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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="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/regex-syntax-0.7.2/src/ast/mod.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>mod.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../../normalize.css"><link rel="stylesheet" href="../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../ayu.css" disabled><link rel="stylesheet" href="../../../dark.css" disabled><link rel="stylesheet" href="../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../storage.js"></script><script defer src="../../../source-script.js"></script><script defer src="../../../source-files.js"></script><script defer src="../../../main.js"></script><noscript><link rel="stylesheet" href="../../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../../regex_syntax/index.html"><div class="logo-container"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../../regex_syntax/index.html"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span>
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 </pre><pre class="rust"><code><span class="doccomment">/*!
 Defines an abstract syntax for regular expressions.
 */

 </span><span class="kw">use </span>core::cmp::Ordering;

 <span class="kw">use </span>alloc::{boxed::Box, string::String, vec, vec::Vec};

 <span class="kw">pub use </span><span class="kw">crate</span>::ast::visitor::{visit, Visitor};

 <span class="kw">pub mod </span>parse;
 <span class="kw">pub mod </span>print;
 <span class="kw">mod </span>visitor;

 <span class="doccomment">/// An error that occurred while parsing a regular expression into an abstract
 /// syntax tree.
 ///
 /// Note that not all ASTs represents a valid regular expression. For example,
 /// an AST is constructed without error for `\p{Quux}`, but `Quux` is not a
 /// valid Unicode property name. That particular error is reported when
 /// translating an AST to the high-level intermediate representation (`HIR`).
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Error {
     <span class="doccomment">/// The kind of error.
     </span>kind: ErrorKind,
     <span class="doccomment">/// The original pattern that the parser generated the error from. Every
     /// span in an error is a valid range into this string.
     </span>pattern: String,
     <span class="doccomment">/// The span of this error.
     </span>span: Span,
 }

 <span class="kw">impl </span>Error {
     <span class="doccomment">/// Return the type of this error.
     </span><span class="kw">pub fn </span>kind(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>ErrorKind {
         <span class="kw-2">&amp;</span><span class="self">self</span>.kind
     }

     <span class="doccomment">/// The original pattern string in which this error occurred.
     ///
     /// Every span reported by this error is reported in terms of this string.
     </span><span class="kw">pub fn </span>pattern(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>str {
         <span class="kw-2">&amp;</span><span class="self">self</span>.pattern
     }

     <span class="doccomment">/// Return the span at which this error occurred.
     </span><span class="kw">pub fn </span>span(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>Span {
         <span class="kw-2">&amp;</span><span class="self">self</span>.span
     }

     <span class="doccomment">/// Return an auxiliary span. This span exists only for some errors that
     /// benefit from being able to point to two locations in the original
     /// regular expression. For example, &quot;duplicate&quot; errors will have the
     /// main error position set to the duplicate occurrence while its
     /// auxiliary span will be set to the initial occurrence.
     </span><span class="kw">pub fn </span>auxiliary_span(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="kw-2">&amp;</span>Span&gt; {
         <span class="kw">use </span><span class="self">self</span>::ErrorKind::<span class="kw-2">*</span>;
         <span class="kw">match </span><span class="self">self</span>.kind {
             FlagDuplicate { <span class="kw-2">ref </span>original } =&gt; <span class="prelude-val">Some</span>(original),
             FlagRepeatedNegation { <span class="kw-2">ref </span>original, .. } =&gt; <span class="prelude-val">Some</span>(original),
             GroupNameDuplicate { <span class="kw-2">ref </span>original, .. } =&gt; <span class="prelude-val">Some</span>(original),
             <span class="kw">_ </span>=&gt; <span class="prelude-val">None</span>,
         }
     }
 }

 <span class="doccomment">/// The type of an error that occurred while building an AST.
 ///
 /// This error type is marked as `non_exhaustive`. This means that adding a
 /// new variant is not considered a breaking change.
 </span><span class="attribute">#[non_exhaustive]
 #[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>ErrorKind {
     <span class="doccomment">/// The capturing group limit was exceeded.
     ///
     /// Note that this represents a limit on the total number of capturing
     /// groups in a regex and not necessarily the number of nested capturing
     /// groups. That is, the nest limit can be low and it is still possible for
     /// this error to occur.
     </span>CaptureLimitExceeded,
     <span class="doccomment">/// An invalid escape sequence was found in a character class set.
     </span>ClassEscapeInvalid,
     <span class="doccomment">/// An invalid character class range was found. An invalid range is any
     /// range where the start is greater than the end.
     </span>ClassRangeInvalid,
     <span class="doccomment">/// An invalid range boundary was found in a character class. Range
     /// boundaries must be a single literal codepoint, but this error indicates
     /// that something else was found, such as a nested class.
     </span>ClassRangeLiteral,
     <span class="doccomment">/// An opening `[` was found with no corresponding closing `]`.
     </span>ClassUnclosed,
     <span class="doccomment">/// Note that this error variant is no longer used. Namely, a decimal
     /// number can only appear as a repetition quantifier. When the number
     /// in a repetition quantifier is empty, then it gets its own specialized
     /// error, `RepetitionCountDecimalEmpty`.
     </span>DecimalEmpty,
     <span class="doccomment">/// An invalid decimal number was given where one was expected.
     </span>DecimalInvalid,
     <span class="doccomment">/// A bracketed hex literal was empty.
     </span>EscapeHexEmpty,
     <span class="doccomment">/// A bracketed hex literal did not correspond to a Unicode scalar value.
     </span>EscapeHexInvalid,
     <span class="doccomment">/// An invalid hexadecimal digit was found.
     </span>EscapeHexInvalidDigit,
     <span class="doccomment">/// EOF was found before an escape sequence was completed.
     </span>EscapeUnexpectedEof,
     <span class="doccomment">/// An unrecognized escape sequence.
     </span>EscapeUnrecognized,
     <span class="doccomment">/// A dangling negation was used when setting flags, e.g., `i-`.
     </span>FlagDanglingNegation,
     <span class="doccomment">/// A flag was used twice, e.g., `i-i`.
     </span>FlagDuplicate {
         <span class="doccomment">/// The position of the original flag. The error position
         /// points to the duplicate flag.
         </span>original: Span,
     },
     <span class="doccomment">/// The negation operator was used twice, e.g., `-i-s`.
     </span>FlagRepeatedNegation {
         <span class="doccomment">/// The position of the original negation operator. The error position
         /// points to the duplicate negation operator.
         </span>original: Span,
     },
     <span class="doccomment">/// Expected a flag but got EOF, e.g., `(?`.
     </span>FlagUnexpectedEof,
     <span class="doccomment">/// Unrecognized flag, e.g., `a`.
     </span>FlagUnrecognized,
     <span class="doccomment">/// A duplicate capture name was found.
     </span>GroupNameDuplicate {
         <span class="doccomment">/// The position of the initial occurrence of the capture name. The
         /// error position itself points to the duplicate occurrence.
         </span>original: Span,
     },
     <span class="doccomment">/// A capture group name is empty, e.g., `(?P&lt;&gt;abc)`.
     </span>GroupNameEmpty,
     <span class="doccomment">/// An invalid character was seen for a capture group name. This includes
     /// errors where the first character is a digit (even though subsequent
     /// characters are allowed to be digits).
     </span>GroupNameInvalid,
     <span class="doccomment">/// A closing `&gt;` could not be found for a capture group name.
     </span>GroupNameUnexpectedEof,
     <span class="doccomment">/// An unclosed group, e.g., `(ab`.
     ///
     /// The span of this error corresponds to the unclosed parenthesis.
     </span>GroupUnclosed,
     <span class="doccomment">/// An unopened group, e.g., `ab)`.
     </span>GroupUnopened,
     <span class="doccomment">/// The nest limit was exceeded. The limit stored here is the limit
     /// configured in the parser.
     </span>NestLimitExceeded(u32),
     <span class="doccomment">/// The range provided in a counted repetition operator is invalid. The
     /// range is invalid if the start is greater than the end.
     </span>RepetitionCountInvalid,
     <span class="doccomment">/// An opening `{` was not followed by a valid decimal value.
     /// For example, `x{}` or `x{]}` would fail.
     </span>RepetitionCountDecimalEmpty,
     <span class="doccomment">/// An opening `{` was found with no corresponding closing `}`.
     </span>RepetitionCountUnclosed,
     <span class="doccomment">/// A repetition operator was applied to a missing sub-expression. This
     /// occurs, for example, in the regex consisting of just a `*` or even
     /// `(?i)*`. It is, however, possible to create a repetition operating on
     /// an empty sub-expression. For example, `()*` is still considered valid.
     </span>RepetitionMissing,
     <span class="doccomment">/// The Unicode class is not valid. This typically occurs when a `\p` is
     /// followed by something other than a `{`.
     </span>UnicodeClassInvalid,
     <span class="doccomment">/// When octal support is disabled, this error is produced when an octal
     /// escape is used. The octal escape is assumed to be an invocation of
     /// a backreference, which is the common case.
     </span>UnsupportedBackreference,
     <span class="doccomment">/// When syntax similar to PCRE&#39;s look-around is used, this error is
     /// returned. Some example syntaxes that are rejected include, but are
     /// not necessarily limited to, `(?=re)`, `(?!re)`, `(?&lt;=re)` and
     /// `(?&lt;!re)`. Note that all of these syntaxes are otherwise invalid; this
     /// error is used to improve the user experience.
     </span>UnsupportedLookAround,
 }

 <span class="attribute">#[cfg(feature = <span class="string">&quot;std&quot;</span>)]
 </span><span class="kw">impl </span>std::error::Error <span class="kw">for </span>Error {}

 <span class="kw">impl </span>core::fmt::Display <span class="kw">for </span>Error {
     <span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>core::fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; core::fmt::Result {
         <span class="kw">crate</span>::error::Formatter::from(<span class="self">self</span>).fmt(f)
     }
 }

 <span class="kw">impl </span>core::fmt::Display <span class="kw">for </span>ErrorKind {
     <span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>core::fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; core::fmt::Result {
         <span class="kw">use </span><span class="self">self</span>::ErrorKind::<span class="kw-2">*</span>;
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             CaptureLimitExceeded =&gt; <span class="macro">write!</span>(
                 f,
                 <span class="string">&quot;exceeded the maximum number of \
                  capturing groups ({})&quot;</span>,
                 u32::MAX
             ),
             ClassEscapeInvalid =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;invalid escape sequence found in character class&quot;</span>)
             }
             ClassRangeInvalid =&gt; <span class="macro">write!</span>(
                 f,
                 <span class="string">&quot;invalid character class range, \
                  the start must be &lt;= the end&quot;
             </span>),
             ClassRangeLiteral =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;invalid range boundary, must be a literal&quot;</span>)
             }
             ClassUnclosed =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;unclosed character class&quot;</span>),
             DecimalEmpty =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;decimal literal empty&quot;</span>),
             DecimalInvalid =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;decimal literal invalid&quot;</span>),
             EscapeHexEmpty =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;hexadecimal literal empty&quot;</span>),
             EscapeHexInvalid =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;hexadecimal literal is not a Unicode scalar value&quot;</span>)
             }
             EscapeHexInvalidDigit =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;invalid hexadecimal digit&quot;</span>),
             EscapeUnexpectedEof =&gt; <span class="macro">write!</span>(
                 f,
                 <span class="string">&quot;incomplete escape sequence, \
                  reached end of pattern prematurely&quot;
             </span>),
             EscapeUnrecognized =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;unrecognized escape sequence&quot;</span>),
             FlagDanglingNegation =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;dangling flag negation operator&quot;</span>)
             }
             FlagDuplicate { .. } =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;duplicate flag&quot;</span>),
             FlagRepeatedNegation { .. } =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;flag negation operator repeated&quot;</span>)
             }
             FlagUnexpectedEof =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;expected flag but got end of regex&quot;</span>)
             }
             FlagUnrecognized =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;unrecognized flag&quot;</span>),
             GroupNameDuplicate { .. } =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;duplicate capture group name&quot;</span>)
             }
             GroupNameEmpty =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;empty capture group name&quot;</span>),
             GroupNameInvalid =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;invalid capture group character&quot;</span>),
             GroupNameUnexpectedEof =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;unclosed capture group name&quot;</span>),
             GroupUnclosed =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;unclosed group&quot;</span>),
             GroupUnopened =&gt; <span class="macro">write!</span>(f, <span class="string">&quot;unopened group&quot;</span>),
             NestLimitExceeded(limit) =&gt; <span class="macro">write!</span>(
                 f,
                 <span class="string">&quot;exceed the maximum number of \
                  nested parentheses/brackets ({})&quot;</span>,
                 limit
             ),
             RepetitionCountInvalid =&gt; <span class="macro">write!</span>(
                 f,
                 <span class="string">&quot;invalid repetition count range, \
                  the start must be &lt;= the end&quot;
             </span>),
             RepetitionCountDecimalEmpty =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;repetition quantifier expects a valid decimal&quot;</span>)
             }
             RepetitionCountUnclosed =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;unclosed counted repetition&quot;</span>)
             }
             RepetitionMissing =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;repetition operator missing expression&quot;</span>)
             }
             UnicodeClassInvalid =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;invalid Unicode character class&quot;</span>)
             }
             UnsupportedBackreference =&gt; {
                 <span class="macro">write!</span>(f, <span class="string">&quot;backreferences are not supported&quot;</span>)
             }
             UnsupportedLookAround =&gt; <span class="macro">write!</span>(
                 f,
                 <span class="string">&quot;look-around, including look-ahead and look-behind, \
                  is not supported&quot;
             </span>),
         }
     }
 }

 <span class="doccomment">/// Span represents the position information of a single AST item.
 ///
 /// All span positions are absolute byte offsets that can be used on the
 /// original regular expression that was parsed.
 </span><span class="attribute">#[derive(Clone, Copy, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Span {
     <span class="doccomment">/// The start byte offset.
     </span><span class="kw">pub </span>start: Position,
     <span class="doccomment">/// The end byte offset.
     </span><span class="kw">pub </span>end: Position,
 }

 <span class="kw">impl </span>core::fmt::Debug <span class="kw">for </span>Span {
     <span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>core::fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; core::fmt::Result {
         <span class="macro">write!</span>(f, <span class="string">&quot;Span({:?}, {:?})&quot;</span>, <span class="self">self</span>.start, <span class="self">self</span>.end)
     }
 }

 <span class="kw">impl </span>Ord <span class="kw">for </span>Span {
     <span class="kw">fn </span>cmp(<span class="kw-2">&amp;</span><span class="self">self</span>, other: <span class="kw-2">&amp;</span>Span) -&gt; Ordering {
         (<span class="kw-2">&amp;</span><span class="self">self</span>.start, <span class="kw-2">&amp;</span><span class="self">self</span>.end).cmp(<span class="kw-2">&amp;</span>(<span class="kw-2">&amp;</span>other.start, <span class="kw-2">&amp;</span>other.end))
     }
 }

 <span class="kw">impl </span>PartialOrd <span class="kw">for </span>Span {
     <span class="kw">fn </span>partial_cmp(<span class="kw-2">&amp;</span><span class="self">self</span>, other: <span class="kw-2">&amp;</span>Span) -&gt; <span class="prelude-ty">Option</span>&lt;Ordering&gt; {
         <span class="prelude-val">Some</span>(<span class="self">self</span>.cmp(other))
     }
 }

 <span class="doccomment">/// A single position in a regular expression.
 ///
 /// A position encodes one half of a span, and include the byte offset, line
 /// number and column number.
 </span><span class="attribute">#[derive(Clone, Copy, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Position {
     <span class="doccomment">/// The absolute offset of this position, starting at `0` from the
     /// beginning of the regular expression pattern string.
     </span><span class="kw">pub </span>offset: usize,
     <span class="doccomment">/// The line number, starting at `1`.
     </span><span class="kw">pub </span>line: usize,
     <span class="doccomment">/// The approximate column number, starting at `1`.
     </span><span class="kw">pub </span>column: usize,
 }

 <span class="kw">impl </span>core::fmt::Debug <span class="kw">for </span>Position {
     <span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>core::fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; core::fmt::Result {
         <span class="macro">write!</span>(
             f,
             <span class="string">&quot;Position(o: {:?}, l: {:?}, c: {:?})&quot;</span>,
             <span class="self">self</span>.offset, <span class="self">self</span>.line, <span class="self">self</span>.column
         )
     }
 }

 <span class="kw">impl </span>Ord <span class="kw">for </span>Position {
     <span class="kw">fn </span>cmp(<span class="kw-2">&amp;</span><span class="self">self</span>, other: <span class="kw-2">&amp;</span>Position) -&gt; Ordering {
         <span class="self">self</span>.offset.cmp(<span class="kw-2">&amp;</span>other.offset)
     }
 }

 <span class="kw">impl </span>PartialOrd <span class="kw">for </span>Position {
     <span class="kw">fn </span>partial_cmp(<span class="kw-2">&amp;</span><span class="self">self</span>, other: <span class="kw-2">&amp;</span>Position) -&gt; <span class="prelude-ty">Option</span>&lt;Ordering&gt; {
         <span class="prelude-val">Some</span>(<span class="self">self</span>.cmp(other))
     }
 }

 <span class="kw">impl </span>Span {
     <span class="doccomment">/// Create a new span with the given positions.
     </span><span class="kw">pub fn </span>new(start: Position, end: Position) -&gt; Span {
         Span { start, end }
     }

     <span class="doccomment">/// Create a new span using the given position as the start and end.
     </span><span class="kw">pub fn </span>splat(pos: Position) -&gt; Span {
         Span::new(pos, pos)
     }

     <span class="doccomment">/// Create a new span by replacing the starting the position with the one
     /// given.
     </span><span class="kw">pub fn </span>with_start(<span class="self">self</span>, pos: Position) -&gt; Span {
         Span { start: pos, ..<span class="self">self </span>}
     }

     <span class="doccomment">/// Create a new span by replacing the ending the position with the one
     /// given.
     </span><span class="kw">pub fn </span>with_end(<span class="self">self</span>, pos: Position) -&gt; Span {
         Span { end: pos, ..<span class="self">self </span>}
     }

     <span class="doccomment">/// Returns true if and only if this span occurs on a single line.
     </span><span class="kw">pub fn </span>is_one_line(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="self">self</span>.start.line == <span class="self">self</span>.end.line
     }

     <span class="doccomment">/// Returns true if and only if this span is empty. That is, it points to
     /// a single position in the concrete syntax of a regular expression.
     </span><span class="kw">pub fn </span>is_empty(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="self">self</span>.start.offset == <span class="self">self</span>.end.offset
     }
 }

 <span class="kw">impl </span>Position {
     <span class="doccomment">/// Create a new position with the given information.
     ///
     /// `offset` is the absolute offset of the position, starting at `0` from
     /// the beginning of the regular expression pattern string.
     ///
     /// `line` is the line number, starting at `1`.
     ///
     /// `column` is the approximate column number, starting at `1`.
     </span><span class="kw">pub fn </span>new(offset: usize, line: usize, column: usize) -&gt; Position {
         Position { offset, line, column }
     }
 }

 <span class="doccomment">/// An abstract syntax tree for a singular expression along with comments
 /// found.
 ///
 /// Comments are not stored in the tree itself to avoid complexity. Each
 /// comment contains a span of precisely where it occurred in the original
 /// regular expression.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>WithComments {
     <span class="doccomment">/// The actual ast.
     </span><span class="kw">pub </span>ast: Ast,
     <span class="doccomment">/// All comments found in the original regular expression.
     </span><span class="kw">pub </span>comments: Vec&lt;Comment&gt;,
 }

 <span class="doccomment">/// A comment from a regular expression with an associated span.
 ///
 /// A regular expression can only contain comments when the `x` flag is
 /// enabled.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Comment {
     <span class="doccomment">/// The span of this comment, including the beginning `#` and ending `\n`.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The comment text, starting with the first character following the `#`
     /// and ending with the last character preceding the `\n`.
     </span><span class="kw">pub </span>comment: String,
 }

 <span class="doccomment">/// An abstract syntax tree for a single regular expression.
 ///
 /// An `Ast`&#39;s `fmt::Display` implementation uses constant stack space and heap
 /// space proportional to the size of the `Ast`.
 ///
 /// This type defines its own destructor that uses constant stack space and
 /// heap space proportional to the size of the `Ast`.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>Ast {
     <span class="doccomment">/// An empty regex that matches everything.
     </span>Empty(Span),
     <span class="doccomment">/// A set of flags, e.g., `(?is)`.
     </span>Flags(SetFlags),
     <span class="doccomment">/// A single character literal, which includes escape sequences.
     </span>Literal(Literal),
     <span class="doccomment">/// The &quot;any character&quot; class.
     </span>Dot(Span),
     <span class="doccomment">/// A single zero-width assertion.
     </span>Assertion(Assertion),
     <span class="doccomment">/// A single character class. This includes all forms of character classes
     /// except for `.`. e.g., `\d`, `\pN`, `[a-z]` and `[[:alpha:]]`.
     </span>Class(Class),
     <span class="doccomment">/// A repetition operator applied to an arbitrary regular expression.
     </span>Repetition(Repetition),
     <span class="doccomment">/// A grouped regular expression.
     </span>Group(Group),
     <span class="doccomment">/// An alternation of regular expressions.
     </span>Alternation(Alternation),
     <span class="doccomment">/// A concatenation of regular expressions.
     </span>Concat(Concat),
 }

 <span class="kw">impl </span>Ast {
     <span class="doccomment">/// Return the span of this abstract syntax tree.
     </span><span class="kw">pub fn </span>span(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>Span {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             Ast::Empty(<span class="kw-2">ref </span>span) =&gt; span,
             Ast::Flags(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             Ast::Literal(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             Ast::Dot(<span class="kw-2">ref </span>span) =&gt; span,
             Ast::Assertion(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             Ast::Class(<span class="kw-2">ref </span>x) =&gt; x.span(),
             Ast::Repetition(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             Ast::Group(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             Ast::Alternation(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             Ast::Concat(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
         }
     }

     <span class="doccomment">/// Return true if and only if this Ast is empty.
     </span><span class="kw">pub fn </span>is_empty(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             Ast::Empty(<span class="kw">_</span>) =&gt; <span class="bool-val">true</span>,
             <span class="kw">_ </span>=&gt; <span class="bool-val">false</span>,
         }
     }

     <span class="doccomment">/// Returns true if and only if this AST has any (including possibly empty)
     /// subexpressions.
     </span><span class="kw">fn </span>has_subexprs(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             Ast::Empty(<span class="kw">_</span>)
             | Ast::Flags(<span class="kw">_</span>)
             | Ast::Literal(<span class="kw">_</span>)
             | Ast::Dot(<span class="kw">_</span>)
             | Ast::Assertion(<span class="kw">_</span>) =&gt; <span class="bool-val">false</span>,
             Ast::Class(<span class="kw">_</span>)
             | Ast::Repetition(<span class="kw">_</span>)
             | Ast::Group(<span class="kw">_</span>)
             | Ast::Alternation(<span class="kw">_</span>)
             | Ast::Concat(<span class="kw">_</span>) =&gt; <span class="bool-val">true</span>,
         }
     }
 }

 <span class="doccomment">/// Print a display representation of this Ast.
 ///
 /// This does not preserve any of the original whitespace formatting that may
 /// have originally been present in the concrete syntax from which this Ast
 /// was generated.
 ///
 /// This implementation uses constant stack space and heap space proportional
 /// to the size of the `Ast`.
 </span><span class="kw">impl </span>core::fmt::Display <span class="kw">for </span>Ast {
     <span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, f: <span class="kw-2">&amp;mut </span>core::fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; core::fmt::Result {
         <span class="kw">use </span><span class="kw">crate</span>::ast::print::Printer;
         Printer::new().print(<span class="self">self</span>, f)
     }
 }

 <span class="doccomment">/// An alternation of regular expressions.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Alternation {
     <span class="doccomment">/// The span of this alternation.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The alternate regular expressions.
     </span><span class="kw">pub </span>asts: Vec&lt;Ast&gt;,
 }

 <span class="kw">impl </span>Alternation {
     <span class="doccomment">/// Return this alternation as an AST.
     ///
     /// If this alternation contains zero ASTs, then Ast::Empty is
     /// returned. If this alternation contains exactly 1 AST, then the
     /// corresponding AST is returned. Otherwise, Ast::Alternation is returned.
     </span><span class="kw">pub fn </span>into_ast(<span class="kw-2">mut </span><span class="self">self</span>) -&gt; Ast {
         <span class="kw">match </span><span class="self">self</span>.asts.len() {
             <span class="number">0 </span>=&gt; Ast::Empty(<span class="self">self</span>.span),
             <span class="number">1 </span>=&gt; <span class="self">self</span>.asts.pop().unwrap(),
             <span class="kw">_ </span>=&gt; Ast::Alternation(<span class="self">self</span>),
         }
     }
 }

 <span class="doccomment">/// A concatenation of regular expressions.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Concat {
     <span class="doccomment">/// The span of this concatenation.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The concatenation regular expressions.
     </span><span class="kw">pub </span>asts: Vec&lt;Ast&gt;,
 }

 <span class="kw">impl </span>Concat {
     <span class="doccomment">/// Return this concatenation as an AST.
     ///
     /// If this concatenation contains zero ASTs, then Ast::Empty is
     /// returned. If this concatenation contains exactly 1 AST, then the
     /// corresponding AST is returned. Otherwise, Ast::Concat is returned.
     </span><span class="kw">pub fn </span>into_ast(<span class="kw-2">mut </span><span class="self">self</span>) -&gt; Ast {
         <span class="kw">match </span><span class="self">self</span>.asts.len() {
             <span class="number">0 </span>=&gt; Ast::Empty(<span class="self">self</span>.span),
             <span class="number">1 </span>=&gt; <span class="self">self</span>.asts.pop().unwrap(),
             <span class="kw">_ </span>=&gt; Ast::Concat(<span class="self">self</span>),
         }
     }
 }

 <span class="doccomment">/// A single literal expression.
 ///
 /// A literal corresponds to a single Unicode scalar value. Literals may be
 /// represented in their literal form, e.g., `a` or in their escaped form,
 /// e.g., `\x61`.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Literal {
     <span class="doccomment">/// The span of this literal.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The kind of this literal.
     </span><span class="kw">pub </span>kind: LiteralKind,
     <span class="doccomment">/// The Unicode scalar value corresponding to this literal.
     </span><span class="kw">pub </span>c: char,
 }

 <span class="kw">impl </span>Literal {
     <span class="doccomment">/// If this literal was written as a `\x` hex escape, then this returns
     /// the corresponding byte value. Otherwise, this returns `None`.
     </span><span class="kw">pub fn </span>byte(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Option</span>&lt;u8&gt; {
         <span class="kw">match </span><span class="self">self</span>.kind {
             LiteralKind::HexFixed(HexLiteralKind::X) =&gt; {
                 u8::try_from(<span class="self">self</span>.c).ok()
             }
             <span class="kw">_ </span>=&gt; <span class="prelude-val">None</span>,
         }
     }
 }

 <span class="doccomment">/// The kind of a single literal expression.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>LiteralKind {
     <span class="doccomment">/// The literal is written verbatim, e.g., `a` or `☃`.
     </span>Verbatim,
     <span class="doccomment">/// The literal is written as an escape because it is otherwise a special
     /// regex meta character, e.g., `\*` or `\[`.
     </span>Meta,
     <span class="doccomment">/// The literal is written as an escape despite the fact that the escape is
     /// unnecessary, e.g., `\%` or `\/`.
     </span>Superfluous,
     <span class="doccomment">/// The literal is written as an octal escape, e.g., `\141`.
     </span>Octal,
     <span class="doccomment">/// The literal is written as a hex code with a fixed number of digits
     /// depending on the type of the escape, e.g., `\x61` or or `\u0061` or
     /// `\U00000061`.
     </span>HexFixed(HexLiteralKind),
     <span class="doccomment">/// The literal is written as a hex code with a bracketed number of
     /// digits. The only restriction is that the bracketed hex code must refer
     /// to a valid Unicode scalar value.
     </span>HexBrace(HexLiteralKind),
     <span class="doccomment">/// The literal is written as a specially recognized escape, e.g., `\f`
     /// or `\n`.
     </span>Special(SpecialLiteralKind),
 }

 <span class="doccomment">/// The type of a special literal.
 ///
 /// A special literal is a special escape sequence recognized by the regex
 /// parser, e.g., `\f` or `\n`.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>SpecialLiteralKind {
     <span class="doccomment">/// Bell, spelled `\a` (`\x07`).
     </span>Bell,
     <span class="doccomment">/// Form feed, spelled `\f` (`\x0C`).
     </span>FormFeed,
     <span class="doccomment">/// Tab, spelled `\t` (`\x09`).
     </span>Tab,
     <span class="doccomment">/// Line feed, spelled `\n` (`\x0A`).
     </span>LineFeed,
     <span class="doccomment">/// Carriage return, spelled `\r` (`\x0D`).
     </span>CarriageReturn,
     <span class="doccomment">/// Vertical tab, spelled `\v` (`\x0B`).
     </span>VerticalTab,
     <span class="doccomment">/// Space, spelled `\ ` (`\x20`). Note that this can only appear when
     /// parsing in verbose mode.
     </span>Space,
 }

 <span class="doccomment">/// The type of a Unicode hex literal.
 ///
 /// Note that all variants behave the same when used with brackets. They only
 /// differ when used without brackets in the number of hex digits that must
 /// follow.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>HexLiteralKind {
     <span class="doccomment">/// A `\x` prefix. When used without brackets, this form is limited to
     /// two digits.
     </span>X,
     <span class="doccomment">/// A `\u` prefix. When used without brackets, this form is limited to
     /// four digits.
     </span>UnicodeShort,
     <span class="doccomment">/// A `\U` prefix. When used without brackets, this form is limited to
     /// eight digits.
     </span>UnicodeLong,
 }

 <span class="kw">impl </span>HexLiteralKind {
     <span class="doccomment">/// The number of digits that must be used with this literal form when
     /// used without brackets. When used with brackets, there is no
     /// restriction on the number of digits.
     </span><span class="kw">pub fn </span>digits(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; u32 {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             HexLiteralKind::X =&gt; <span class="number">2</span>,
             HexLiteralKind::UnicodeShort =&gt; <span class="number">4</span>,
             HexLiteralKind::UnicodeLong =&gt; <span class="number">8</span>,
         }
     }
 }

 <span class="doccomment">/// A single character class expression.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>Class {
     <span class="doccomment">/// A Unicode character class, e.g., `\pL` or `\p{Greek}`.
     </span>Unicode(ClassUnicode),
     <span class="doccomment">/// A perl character class, e.g., `\d` or `\W`.
     </span>Perl(ClassPerl),
     <span class="doccomment">/// A bracketed character class set, which may contain zero or more
     /// character ranges and/or zero or more nested classes. e.g.,
     /// `[a-zA-Z\pL]`.
     </span>Bracketed(ClassBracketed),
 }

 <span class="kw">impl </span>Class {
     <span class="doccomment">/// Return the span of this character class.
     </span><span class="kw">pub fn </span>span(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>Span {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             Class::Perl(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             Class::Unicode(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             Class::Bracketed(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
         }
     }
 }

 <span class="doccomment">/// A Perl character class.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>ClassPerl {
     <span class="doccomment">/// The span of this class.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The kind of Perl class.
     </span><span class="kw">pub </span>kind: ClassPerlKind,
     <span class="doccomment">/// Whether the class is negated or not. e.g., `\d` is not negated but
     /// `\D` is.
     </span><span class="kw">pub </span>negated: bool,
 }

 <span class="doccomment">/// The available Perl character classes.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>ClassPerlKind {
     <span class="doccomment">/// Decimal numbers.
     </span>Digit,
     <span class="doccomment">/// Whitespace.
     </span>Space,
     <span class="doccomment">/// Word characters.
     </span>Word,
 }

 <span class="doccomment">/// An ASCII character class.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>ClassAscii {
     <span class="doccomment">/// The span of this class.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The kind of ASCII class.
     </span><span class="kw">pub </span>kind: ClassAsciiKind,
     <span class="doccomment">/// Whether the class is negated or not. e.g., `[[:alpha:]]` is not negated
     /// but `[[:^alpha:]]` is.
     </span><span class="kw">pub </span>negated: bool,
 }

 <span class="doccomment">/// The available ASCII character classes.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>ClassAsciiKind {
     <span class="doccomment">/// `[0-9A-Za-z]`
     </span>Alnum,
     <span class="doccomment">/// `[A-Za-z]`
     </span>Alpha,
     <span class="doccomment">/// `[\x00-\x7F]`
     </span>Ascii,
     <span class="doccomment">/// `[ \t]`
     </span>Blank,
     <span class="doccomment">/// `[\x00-\x1F\x7F]`
     </span>Cntrl,
     <span class="doccomment">/// `[0-9]`
     </span>Digit,
     <span class="doccomment">/// `[!-~]`
     </span>Graph,
     <span class="doccomment">/// `[a-z]`
     </span>Lower,
     <span class="doccomment">/// `[ -~]`
     </span>Print,
     <span class="doccomment">/// `[!-/:-@\[-`{-~]`
     </span>Punct,
     <span class="doccomment">/// `[\t\n\v\f\r ]`
     </span>Space,
     <span class="doccomment">/// `[A-Z]`
     </span>Upper,
     <span class="doccomment">/// `[0-9A-Za-z_]`
     </span>Word,
     <span class="doccomment">/// `[0-9A-Fa-f]`
     </span>Xdigit,
 }

 <span class="kw">impl </span>ClassAsciiKind {
     <span class="doccomment">/// Return the corresponding ClassAsciiKind variant for the given name.
     ///
     /// The name given should correspond to the lowercase version of the
     /// variant name. e.g., `cntrl` is the name for `ClassAsciiKind::Cntrl`.
     ///
     /// If no variant with the corresponding name exists, then `None` is
     /// returned.
     </span><span class="kw">pub fn </span>from_name(name: <span class="kw-2">&amp;</span>str) -&gt; <span class="prelude-ty">Option</span>&lt;ClassAsciiKind&gt; {
         <span class="kw">use </span><span class="self">self</span>::ClassAsciiKind::<span class="kw-2">*</span>;
         <span class="kw">match </span>name {
             <span class="string">&quot;alnum&quot; </span>=&gt; <span class="prelude-val">Some</span>(Alnum),
             <span class="string">&quot;alpha&quot; </span>=&gt; <span class="prelude-val">Some</span>(Alpha),
             <span class="string">&quot;ascii&quot; </span>=&gt; <span class="prelude-val">Some</span>(Ascii),
             <span class="string">&quot;blank&quot; </span>=&gt; <span class="prelude-val">Some</span>(Blank),
             <span class="string">&quot;cntrl&quot; </span>=&gt; <span class="prelude-val">Some</span>(Cntrl),
             <span class="string">&quot;digit&quot; </span>=&gt; <span class="prelude-val">Some</span>(Digit),
             <span class="string">&quot;graph&quot; </span>=&gt; <span class="prelude-val">Some</span>(Graph),
             <span class="string">&quot;lower&quot; </span>=&gt; <span class="prelude-val">Some</span>(Lower),
             <span class="string">&quot;print&quot; </span>=&gt; <span class="prelude-val">Some</span>(Print),
             <span class="string">&quot;punct&quot; </span>=&gt; <span class="prelude-val">Some</span>(Punct),
             <span class="string">&quot;space&quot; </span>=&gt; <span class="prelude-val">Some</span>(Space),
             <span class="string">&quot;upper&quot; </span>=&gt; <span class="prelude-val">Some</span>(Upper),
             <span class="string">&quot;word&quot; </span>=&gt; <span class="prelude-val">Some</span>(Word),
             <span class="string">&quot;xdigit&quot; </span>=&gt; <span class="prelude-val">Some</span>(Xdigit),
             <span class="kw">_ </span>=&gt; <span class="prelude-val">None</span>,
         }
     }
 }

 <span class="doccomment">/// A Unicode character class.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>ClassUnicode {
     <span class="doccomment">/// The span of this class.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// Whether this class is negated or not.
     ///
     /// Note: be careful when using this attribute. This specifically refers
     /// to whether the class is written as `\p` or `\P`, where the latter
     /// is `negated = true`. However, it also possible to write something like
     /// `\P{scx!=Katakana}` which is actually equivalent to
     /// `\p{scx=Katakana}` and is therefore not actually negated even though
     /// `negated = true` here. To test whether this class is truly negated
     /// or not, use the `is_negated` method.
     </span><span class="kw">pub </span>negated: bool,
     <span class="doccomment">/// The kind of Unicode class.
     </span><span class="kw">pub </span>kind: ClassUnicodeKind,
 }

 <span class="kw">impl </span>ClassUnicode {
     <span class="doccomment">/// Returns true if this class has been negated.
     ///
     /// Note that this takes the Unicode op into account, if it&#39;s present.
     /// e.g., `is_negated` for `\P{scx!=Katakana}` will return `false`.
     </span><span class="kw">pub fn </span>is_negated(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="self">self</span>.kind {
             ClassUnicodeKind::NamedValue {
                 op: ClassUnicodeOpKind::NotEqual,
                 ..
             } =&gt; !<span class="self">self</span>.negated,
             <span class="kw">_ </span>=&gt; <span class="self">self</span>.negated,
         }
     }
 }

 <span class="doccomment">/// The available forms of Unicode character classes.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>ClassUnicodeKind {
     <span class="doccomment">/// A one letter abbreviated class, e.g., `\pN`.
     </span>OneLetter(char),
     <span class="doccomment">/// A binary property, general category or script. The string may be
     /// empty.
     </span>Named(String),
     <span class="doccomment">/// A property name and an associated value.
     </span>NamedValue {
         <span class="doccomment">/// The type of Unicode op used to associate `name` with `value`.
         </span>op: ClassUnicodeOpKind,
         <span class="doccomment">/// The property name (which may be empty).
         </span>name: String,
         <span class="doccomment">/// The property value (which may be empty).
         </span>value: String,
     },
 }

 <span class="doccomment">/// The type of op used in a Unicode character class.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>ClassUnicodeOpKind {
     <span class="doccomment">/// A property set to a specific value, e.g., `\p{scx=Katakana}`.
     </span>Equal,
     <span class="doccomment">/// A property set to a specific value using a colon, e.g.,
     /// `\p{scx:Katakana}`.
     </span>Colon,
     <span class="doccomment">/// A property that isn&#39;t a particular value, e.g., `\p{scx!=Katakana}`.
     </span>NotEqual,
 }

 <span class="kw">impl </span>ClassUnicodeOpKind {
     <span class="doccomment">/// Whether the op is an equality op or not.
     </span><span class="kw">pub fn </span>is_equal(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             ClassUnicodeOpKind::Equal | ClassUnicodeOpKind::Colon =&gt; <span class="bool-val">true</span>,
             <span class="kw">_ </span>=&gt; <span class="bool-val">false</span>,
         }
     }
 }

 <span class="doccomment">/// A bracketed character class, e.g., `[a-z0-9]`.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>ClassBracketed {
     <span class="doccomment">/// The span of this class.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// Whether this class is negated or not. e.g., `[a]` is not negated but
     /// `[^a]` is.
     </span><span class="kw">pub </span>negated: bool,
     <span class="doccomment">/// The type of this set. A set is either a normal union of things, e.g.,
     /// `[abc]` or a result of applying set operations, e.g., `[\pL--c]`.
     </span><span class="kw">pub </span>kind: ClassSet,
 }

 <span class="doccomment">/// A character class set.
 ///
 /// This type corresponds to the internal structure of a bracketed character
 /// class. That is, every bracketed character is one of two types: a union of
 /// items (literals, ranges, other bracketed classes) or a tree of binary set
 /// operations.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>ClassSet {
     <span class="doccomment">/// An item, which can be a single literal, range, nested character class
     /// or a union of items.
     </span>Item(ClassSetItem),
     <span class="doccomment">/// A single binary operation (i.e., &amp;&amp;, -- or ~~).
     </span>BinaryOp(ClassSetBinaryOp),
 }

 <span class="kw">impl </span>ClassSet {
     <span class="doccomment">/// Build a set from a union.
     </span><span class="kw">pub fn </span>union(ast: ClassSetUnion) -&gt; ClassSet {
         ClassSet::Item(ClassSetItem::Union(ast))
     }

     <span class="doccomment">/// Return the span of this character class set.
     </span><span class="kw">pub fn </span>span(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>Span {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             ClassSet::Item(<span class="kw-2">ref </span>x) =&gt; x.span(),
             ClassSet::BinaryOp(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
         }
     }

     <span class="doccomment">/// Return true if and only if this class set is empty.
     </span><span class="kw">fn </span>is_empty(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             ClassSet::Item(ClassSetItem::Empty(<span class="kw">_</span>)) =&gt; <span class="bool-val">true</span>,
             <span class="kw">_ </span>=&gt; <span class="bool-val">false</span>,
         }
     }
 }

 <span class="doccomment">/// A single component of a character class set.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>ClassSetItem {
     <span class="doccomment">/// An empty item.
     ///
     /// Note that a bracketed character class cannot contain a single empty
     /// item. Empty items can appear when using one of the binary operators.
     /// For example, `[&amp;&amp;]` is the intersection of two empty classes.
     </span>Empty(Span),
     <span class="doccomment">/// A single literal.
     </span>Literal(Literal),
     <span class="doccomment">/// A range between two literals.
     </span>Range(ClassSetRange),
     <span class="doccomment">/// An ASCII character class, e.g., `[:alnum:]` or `[:punct:]`.
     </span>Ascii(ClassAscii),
     <span class="doccomment">/// A Unicode character class, e.g., `\pL` or `\p{Greek}`.
     </span>Unicode(ClassUnicode),
     <span class="doccomment">/// A perl character class, e.g., `\d` or `\W`.
     </span>Perl(ClassPerl),
     <span class="doccomment">/// A bracketed character class set, which may contain zero or more
     /// character ranges and/or zero or more nested classes. e.g.,
     /// `[a-zA-Z\pL]`.
     </span>Bracketed(Box&lt;ClassBracketed&gt;),
     <span class="doccomment">/// A union of items.
     </span>Union(ClassSetUnion),
 }

 <span class="kw">impl </span>ClassSetItem {
     <span class="doccomment">/// Return the span of this character class set item.
     </span><span class="kw">pub fn </span>span(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="kw-2">&amp;</span>Span {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             ClassSetItem::Empty(<span class="kw-2">ref </span>span) =&gt; span,
             ClassSetItem::Literal(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             ClassSetItem::Range(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             ClassSetItem::Ascii(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             ClassSetItem::Perl(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             ClassSetItem::Unicode(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             ClassSetItem::Bracketed(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
             ClassSetItem::Union(<span class="kw-2">ref </span>x) =&gt; <span class="kw-2">&amp;</span>x.span,
         }
     }
 }

 <span class="doccomment">/// A single character class range in a set.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>ClassSetRange {
     <span class="doccomment">/// The span of this range.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The start of this range.
     </span><span class="kw">pub </span>start: Literal,
     <span class="doccomment">/// The end of this range.
     </span><span class="kw">pub </span>end: Literal,
 }

 <span class="kw">impl </span>ClassSetRange {
     <span class="doccomment">/// Returns true if and only if this character class range is valid.
     ///
     /// The only case where a range is invalid is if its start is greater than
     /// its end.
     </span><span class="kw">pub fn </span>is_valid(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="self">self</span>.start.c &lt;= <span class="self">self</span>.end.c
     }
 }

 <span class="doccomment">/// A union of items inside a character class set.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>ClassSetUnion {
     <span class="doccomment">/// The span of the items in this operation. e.g., the `a-z0-9` in
     /// `[^a-z0-9]`
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The sequence of items that make up this union.
     </span><span class="kw">pub </span>items: Vec&lt;ClassSetItem&gt;,
 }

 <span class="kw">impl </span>ClassSetUnion {
     <span class="doccomment">/// Push a new item in this union.
     ///
     /// The ending position of this union&#39;s span is updated to the ending
     /// position of the span of the item given. If the union is empty, then
     /// the starting position of this union is set to the starting position
     /// of this item.
     ///
     /// In other words, if you only use this method to add items to a union
     /// and you set the spans on each item correctly, then you should never
     /// need to adjust the span of the union directly.
     </span><span class="kw">pub fn </span>push(<span class="kw-2">&amp;mut </span><span class="self">self</span>, item: ClassSetItem) {
         <span class="kw">if </span><span class="self">self</span>.items.is_empty() {
             <span class="self">self</span>.span.start = item.span().start;
         }
         <span class="self">self</span>.span.end = item.span().end;
         <span class="self">self</span>.items.push(item);
     }

     <span class="doccomment">/// Return this union as a character class set item.
     ///
     /// If this union contains zero items, then an empty union is
     /// returned. If this concatenation contains exactly 1 item, then the
     /// corresponding item is returned. Otherwise, ClassSetItem::Union is
     /// returned.
     </span><span class="kw">pub fn </span>into_item(<span class="kw-2">mut </span><span class="self">self</span>) -&gt; ClassSetItem {
         <span class="kw">match </span><span class="self">self</span>.items.len() {
             <span class="number">0 </span>=&gt; ClassSetItem::Empty(<span class="self">self</span>.span),
             <span class="number">1 </span>=&gt; <span class="self">self</span>.items.pop().unwrap(),
             <span class="kw">_ </span>=&gt; ClassSetItem::Union(<span class="self">self</span>),
         }
     }
 }

 <span class="doccomment">/// A Unicode character class set operation.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>ClassSetBinaryOp {
     <span class="doccomment">/// The span of this operation. e.g., the `a-z--[h-p]` in `[a-z--h-p]`.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The type of this set operation.
     </span><span class="kw">pub </span>kind: ClassSetBinaryOpKind,
     <span class="doccomment">/// The left hand side of the operation.
     </span><span class="kw">pub </span>lhs: Box&lt;ClassSet&gt;,
     <span class="doccomment">/// The right hand side of the operation.
     </span><span class="kw">pub </span>rhs: Box&lt;ClassSet&gt;,
 }

 <span class="doccomment">/// The type of a Unicode character class set operation.
 ///
 /// Note that this doesn&#39;t explicitly represent union since there is no
 /// explicit union operator. Concatenation inside a character class corresponds
 /// to the union operation.
 </span><span class="attribute">#[derive(Clone, Copy, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>ClassSetBinaryOpKind {
     <span class="doccomment">/// The intersection of two sets, e.g., `\pN&amp;&amp;[a-z]`.
     </span>Intersection,
     <span class="doccomment">/// The difference of two sets, e.g., `\pN--[0-9]`.
     </span>Difference,
     <span class="doccomment">/// The symmetric difference of two sets. The symmetric difference is the
     /// set of elements belonging to one but not both sets.
     /// e.g., `[\pL~~[:ascii:]]`.
     </span>SymmetricDifference,
 }

 <span class="doccomment">/// A single zero-width assertion.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Assertion {
     <span class="doccomment">/// The span of this assertion.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The assertion kind, e.g., `\b` or `^`.
     </span><span class="kw">pub </span>kind: AssertionKind,
 }

 <span class="doccomment">/// An assertion kind.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>AssertionKind {
     <span class="doccomment">/// `^`
     </span>StartLine,
     <span class="doccomment">/// `$`
     </span>EndLine,
     <span class="doccomment">/// `\A`
     </span>StartText,
     <span class="doccomment">/// `\z`
     </span>EndText,
     <span class="doccomment">/// `\b`
     </span>WordBoundary,
     <span class="doccomment">/// `\B`
     </span>NotWordBoundary,
 }

 <span class="doccomment">/// A repetition operation applied to a regular expression.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Repetition {
     <span class="doccomment">/// The span of this operation.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The actual operation.
     </span><span class="kw">pub </span>op: RepetitionOp,
     <span class="doccomment">/// Whether this operation was applied greedily or not.
     </span><span class="kw">pub </span>greedy: bool,
     <span class="doccomment">/// The regular expression under repetition.
     </span><span class="kw">pub </span>ast: Box&lt;Ast&gt;,
 }

 <span class="doccomment">/// The repetition operator itself.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>RepetitionOp {
     <span class="doccomment">/// The span of this operator. This includes things like `+`, `*?` and
     /// `{m,n}`.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The type of operation.
     </span><span class="kw">pub </span>kind: RepetitionKind,
 }

 <span class="doccomment">/// The kind of a repetition operator.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>RepetitionKind {
     <span class="doccomment">/// `?`
     </span>ZeroOrOne,
     <span class="doccomment">/// `*`
     </span>ZeroOrMore,
     <span class="doccomment">/// `+`
     </span>OneOrMore,
     <span class="doccomment">/// `{m,n}`
     </span>Range(RepetitionRange),
 }

 <span class="doccomment">/// A range repetition operator.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>RepetitionRange {
     <span class="doccomment">/// `{m}`
     </span>Exactly(u32),
     <span class="doccomment">/// `{m,}`
     </span>AtLeast(u32),
     <span class="doccomment">/// `{m,n}`
     </span>Bounded(u32, u32),
 }

 <span class="kw">impl </span>RepetitionRange {
     <span class="doccomment">/// Returns true if and only if this repetition range is valid.
     ///
     /// The only case where a repetition range is invalid is if it is bounded
     /// and its start is greater than its end.
     </span><span class="kw">pub fn </span>is_valid(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             RepetitionRange::Bounded(s, e) <span class="kw">if </span>s &gt; e =&gt; <span class="bool-val">false</span>,
             <span class="kw">_ </span>=&gt; <span class="bool-val">true</span>,
         }
     }
 }

 <span class="doccomment">/// A grouped regular expression.
 ///
 /// This includes both capturing and non-capturing groups. This does **not**
 /// include flag-only groups like `(?is)`, but does contain any group that
 /// contains a sub-expression, e.g., `(a)`, `(?P&lt;name&gt;a)`, `(?:a)` and
 /// `(?is:a)`.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Group {
     <span class="doccomment">/// The span of this group.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The kind of this group.
     </span><span class="kw">pub </span>kind: GroupKind,
     <span class="doccomment">/// The regular expression in this group.
     </span><span class="kw">pub </span>ast: Box&lt;Ast&gt;,
 }

 <span class="kw">impl </span>Group {
     <span class="doccomment">/// If this group is non-capturing, then this returns the (possibly empty)
     /// set of flags. Otherwise, `None` is returned.
     </span><span class="kw">pub fn </span>flags(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Option</span>&lt;<span class="kw-2">&amp;</span>Flags&gt; {
         <span class="kw">match </span><span class="self">self</span>.kind {
             GroupKind::NonCapturing(<span class="kw-2">ref </span>flags) =&gt; <span class="prelude-val">Some</span>(flags),
             <span class="kw">_ </span>=&gt; <span class="prelude-val">None</span>,
         }
     }

     <span class="doccomment">/// Returns true if and only if this group is capturing.
     </span><span class="kw">pub fn </span>is_capturing(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="self">self</span>.kind {
             GroupKind::CaptureIndex(<span class="kw">_</span>) | GroupKind::CaptureName { .. } =&gt; <span class="bool-val">true</span>,
             GroupKind::NonCapturing(<span class="kw">_</span>) =&gt; <span class="bool-val">false</span>,
         }
     }

     <span class="doccomment">/// Returns the capture index of this group, if this is a capturing group.
     ///
     /// This returns a capture index precisely when `is_capturing` is `true`.
     </span><span class="kw">pub fn </span>capture_index(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; <span class="prelude-ty">Option</span>&lt;u32&gt; {
         <span class="kw">match </span><span class="self">self</span>.kind {
             GroupKind::CaptureIndex(i) =&gt; <span class="prelude-val">Some</span>(i),
             GroupKind::CaptureName { <span class="kw-2">ref </span>name, .. } =&gt; <span class="prelude-val">Some</span>(name.index),
             GroupKind::NonCapturing(<span class="kw">_</span>) =&gt; <span class="prelude-val">None</span>,
         }
     }
 }

 <span class="doccomment">/// The kind of a group.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>GroupKind {
     <span class="doccomment">/// `(a)`
     </span>CaptureIndex(u32),
     <span class="doccomment">/// `(?&lt;name&gt;a)` or `(?P&lt;name&gt;a)`
     </span>CaptureName {
         <span class="doccomment">/// True if the `?P&lt;` syntax is used and false if the `?&lt;` syntax is used.
         </span>starts_with_p: bool,
         <span class="doccomment">/// The capture name.
         </span>name: CaptureName,
     },
     <span class="doccomment">/// `(?:a)` and `(?i:a)`
     </span>NonCapturing(Flags),
 }

 <span class="doccomment">/// A capture name.
 ///
 /// This corresponds to the name itself between the angle brackets in, e.g.,
 /// `(?P&lt;foo&gt;expr)`.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>CaptureName {
     <span class="doccomment">/// The span of this capture name.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The capture name.
     </span><span class="kw">pub </span>name: String,
     <span class="doccomment">/// The capture index.
     </span><span class="kw">pub </span>index: u32,
 }

 <span class="doccomment">/// A group of flags that is not applied to a particular regular expression.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>SetFlags {
     <span class="doccomment">/// The span of these flags, including the grouping parentheses.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The actual sequence of flags.
     </span><span class="kw">pub </span>flags: Flags,
 }

 <span class="doccomment">/// A group of flags.
 ///
 /// This corresponds only to the sequence of flags themselves, e.g., `is-u`.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>Flags {
     <span class="doccomment">/// The span of this group of flags.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// A sequence of flag items. Each item is either a flag or a negation
     /// operator.
     </span><span class="kw">pub </span>items: Vec&lt;FlagsItem&gt;,
 }

 <span class="kw">impl </span>Flags {
     <span class="doccomment">/// Add the given item to this sequence of flags.
     ///
     /// If the item was added successfully, then `None` is returned. If the
     /// given item is a duplicate, then `Some(i)` is returned, where
     /// `items[i].kind == item.kind`.
     </span><span class="kw">pub fn </span>add_item(<span class="kw-2">&amp;mut </span><span class="self">self</span>, item: FlagsItem) -&gt; <span class="prelude-ty">Option</span>&lt;usize&gt; {
         <span class="kw">for </span>(i, x) <span class="kw">in </span><span class="self">self</span>.items.iter().enumerate() {
             <span class="kw">if </span>x.kind == item.kind {
                 <span class="kw">return </span><span class="prelude-val">Some</span>(i);
             }
         }
         <span class="self">self</span>.items.push(item);
         <span class="prelude-val">None
     </span>}

     <span class="doccomment">/// Returns the state of the given flag in this set.
     ///
     /// If the given flag is in the set but is negated, then `Some(false)` is
     /// returned.
     ///
     /// If the given flag is in the set and is not negated, then `Some(true)`
     /// is returned.
     ///
     /// Otherwise, `None` is returned.
     </span><span class="kw">pub fn </span>flag_state(<span class="kw-2">&amp;</span><span class="self">self</span>, flag: Flag) -&gt; <span class="prelude-ty">Option</span>&lt;bool&gt; {
         <span class="kw">let </span><span class="kw-2">mut </span>negated = <span class="bool-val">false</span>;
         <span class="kw">for </span>x <span class="kw">in </span><span class="kw-2">&amp;</span><span class="self">self</span>.items {
             <span class="kw">match </span>x.kind {
                 FlagsItemKind::Negation =&gt; {
                     negated = <span class="bool-val">true</span>;
                 }
                 FlagsItemKind::Flag(<span class="kw-2">ref </span>xflag) <span class="kw">if </span>xflag == <span class="kw-2">&amp;</span>flag =&gt; {
                     <span class="kw">return </span><span class="prelude-val">Some</span>(!negated);
                 }
                 <span class="kw">_ </span>=&gt; {}
             }
         }
         <span class="prelude-val">None
     </span>}
 }

 <span class="doccomment">/// A single item in a group of flags.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub struct </span>FlagsItem {
     <span class="doccomment">/// The span of this item.
     </span><span class="kw">pub </span>span: Span,
     <span class="doccomment">/// The kind of this item.
     </span><span class="kw">pub </span>kind: FlagsItemKind,
 }

 <span class="doccomment">/// The kind of an item in a group of flags.
 </span><span class="attribute">#[derive(Clone, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>FlagsItemKind {
     <span class="doccomment">/// A negation operator applied to all subsequent flags in the enclosing
     /// group.
     </span>Negation,
     <span class="doccomment">/// A single flag in a group.
     </span>Flag(Flag),
 }

 <span class="kw">impl </span>FlagsItemKind {
     <span class="doccomment">/// Returns true if and only if this item is a negation operator.
     </span><span class="kw">pub fn </span>is_negation(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             FlagsItemKind::Negation =&gt; <span class="bool-val">true</span>,
             <span class="kw">_ </span>=&gt; <span class="bool-val">false</span>,
         }
     }
 }

 <span class="doccomment">/// A single flag.
 </span><span class="attribute">#[derive(Clone, Copy, Debug, Eq, PartialEq)]
 </span><span class="kw">pub enum </span>Flag {
     <span class="doccomment">/// `i`
     </span>CaseInsensitive,
     <span class="doccomment">/// `m`
     </span>MultiLine,
     <span class="doccomment">/// `s`
     </span>DotMatchesNewLine,
     <span class="doccomment">/// `U`
     </span>SwapGreed,
     <span class="doccomment">/// `u`
     </span>Unicode,
     <span class="doccomment">/// `R`
     </span>CRLF,
     <span class="doccomment">/// `x`
     </span>IgnoreWhitespace,
 }

 <span class="doccomment">/// A custom `Drop` impl is used for `Ast` such that it uses constant stack
 /// space but heap space proportional to the depth of the `Ast`.
 </span><span class="kw">impl </span>Drop <span class="kw">for </span>Ast {
     <span class="kw">fn </span>drop(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
         <span class="kw">use </span>core::mem;

         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             Ast::Empty(<span class="kw">_</span>)
             | Ast::Flags(<span class="kw">_</span>)
             | Ast::Literal(<span class="kw">_</span>)
             | Ast::Dot(<span class="kw">_</span>)
             | Ast::Assertion(<span class="kw">_</span>)
             <span class="comment">// Classes are recursive, so they get their own Drop impl.
             </span>| Ast::Class(<span class="kw">_</span>) =&gt; <span class="kw">return</span>,
             Ast::Repetition(<span class="kw-2">ref </span>x) <span class="kw">if </span>!x.ast.has_subexprs() =&gt; <span class="kw">return</span>,
             Ast::Group(<span class="kw-2">ref </span>x) <span class="kw">if </span>!x.ast.has_subexprs() =&gt; <span class="kw">return</span>,
             Ast::Alternation(<span class="kw-2">ref </span>x) <span class="kw">if </span>x.asts.is_empty() =&gt; <span class="kw">return</span>,
             Ast::Concat(<span class="kw-2">ref </span>x) <span class="kw">if </span>x.asts.is_empty() =&gt; <span class="kw">return</span>,
             <span class="kw">_ </span>=&gt; {}
         }

         <span class="kw">let </span>empty_span = || Span::splat(Position::new(<span class="number">0</span>, <span class="number">0</span>, <span class="number">0</span>));
         <span class="kw">let </span>empty_ast = || Ast::Empty(empty_span());
         <span class="kw">let </span><span class="kw-2">mut </span>stack = <span class="macro">vec!</span>[mem::replace(<span class="self">self</span>, empty_ast())];
         <span class="kw">while let </span><span class="prelude-val">Some</span>(<span class="kw-2">mut </span>ast) = stack.pop() {
             <span class="kw">match </span>ast {
                 Ast::Empty(<span class="kw">_</span>)
                 | Ast::Flags(<span class="kw">_</span>)
                 | Ast::Literal(<span class="kw">_</span>)
                 | Ast::Dot(<span class="kw">_</span>)
                 | Ast::Assertion(<span class="kw">_</span>)
                 <span class="comment">// Classes are recursive, so they get their own Drop impl.
                 </span>| Ast::Class(<span class="kw">_</span>) =&gt; {}
                 Ast::Repetition(<span class="kw-2">ref mut </span>x) =&gt; {
                     stack.push(mem::replace(<span class="kw-2">&amp;mut </span>x.ast, empty_ast()));
                 }
                 Ast::Group(<span class="kw-2">ref mut </span>x) =&gt; {
                     stack.push(mem::replace(<span class="kw-2">&amp;mut </span>x.ast, empty_ast()));
                 }
                 Ast::Alternation(<span class="kw-2">ref mut </span>x) =&gt; {
                     stack.extend(x.asts.drain(..));
                 }
                 Ast::Concat(<span class="kw-2">ref mut </span>x) =&gt; {
                     stack.extend(x.asts.drain(..));
                 }
             }
         }
     }
 }

 <span class="doccomment">/// A custom `Drop` impl is used for `ClassSet` such that it uses constant
 /// stack space but heap space proportional to the depth of the `ClassSet`.
 </span><span class="kw">impl </span>Drop <span class="kw">for </span>ClassSet {
     <span class="kw">fn </span>drop(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
         <span class="kw">use </span>core::mem;

         <span class="kw">match </span><span class="kw-2">*</span><span class="self">self </span>{
             ClassSet::Item(<span class="kw-2">ref </span>item) =&gt; <span class="kw">match </span><span class="kw-2">*</span>item {
                 ClassSetItem::Empty(<span class="kw">_</span>)
                 | ClassSetItem::Literal(<span class="kw">_</span>)
                 | ClassSetItem::Range(<span class="kw">_</span>)
                 | ClassSetItem::Ascii(<span class="kw">_</span>)
                 | ClassSetItem::Unicode(<span class="kw">_</span>)
                 | ClassSetItem::Perl(<span class="kw">_</span>) =&gt; <span class="kw">return</span>,
                 ClassSetItem::Bracketed(<span class="kw-2">ref </span>x) =&gt; {
                     <span class="kw">if </span>x.kind.is_empty() {
                         <span class="kw">return</span>;
                     }
                 }
                 ClassSetItem::Union(<span class="kw-2">ref </span>x) =&gt; {
                     <span class="kw">if </span>x.items.is_empty() {
                         <span class="kw">return</span>;
                     }
                 }
             },
             ClassSet::BinaryOp(<span class="kw-2">ref </span>op) =&gt; {
                 <span class="kw">if </span>op.lhs.is_empty() &amp;&amp; op.rhs.is_empty() {
                     <span class="kw">return</span>;
                 }
             }
         }

         <span class="kw">let </span>empty_span = || Span::splat(Position::new(<span class="number">0</span>, <span class="number">0</span>, <span class="number">0</span>));
         <span class="kw">let </span>empty_set = || ClassSet::Item(ClassSetItem::Empty(empty_span()));
         <span class="kw">let </span><span class="kw-2">mut </span>stack = <span class="macro">vec!</span>[mem::replace(<span class="self">self</span>, empty_set())];
         <span class="kw">while let </span><span class="prelude-val">Some</span>(<span class="kw-2">mut </span>set) = stack.pop() {
             <span class="kw">match </span>set {
                 ClassSet::Item(<span class="kw-2">ref mut </span>item) =&gt; <span class="kw">match </span><span class="kw-2">*</span>item {
                     ClassSetItem::Empty(<span class="kw">_</span>)
                     | ClassSetItem::Literal(<span class="kw">_</span>)
                     | ClassSetItem::Range(<span class="kw">_</span>)
                     | ClassSetItem::Ascii(<span class="kw">_</span>)
                     | ClassSetItem::Unicode(<span class="kw">_</span>)
                     | ClassSetItem::Perl(<span class="kw">_</span>) =&gt; {}
                     ClassSetItem::Bracketed(<span class="kw-2">ref mut </span>x) =&gt; {
                         stack.push(mem::replace(<span class="kw-2">&amp;mut </span>x.kind, empty_set()));
                     }
                     ClassSetItem::Union(<span class="kw-2">ref mut </span>x) =&gt; {
                         stack.extend(x.items.drain(..).map(ClassSet::Item));
                     }
                 },
                 ClassSet::BinaryOp(<span class="kw-2">ref mut </span>op) =&gt; {
                     stack.push(mem::replace(<span class="kw-2">&amp;mut </span>op.lhs, empty_set()));
                     stack.push(mem::replace(<span class="kw-2">&amp;mut </span>op.rhs, empty_set()));
                 }
             }
         }
     }
 }

 <span class="attribute">#[cfg(test)]
 </span><span class="kw">mod </span>tests {
     <span class="kw">use super</span>::<span class="kw-2">*</span>;

     <span class="comment">// We use a thread with an explicit stack size to test that our destructor
     // for Ast can handle arbitrarily sized expressions in constant stack
     // space. In case we run on a platform without threads (WASM?), we limit
     // this test to Windows/Unix.
     </span><span class="attribute">#[test]
     #[cfg(any(unix, windows))]
     </span><span class="kw">fn </span>no_stack_overflow_on_drop() {
         <span class="kw">use </span>std::thread;

         <span class="kw">let </span>run = || {
             <span class="kw">let </span>span = || Span::splat(Position::new(<span class="number">0</span>, <span class="number">0</span>, <span class="number">0</span>));
             <span class="kw">let </span><span class="kw-2">mut </span>ast = Ast::Empty(span());
             <span class="kw">for </span>i <span class="kw">in </span><span class="number">0</span>..<span class="number">200 </span>{
                 ast = Ast::Group(Group {
                     span: span(),
                     kind: GroupKind::CaptureIndex(i),
                     ast: Box::new(ast),
                 });
             }
             <span class="macro">assert!</span>(!ast.is_empty());
         };

         <span class="comment">// We run our test on a thread with a small stack size so we can
         // force the issue more easily.
         //
         // NOTE(2023-03-21): It turns out that some platforms (like FreeBSD)
         // will just barf with very small stack sizes. So we bump this up a bit
         // to give more room to breath. When I did this, I confirmed that if
         // I remove the custom `Drop` impl for `Ast`, then this test does
         // indeed still fail with a stack overflow. (At the time of writing, I
         // had to bump it all the way up to 32K before the test would pass even
         // without the custom `Drop` impl. So 16K seems like a safe number
         // here.)
         //
         // See: https://github.com/rust-lang/regex/issues/967
         </span>thread::Builder::new()
             .stack_size(<span class="number">16 </span>&lt;&lt; <span class="number">10</span>)
             .spawn(run)
             .unwrap()
             .join()
             .unwrap();
     }
 }
 </code></pre></div>
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