<!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="Provides non-deterministic finite automata (NFA) and regex engines that use them."><title>regex_automata::nfa - 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-5bc39a1768837dd0.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="1.77.1 (7cf61ebde 2024-03-27)"data-channel="1.77.1"data-search-js="search-dd67cee4cfa65049.js"data-settings-js="settings-4313503d2e1961c2.js"><scriptsrc="../../static.files/storage-4c98445ec4002617.js"></script><scriptdefersrc="../sidebar-items.js"></script><scriptdefersrc="../../static.files/main-48f368f3872407c8.js"></script><noscript><linkrel="stylesheet"href="../../static.files/noscript-04d5337699b92874.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"title="show sidebar"></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 nfa</a></h2><divclass="sidebar-elems"><section><ulclass="block"><li><ahref="#modules">Modules</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="#">nfa</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/nfa/mod.rs.html#1-55">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>Provides non-deterministic finite automata (NFA) and regex engines that use
them.</p>
<p>While NFAs and DFAs (deterministic finite automata) have equivalent <em>theoretical</em>
power, their usage in practice tends to result in different engineering trade
offs. While this isn’t meant to be a comprehensive treatment of the topic, here
are a few key trade offs that are, at minimum, true for this crate:</p>
<ul>
<li>NFAs tend to be represented sparsely where as DFAs are represented densely.
Sparse representations use less memory, but are slower to traverse. Conversely,
dense representations use more memory, but are faster to traverse. (Sometimes
these lines are blurred. For example, an <code>NFA</code> might choose to represent a
particular state in a dense fashion, and a DFA can be built using a sparse
representation via <ahref="crate::dfa::sparse::DFA"><code>sparse::DFA</code></a>.</li>
<li>NFAs have espilon transitions and DFAs don’t. In practice, this means that
handling a single byte in a haystack with an NFA at search time may require
visiting multiple NFA states. In a DFA, each byte only requires visiting
a single state. Stated differently, NFAs require a variable number of CPU
instructions to process one byte in a haystack where as a DFA uses a constant
number of CPU instructions to process one byte.</li>
<li>NFAs are generally easier to amend with secondary storage. For example, the
<ahref="thompson/pikevm/struct.PikeVM.html"title="struct regex_automata::nfa::thompson::pikevm::PikeVM"><code>thompson::pikevm::PikeVM</code></a> uses an NFA to match, but also uses additional
memory beyond the model of a finite state machine to track offsets for matching
capturing groups. Conversely, the most a DFA can do is report the offset (and
pattern ID) at which a match occurred. This is generally why we also compile
DFAs in reverse, so that we can run them after finding the end of a match to
also find the start of a match.</li>
<li>NFAs take worst case linear time to build, but DFAs take worst case
</div></details><h2id="modules"class="section-header">Modules<ahref="#modules"class="anchor">§</a></h2><ulclass="item-table"><li><divclass="item-name"><aclass="mod"href="thompson/index.html"title="mod regex_automata::nfa::thompson">thompson</a></div><divclass="desc docblock-short">Defines a Thompson NFA and provides the <ahref="thompson/pikevm/struct.PikeVM.html"title="struct regex_automata::nfa::thompson::pikevm::PikeVM"><code>PikeVM</code></a> and
