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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Provides literal extraction from `Hir` expressions."><title>regex_syntax::hir::literal - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../../../" data-static-root-path="../../../static.files/" data-current-crate="regex_syntax" 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" ><script src="../../../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="../sidebar-items.js"></script><script defer src="../../../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../../../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc mod"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">&#9776;</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../../regex_syntax/index.html">regex_syntax</a><span class="version">0.8.2</span></h2></div><h2 class="location"><a href="#">Module literal</a></h2><div class="sidebar-elems"><section><ul class="block"><li><a href="#structs">Structs</a></li><li><a href="#enums">Enums</a></li><li><a href="#functions">Functions</a></li></ul></section><h2><a href="../index.html">In regex_syntax::hir</a></h2></div></nav><div class="sidebar-resizer"></div>
<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../../../regex_syntax/all.html" title="show sidebar"></a></div><input class="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"><div id="help-button" tabindex="-1"><a href="../../../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Module <a href="../../index.html">regex_syntax</a>::<wbr><a href="../index.html">hir</a>::<wbr><a class="mod" href="#">literal</a><button id="copy-path" title="Copy item path to clipboard"><img src="../../../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../../../src/regex_syntax/hir/literal.rs.html#1-3214">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>&#x2212;</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Provides literal extraction from <code>Hir</code> expressions.</p>
<p>An <a href="struct.Extractor.html" title="struct regex_syntax::hir::literal::Extractor"><code>Extractor</code></a> pulls literals out of <a href="../struct.Hir.html" title="struct regex_syntax::hir::Hir"><code>Hir</code></a> expressions and returns a
<a href="struct.Seq.html" title="struct regex_syntax::hir::literal::Seq"><code>Seq</code></a> of <a href="struct.Literal.html" title="struct regex_syntax::hir::literal::Literal"><code>Literal</code></a>s.</p>
<p>The purpose of literal extraction is generally to provide avenues for
optimizing regex searches. The main idea is that substring searches can be an
order of magnitude faster than a regex search. Therefore, if one can execute
a substring search to find candidate match locations and only run the regex
search at those locations, then it is possible for huge improvements in
performance to be realized.</p>
<p>With that said, literal optimizations are generally a black art because even
though substring search is generally faster, if the number of candidates
produced is high, then it can create a lot of overhead by ping-ponging between
the substring search and the regex search.</p>
<p>Here are some heuristics that might be used to help increase the chances of
effective literal optimizations:</p>
<ul>
<li>Stick to small <a href="struct.Seq.html" title="struct regex_syntax::hir::literal::Seq"><code>Seq</code></a>s. If you search for too many literals, it’s likely
to lead to substring search that is only a little faster than a regex search,
and thus the overhead of using literal optimizations in the first place might
make things slower overall.</li>
<li>The literals in your <a href="struct.Seq.html" title="struct regex_syntax::hir::literal::Seq"><code>Seq</code></a> shouldn’t be too short. In general, longer is
better. A sequence corresponding to single bytes that occur frequently in the
haystack, for example, is probably a bad literal optimization because it’s
likely to produce many false positive candidates. Longer literals are less
likely to match, and thus probably produce fewer false positives.</li>
<li>If it’s possible to estimate the approximate frequency of each byte according
to some pre-computed background distribution, it is possible to compute a score
of how “good” a <code>Seq</code> is. If a <code>Seq</code> isn’t good enough, you might consider
skipping the literal optimization and just use the regex engine.</li>
</ul>
<p>(It should be noted that there are always pathological cases that can make
any kind of literal optimization be a net slower result. This is why it
might be a good idea to be conservative, or to even provide a means for
literal optimizations to be dynamically disabled if they are determined to be
ineffective according to some measure.)</p>
<p>You’re encouraged to explore the methods on <a href="struct.Seq.html" title="struct regex_syntax::hir::literal::Seq"><code>Seq</code></a>, which permit shrinking
the size of sequences in a preference-order preserving fashion.</p>
<p>Finally, note that it isn’t strictly necessary to use an <a href="struct.Extractor.html" title="struct regex_syntax::hir::literal::Extractor"><code>Extractor</code></a>. Namely,
an <code>Extractor</code> only uses public APIs of the <a href="struct.Seq.html" title="struct regex_syntax::hir::literal::Seq"><code>Seq</code></a> and <a href="struct.Literal.html" title="struct regex_syntax::hir::literal::Literal"><code>Literal</code></a> types,
so it is possible to implement your own extractor. For example, for n-grams
or “inner” literals (i.e., not prefix or suffix literals). The <code>Extractor</code>
is mostly responsible for the case analysis over <code>Hir</code> expressions. Much of
the “trickier” parts are how to combine literal sequences, and that is all
implemented on <a href="struct.Seq.html" title="struct regex_syntax::hir::literal::Seq"><code>Seq</code></a>.</p>
</div></details><h2 id="structs" class="section-header"><a href="#structs">Structs</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.Extractor.html" title="struct regex_syntax::hir::literal::Extractor">Extractor</a></div><div class="desc docblock-short">Extracts prefix or suffix literal sequences from <a href="../struct.Hir.html" title="struct regex_syntax::hir::Hir"><code>Hir</code></a> expressions.</div></li><li><div class="item-name"><a class="struct" href="struct.Literal.html" title="struct regex_syntax::hir::literal::Literal">Literal</a></div><div class="desc docblock-short">A single literal extracted from an <a href="../struct.Hir.html" title="struct regex_syntax::hir::Hir"><code>Hir</code></a> expression.</div></li><li><div class="item-name"><a class="struct" href="struct.Seq.html" title="struct regex_syntax::hir::literal::Seq">Seq</a></div><div class="desc docblock-short">A sequence of literals.</div></li></ul><h2 id="enums" class="section-header"><a href="#enums">Enums</a></h2><ul class="item-table"><li><div class="item-name"><a class="enum" href="enum.ExtractKind.html" title="enum regex_syntax::hir::literal::ExtractKind">ExtractKind</a></div><div class="desc docblock-short">The kind of literals to extract from an <a href="../struct.Hir.html" title="struct regex_syntax::hir::Hir"><code>Hir</code></a> expression.</div></li></ul><h2 id="functions" class="section-header"><a href="#functions">Functions</a></h2><ul class="item-table"><li><div class="item-name"><a class="fn" href="fn.rank.html" title="fn regex_syntax::hir::literal::rank">rank</a></div><div class="desc docblock-short">Returns the “rank” of the given byte.</div></li></ul></section></div></main></body></html>
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