used up by this cache. To
+    /// compute that, use `std::mem::size_of::<Cache>()`.
+    pub fn memory_usage(&self) -> usize {
+        self.forward.memory_usage() + self.reverse.memory_usage()
+    }
+
+    /// Return references to the forward and reverse caches, respectively.
+    pub fn as_parts(&self) -> (&dfa::Cache, &dfa::Cache) {
+        (&self.forward, &self.reverse)
+    }
+
+    /// Return mutable references to the forward and reverse caches,
+    /// respectively.
+    pub fn as_parts_mut(&mut self) -> (&mut dfa::Cache, &mut dfa::Cache) {
+        (&mut self.forward, &mut self.reverse)
+    }
+}
+
+/// The configuration used for compiling a hybrid NFA/DFA regex.
+///
+/// A regex configuration is a simple data object that is typically used with
+/// [`Builder::configure`].
+#[derive(Clone, Copy, Debug, Default)]
+pub struct Config {
+    utf8: Option<bool>,
+}
+
+impl Config {
+    /// Return a new default regex compiler configuration.
+    pub fn new() -> Config {
+        Config::default()
+    }
+
+    /// Whether to enable UTF-8 mode or not.
+    ///
+    /// When UTF-8 mode is enabled (the default) and an empty match is seen,
+    /// the iterators on [`Regex`] will always start the next search at the
+    /// next UTF-8 encoded codepoint when searching valid UTF-8. When UTF-8
+    /// mode is disabled, such searches are begun at the next byte offset.
+    ///
+    /// If this mode is enabled and invalid UTF-8 is given to search, then
+    /// behavior is unspecified.
+    ///
+    /// Generally speaking, one should enable this when
+    /// [`SyntaxConfig::utf8`](crate::SyntaxConfig::utf8)
+    /// and
+    /// [`thompson::Config::utf8`](crate::nfa::thompson::Config::utf8)
+    /// are enabled, and disable it otherwise.
+    ///
+    /// # Example
+    ///
+    /// This example demonstrates the differences between when this option is
+    /// enabled and disabled. The differences only arise when the regex can
+    /// return matches of length zero.
+    ///
+    /// In this first snippet, we show the results when UTF-8 mode is disabled.
+    ///
+    /// ```
+    /// use regex_automata::{hybrid::regex::Regex, MultiMatch};
+    ///
+    /// let re = Regex::builder()
+    ///     .configure(Regex::config().utf8(false))
+    ///     .build(r"")?;
+    /// let mut cache = re.create_cache();
+    ///
+    /// let haystack = "a☃z".as_bytes();
+    /// let mut it = re.find_leftmost_iter(&mut cache, haystack);
+    /// assert_eq!(Some(MultiMatch::must(0, 0, 0)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 1, 1)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 2, 2)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 3, 3)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 4, 4)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 5, 5)), it.next());
+    /// assert_eq!(None, it.next());
+    ///
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    ///
+    /// And in this snippet, we execute the same search on the same haystack,
+    /// but with UTF-8 mode enabled. Notice that byte offsets that would
+    /// otherwise split the encoding of `☃` are not returned.
+    ///
+    /// ```
+    /// use regex_automata::{hybrid::regex::Regex, MultiMatch};
+    ///
+    /// let re = Regex::builder()
+    ///     .configure(Regex::config().utf8(true))
+    ///     .build(r"")?;
+    /// let mut cache = re.create_cache();
+    ///
+    /// let haystack = "a☃z".as_bytes();
+    /// let mut it = re.find_leftmost_iter(&mut cache, haystack);
+    /// assert_eq!(Some(MultiMatch::must(0, 0, 0)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 1, 1)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 4, 4)), it.next());
+    /// assert_eq!(Some(MultiMatch::must(0, 5, 5)), it.next());
+    /// assert_eq!(None, it.next());
+    ///
+    /// # Ok::<(), Box<dyn std::error::Error>>(())
+    /// ```
+    pub fn utf8(mut self, yes: bool) -> Config {
+        self.utf8 = Some(yes);
+        self
+    }
+
+    /// Returns true if and only if this configuration has UTF-8 mode enabled.
+    ///
+    /// When UTF-8 mode is enabled and an empty match is seen, the iterators on
+    /// [`Regex`] will always start the next search at the next UTF-8 encoded
+    /// codepoint. When UTF-8 mode is disabled, such searches are begun at the
+    /// next byte offset.
+    pub fn get_utf8(&self) -> bool {
+        self.utf8.unwrap_or(true)
+    }
+
+    /// Overwrite the default configuration such that the options in `o` are
+    /// always used. If an option in `o` is not set, then the corresponding
+    /// option in `self` is used. If it's not set in `self` either, then it
+    /// remains not set.
+    pub(crate) fn overwrite(self, o: Config) -> Config {
+        Config { utf8: o.utf8.or(self.utf8) }
+    }
+}
+
+/// A builder for a regex based on a hybrid NFA/DFA.
+///
+/// This builder permits configuring options for the syntax of a pattern, the
+/// NFA construction, the lazy DFA construction and finally the regex searching
+/// itself. This builder is different from a general purpose regex builder
+/// in that it permits fine grain configuration of the construction process.
+/// The trade off for this is complexity, and the possibility of setting a
+/// configuration that might not make sense. For example, there are three
+/// different UTF-8 modes:
+///
+/// * [`SyntaxConfig::utf8`](crate::SyntaxConfig::utf8) controls whether the
+/// pattern itself can contain sub-expressions that match invalid UTF-8.
+/// * [`nfa::thompson::Config::utf8`](crate::nfa::thompson::Config::utf8)
+/// controls whether the implicit unanchored prefix added to the NFA can
+/// match through invalid UTF-8 or not.
+/// * [`Config::utf8`] controls how the regex iterators themselves advance
+/// the starting position of the next search when a match with zero length is
+/// found.
+///
+/// Generally speaking, callers will want to either enable all of these or
+/// disable all of these.
+///
+/// Internally, building a regex requires building two hybrid NFA/DFAs,
+/// where one is responsible for finding the end of a match and the other is
+/// responsible for finding the start of a match. If you only need to detect
+/// whether something matched, or only the end of a match, then you should use
+/// a [`dfa::Builder`] to construct a single hybrid NFA/DFA, which is cheaper
+/// than building two of them.
+///
+/// # Example
+///
+/// This example shows how to disable UTF-8 mode in the syntax, the NFA and
+/// the regex itself. This is generally what you want for matching on
+/// arbitrary bytes.
+///
+/// ```
+/// use regex_automata::{
+///     hybrid::regex::Regex, nfa::thompson, MultiMatch, SyntaxConfig
+/// };
+///
+/// let re = Regex::builder()
+///     .configure(Regex::config().utf8(false))
+///     .syntax(SyntaxConfig::new().utf8(false))
+///     .thompson(thompson::Config::new().utf8(false))
+///     .build(r"foo(?-u:[^b])ar.*")?;
+/// let mut cache = re.create_cache();
+///
+/// let haystack = b"\xFEfoo\xFFarzz\xE2\x98\xFF\n";
+/// let expected = Some(MultiMatch::must(0, 1, 9));
+/// let got = re.find_leftmost(&mut cache, haystack);
+/// assert_eq!(expected, got);
+/// // Notice that `(?-u:[^b])` matches invalid UTF-8,
+/// // but the subsequent `.*` does not! Disabling UTF-8
+/// // on the syntax permits this. Notice also that the
+/// // search was unanchored and skipped over invalid UTF-8.
+/// // Disabling UTF-8 on the Thompson NFA permits this.
+/// //
+/// // N.B. This example does not show the impact of
+/// // disabling UTF-8 mode on Config, since that
+/// // only impacts regexes that can produce matches of
+/// // length 0.
+/// assert_eq!(b"foo\xFFarzz", &haystack[got.unwrap().range()]);
+///
+/// # Ok::<(), Box<dyn std::error::Error>>(())
+/// ```
+#[derive(Clone, Debug)]
+pub struct Builder {
+    config: Config,
+    dfa: dfa::Builder,
+}
+
+impl Builder {
+    /// Create a new regex builder with the default configuration.
+    pub fn new() -> Builder {
+        Builder { config: Config::default(), dfa: DFA::builder() }
+    }
+
+    /// Build a regex from the given pattern.
+    ///
+    /// If there was a problem parsing or compiling the pattern, then an error
+    /// is returned.
+    pub fn build(&self, pattern: &str) -> Result<Regex, BuildError> {
+        self.build_many(&[pattern])
+    }
+
+    /// Build a regex from the given patterns.
+    pub fn build_many<P: AsRef<str>>(
+        &self,
+        patterns: &[P],
+    ) -> Result<Regex, BuildError> {
+        let forward = self.dfa.build_many(patterns)?;
+        let reverse = self
+            .dfa
+            .clone()
+            .configure(
+                DFA::config()
+                    .anchored(true)
+                    .match_kind(MatchKind::All)
+                    .starts_for_each_pattern(true),
+            )
+            .thompson(thompson::Config::new().reverse(true))
+            .build_many(patterns)?;
+        Ok(self.build_from_dfas(forward, reverse))
+    }
+
+    /// Build a regex from its component forward and reverse hybrid NFA/DFAs.
+    fn build_from_dfas(&self, forward: DFA, reverse: DFA) -> Regex {
+        // The congruous method on DFA-backed regexes is exposed, but it's
+        // not clear this builder is useful here since lazy DFAs can't be
+        // serialized and there is only one type of them.
+        let utf8 = self.config.get_utf8();
+        Regex { pre: None, forward, reverse, utf8 }
+    }
+
+    /// Apply the given regex configuration options to this builder.
+    pub fn configure(&mut self, config: Config) -> &mut Builder {
+        self.config = self.config.overwrite(config);
+        self
+    }
+
+    /// Set the syntax configuration for this builder using
+    /// [`SyntaxConfig`](crate::SyntaxConfig).
+    ///
+    /// This permits setting things like case insensitivity, Unicode and multi
+    /// line mode.
+    pub fn syntax(
+        &mut self,
+        config: crate::util::syntax::SyntaxConfig,
+    ) -> &mut Builder {
+        self.dfa.syntax(config);
+        self
+    }
+
+    /// Set the Thompson NFA configuration for this builder using
+    /// [`nfa::thompson::Config`](thompson::Config).
+    ///
+    /// This permits setting things like whether additional time should be
+    /// spent shrinking the size of the NFA.
+    pub fn thompson(&mut self, config: thompson::Config) -> &mut Builder {
+        self.dfa.thompson(config);
+        self
+    }
+
+    /// Set the lazy DFA compilation configuration for this builder using
+    /// [`dfa::Config`](dfa::Config).
+    ///
+    /// This permits setting things like whether Unicode word boundaries should
+    /// be heuristically supported or settings how the behavior of the cache.
+    pub fn dfa(&mut self, config: dfa::Config) -> &mut Builder {
+        self.dfa.configure(config);
+        self
+    }
+}
+
+impl Default for Builder {
+    fn default() -> Builder {
+        Builder::new()
+    }
+}
+
+#[inline(always)]
+fn next_unwrap(
+    item: Option<Result<MultiMatch, MatchError>>,
+) -> Option<MultiMatch> {
+    match item {
+        None => None,
+        Some(Ok(m)) => Some(m),
+        Some(Err(err)) => panic!(
+            "unexpected regex search error: {}\n\
+             to handle search errors, use try_ methods",
+            err,
+        ),
+    }
+}