<!DOCTYPE html><htmllang="en"><head><metacharset="utf-8"><metaname="viewport"content="width=device-width, initial-scale=1.0"><metaname="generator"content="rustdoc"><metaname="description"content="A module for building and searching with lazy deterministic finite automata (DFAs)."><title>regex_automata::hybrid - Rust</title><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><linkrel="preload"as="font"type="font/woff2"crossoriginhref="../../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><linkrel="stylesheet"href="../../static.files/normalize-76eba96aa4d2e634.css"><linkrel="stylesheet"href="../../static.files/rustdoc-ac92e1bbe349e143.css"><metaname="rustdoc-vars"data-root-path="../../"data-static-root-path="../../static.files/"data-current-crate="regex_automata"data-themes=""data-resource-suffix=""data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)"data-channel="1.76.0"data-search-js="search-2b6ce74ff89ae146.js"data-settings-js="settings-4313503d2e1961c2.js"><scriptsrc="../../static.files/storage-f2adc0d6ca4d09fb.js"></script><scriptdefersrc="../sidebar-items.js"></script><scriptdefersrc="../../static.files/main-305769736d49e732.js"></script><noscript><linkrel="stylesheet"href="../../static.files/noscript-feafe1bb7466e4bd.css"></noscript><linkrel="alternate icon"type="image/png"href="../../static.files/favicon-16x16-8b506e7a72182f1c.png"><linkrel="alternate icon"type="image/png"href="../../static.files/favicon-32x32-422f7d1d52889060.png"><linkrel="icon"type="image/svg+xml"href="../../static.files/favicon-2c020d218678b618.svg"></head><bodyclass="rustdoc mod"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><navclass="mobile-topbar"><buttonclass="sidebar-menu-toggle">☰</button></nav><navclass="sidebar"><divclass="sidebar-crate"><h2><ahref="../../regex_automata/index.html">regex_automata</a><spanclass="version">0.4.6</span></h2></div><h2class="location"><ahref="#">Module hybrid</a></h2><divclass="sidebar-elems"><section><ulclass="block"><li><ahref="#modules">Modules</a></li><li><ahref="#structs">Structs</a></li><li><ahref="#enums">Enums</a></li></ul></section><h2><ahref="../index.html">In crate regex_automata</a></h2></div></nav><divclass="sidebar-resizer"></div>
<main><divclass="width-limiter"><navclass="sub"><formclass="search-form"><span></span><divid="sidebar-button"tabindex="-1"><ahref="../../regex_automata/all.html"title="show sidebar"></a></div><inputclass="search-input"name="search"aria-label="Run search in the documentation"autocomplete="off"spellcheck="false"placeholder="Click or press ‘S’ to search, ‘?’ for more options…"type="search"><divid="help-button"tabindex="-1"><ahref="../../help.html"title="help">?</a></div><divid="settings-menu"tabindex="-1"><ahref="../../settings.html"title="settings"><imgwidth="22"height="22"alt="Change settings"src="../../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><sectionid="main-content"class="content"><divclass="main-heading"><h1>Module <ahref="../index.html">regex_automata</a>::<wbr><aclass="mod"href="#">hybrid</a><buttonid="copy-path"title="Copy item path to clipboard"><imgsrc="../../static.files/clipboard-7571035ce49a181d.svg"width="19"height="18"alt="Copy item path"></button></h1><spanclass="out-of-band"><aclass="src"href="../../src/regex_automata/hybrid/mod.rs.html#1-144">source</a> · <buttonid="toggle-all-docs"title="collapse all docs">[<span>−</span>]</button></span></div><detailsclass="toggle top-doc"open><summaryclass="hideme"><span>Expand description</span></summary><divclass="docblock"><p>A module for building and searching with lazy deterministic finite automata
(DFAs).</p>
<p>Like other modules in this crate, lazy DFAs support a rich regex syntax with
Unicode features. The key feature of a lazy DFA is that it builds itself
incrementally during search, and never uses more than a configured capacity of
memory. Thus, when searching with a lazy DFA, one must supply a mutable “cache”
in which the actual DFA’s transition table is stored.</p>
<p>If you’re looking for fully compiled DFAs, then please see the top-level
<p>This section gives a brief overview of the primary types in this module:</p>
<ul>
<li>A <ahref="regex/struct.Regex.html"title="struct regex_automata::hybrid::regex::Regex"><code>regex::Regex</code></a> provides a way to search for matches of a regular
expression using lazy DFAs. This includes iterating over matches with both the
start and end positions of each match.</li>
<li>A <ahref="dfa/struct.DFA.html"title="struct regex_automata::hybrid::dfa::DFA"><code>dfa::DFA</code></a> provides direct low level access to a lazy DFA.</li>
<p>If you know ahead of time that your pattern would result in a very large DFA
if it was fully compiled, it may be better to use an NFA simulation instead
of a lazy DFA. Either that, or increase the cache capacity of your lazy DFA
to something that is big enough to hold the state machine (likely through
experimentation). The issue here is that if the cache is too small, then it
could wind up being reset too frequently and this might decrease searching
speed significantly.</p>
<h2id="differences-with-fully-compiled-dfas"><ahref="#differences-with-fully-compiled-dfas">Differences with fully compiled DFAs</a></h2>
<p>A <ahref="regex/struct.Regex.html"title="struct regex_automata::hybrid::regex::Regex"><code>hybrid::regex::Regex</code></a> and a
<ahref="crate::dfa::regex::Regex"><code>dfa::regex::Regex</code></a> both have the same capabilities
(and similarly for their underlying DFAs), but they achieve them through
different means. The main difference is that a hybrid or “lazy” regex builds
its DFA lazily during search, where as a fully compiled regex will build its
DFA at construction time. While building a DFA at search time might sound like
it’s slow, it tends to work out where most bytes seen during a search will
reuse pre-built parts of the DFA and thus can be almost as fast as a fully
compiled DFA. The main downside is that searching requires mutable space to
store the DFA, and, in the worst case, a search can result in a new state being
created for each byte seen, which would make searching quite a bit slower.</p>
<p>A fully compiled DFA never has to worry about searches being slower once
it’s built. (Aside from, say, the transition table being so large that it
is subject to harsh CPU cache effects.) However, of course, building a full
DFA can be quite time consuming and memory hungry. Particularly when large
Unicode character classes are used, which tend to translate into very large
DFAs.</p>
<p>A lazy DFA strikes a nice balance <em>in practice</em>, particularly in the
presence of Unicode mode, by only building what is needed. It avoids the
worst case exponential time complexity of DFA compilation by guaranteeing that
it will only build at most one state per byte searched. While the worst
case here can lead to a very high constant, it will never be exponential.</p>
<h2id="syntax"><ahref="#syntax">Syntax</a></h2>
<p>This module supports the same syntax as the <code>regex</code> crate, since they share the
same parser. You can find an exhaustive list of supported syntax in the
<ahref="https://docs.rs/regex/1/regex/#syntax">documentation for the <code>regex</code> crate</a>.</p>
<p>There are two things that are not supported by the lazy DFAs in this module:</p>
<ul>
<li>Capturing groups. The DFAs (and <ahref="regex/struct.Regex.html"title="struct regex_automata::hybrid::regex::Regex"><code>Regex</code></a>es built on top
of them) can only find the offsets of an entire match, but cannot resolve
the offsets of each capturing group. This is because DFAs do not have the
expressive power necessary. Note that it is okay to build a lazy DFA from an
NFA that contains capture groups. The capture groups will simply be ignored.</li>
<li>Unicode word boundaries. These present particularly difficult challenges for
DFA construction and would result in an explosion in the number of states.
One can enable <ahref="dfa/struct.Config.html#method.unicode_word_boundary"title="method regex_automata::hybrid::dfa::Config::unicode_word_boundary"><code>dfa::Config::unicode_word_boundary</code></a> though, which provides
heuristic support for Unicode word boundaries that only works on ASCII text.
Otherwise, one can use <code>(?-u:\b)</code> for an ASCII word boundary, which will work
on any input.</li>
</ul>
<p>There are no plans to lift either of these limitations.</p>
<p>Note that these restrictions are identical to the restrictions on fully
compiled DFAs.</p>
</div></details><h2id="modules"class="section-header"><ahref="#modules">Modules</a></h2><ulclass="item-table"><li><divclass="item-name"><aclass="mod"href="dfa/index.html"title="mod regex_automata::hybrid::dfa">dfa</a></div><divclass="desc docblock-short">Types and routines specific to lazy DFAs.</div></li><li><divclass="item-name"><aclass="mod"href="regex/index.html"title="mod regex_automata::hybrid::regex">regex</a></div><divclass="desc docblock-short">A lazy DFA backed <code>Regex</code>.</div></li></ul><h2id="structs"class="section-header"><ahref="#structs">Structs</a></h2><ulclass="item-table"><li><divclass="item-name"><aclass="struct"href="struct.BuildError.html"title="struct regex_automata::hybrid::BuildError">BuildError</a></div><divclass="desc docblock-short">An error that occurs when initial construction of a lazy DFA fails.</div></li><li><divclass="item-name"><aclass="struct"href="struct.CacheError.html"title="struct regex_automata::hybrid::CacheError">CacheError</a></div><divclass="desc docblock-short">An error that occurs when cache usage has become inefficient.</div></li><li><divclass="item-name"><aclass="struct"href="struct.LazyStateID.html"title="struct regex_automata::hybrid::LazyStateID">LazyStateID</a></div><divclass="desc docblock-short">A state identifier specifically tailored for lazy DFAs.</div></li></ul><h2id="enums"class="section-header"><ahref="#enums">Enums</a></h2><ulclass="item-table"><li><divclass="item-name"><aclass="enum"href="enum.StartError.html"title="enum regex_automata::hybrid::StartError">StartError</a></div><divclass="desc docblock-short">An error that can occur when computing the start state for a search.</div></li></ul></section></div></main></body></html>
