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    // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    /*!
    Defines an abstract syntax for regular expressions.
    */

    use std::cmp::Ordering;
    use std::error;
    use std::fmt;

    pub use ast::visitor::{Visitor, visit};

    pub mod parse;
    pub mod print;
    mod visitor;

    /// An error that occurred while parsing a regular expression into an abstract
    /// syntax tree.
    ///
    /// Note that note 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`).
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct Error {
        /// The kind of error.
        kind: ErrorKind,
        /// The original pattern that the parser generated the error from. Every
        /// span in an error is a valid range into this string.
        pattern: String,
        /// The span of this error.
        span: Span,
    }

    impl Error {
        /// Return the type of this error.
        pub fn kind(&self) -> &ErrorKind {
            &self.kind
        }

        /// The original pattern string in which this error occurred.
        ///
        /// Every span reported by this error is reported in terms of this string.
        pub fn pattern(&self) -> &str {
            &self.pattern
        }

        /// Return the span at which this error occurred.
        pub fn span(&self) -> &Span {
            &self.span
        }

        /// 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, "duplicate" errors will have the
        /// main error position set to the duplicate occurrence while its
        /// auxiliary span will be set to the initial occurrence.
        pub fn auxiliary_span(&self) -> Option<&Span> {
            use self::ErrorKind::*;
            match self.kind {
                FlagDuplicate { ref original } => Some(original),
                FlagRepeatedNegation { ref original, .. } => Some(original),
                GroupNameDuplicate { ref original, .. } => Some(original),
                _ => None,
            }
        }
    }

    /// The type of an error that occurred while building an AST.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum ErrorKind {
        /// 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.
        CaptureLimitExceeded,
        /// An invalid escape sequence was found in a character class set.
        ClassEscapeInvalid,
        /// An invalid character class range was found. An invalid range is any
        /// range where the start is greater than the end.
        ClassRangeInvalid,
        /// 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.
        ClassRangeLiteral,
        /// An opening `[` was found with no corresponding closing `]`.
        ClassUnclosed,
        /// 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`.
        DecimalEmpty,
        /// An invalid decimal number was given where one was expected.
        DecimalInvalid,
        /// A bracketed hex literal was empty.
        EscapeHexEmpty,
        /// A bracketed hex literal did not correspond to a Unicode scalar value.
        EscapeHexInvalid,
        /// An invalid hexadecimal digit was found.
        EscapeHexInvalidDigit,
        /// EOF was found before an escape sequence was completed.
        EscapeUnexpectedEof,
        /// An unrecognized escape sequence.
        EscapeUnrecognized,
        /// A dangling negation was used when setting flags, e.g., `i-`.
        FlagDanglingNegation,
        /// A flag was used twice, e.g., `i-i`.
        FlagDuplicate {
            /// The position of the original flag. The error position
            /// points to the duplicate flag.
            original: Span,
        },
        /// The negation operator was used twice, e.g., `-i-s`.
        FlagRepeatedNegation {
            /// The position of the original negation operator. The error position
            /// points to the duplicate negation operator.
            original: Span,
        },
        /// Expected a flag but got EOF, e.g., `(?`.
        FlagUnexpectedEof,
        /// Unrecognized flag, e.g., `a`.
        FlagUnrecognized,
        /// A duplicate capture name was found.
        GroupNameDuplicate {
            /// The position of the initial occurrence of the capture name. The
            /// error position itself points to the duplicate occurrence.
            original: Span,
        },
        /// A capture group name is empty, e.g., `(?P<>abc)`.
        GroupNameEmpty,
        /// 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).
        GroupNameInvalid,
        /// A closing `>` could not be found for a capture group name.
        GroupNameUnexpectedEof,
        /// An unclosed group, e.g., `(ab`.
        ///
        /// The span of this error corresponds to the unclosed parenthesis.
        GroupUnclosed,
        /// An unopened group, e.g., `ab)`.
        GroupUnopened,
        /// The nest limit was exceeded. The limit stored here is the limit
        /// configured in the parser.
        NestLimitExceeded(u32),
        /// The range provided in a counted repetition operator is invalid. The
        /// range is invalid if the start is greater than the end.
        RepetitionCountInvalid,
        /// An opening `{` was not followed by a valid decimal value.
        /// For example, `x{}` or `x{]}` would fail.
        RepetitionCountDecimalEmpty,
        /// An opening `{` was found with no corresponding closing `}`.
        RepetitionCountUnclosed,
        /// 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.
        RepetitionMissing,
        /// 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.
        UnsupportedBackreference,
        /// When syntax similar to PCRE's look-around is used, this error is
        /// returned. Some example syntaxes that are rejected include, but are
        /// not necessarily limited to, `(?=re)`, `(?!re)`, `(?<=re)` and
        /// `(?<!re)`. Note that all of these syntaxes are otherwise invalid; this
        /// error is used to improve the user experience.
        UnsupportedLookAround,
        /// Hints that destructuring should not be exhaustive.
        ///
        /// This enum may grow additional variants, so this makes sure clients
        /// don't count on exhaustive matching. (Otherwise, adding a new variant
        /// could break existing code.)
        #[doc(hidden)]
        __Nonexhaustive,
    }

    impl error::Error for Error {
        fn description(&self) -> &str {
            use self::ErrorKind::*;
            match self.kind {
                CaptureLimitExceeded => "capture group limit exceeded",
                ClassEscapeInvalid => "invalid escape sequence in character class",
                ClassRangeInvalid => "invalid character class range",
                ClassRangeLiteral => "invalid range boundary, must be a literal",
                ClassUnclosed => "unclosed character class",
                DecimalEmpty => "empty decimal literal",
                DecimalInvalid => "invalid decimal literal",
                EscapeHexEmpty => "empty hexadecimal literal",
                EscapeHexInvalid => "invalid hexadecimal literal",
                EscapeHexInvalidDigit => "invalid hexadecimal digit",
                EscapeUnexpectedEof => "unexpected eof (escape sequence)",
                EscapeUnrecognized => "unrecognized escape sequence",
                FlagDanglingNegation => "dangling flag negation operator",
                FlagDuplicate{..} => "duplicate flag",
                FlagRepeatedNegation{..} => "repeated negation",
                FlagUnexpectedEof => "unexpected eof (flag)",
                FlagUnrecognized => "unrecognized flag",
                GroupNameDuplicate{..} => "duplicate capture group name",
                GroupNameEmpty => "empty capture group name",
                GroupNameInvalid => "invalid capture group name",
                GroupNameUnexpectedEof => "unclosed capture group name",
                GroupUnclosed => "unclosed group",
                GroupUnopened => "unopened group",
                NestLimitExceeded(_) => "nest limit exceeded",
                RepetitionCountInvalid => "invalid repetition count range",
                RepetitionCountUnclosed => "unclosed counted repetition",
                RepetitionMissing => "repetition operator missing expression",
                UnsupportedBackreference => "backreferences are not supported",
                UnsupportedLookAround => "look-around is not supported",
                _ => unreachable!(),
            }
        }
    }

    impl fmt::Display for Error {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            ::error::Formatter::from(self).fmt(f)
        }
    }

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

    /// 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.
    #[derive(Clone, Copy, Eq, PartialEq)]
    pub struct Span {
        /// The start byte offset.
        pub start: Position,
        /// The end byte offset.
        pub end: Position,
    }

    impl fmt::Debug for Span {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Span({:?}, {:?})", self.start, self.end)
        }
    }

    impl Ord for Span {
        fn cmp(&self, other: &Span) -> Ordering {
            (&self.start, &self.end).cmp(&(&other.start, &other.end))
        }
    }

    impl PartialOrd for Span {
        fn partial_cmp(&self, other: &Span) -> Option<Ordering> {
            Some(self.cmp(other))
        }
    }

    /// 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.
    #[derive(Clone, Copy, Eq, PartialEq)]
    pub struct Position {
        /// The absolute offset of this position, starting at `0` from the
        /// beginning of the regular expression pattern string.
        pub offset: usize,
        /// The line number, starting at `1`.
        pub line: usize,
        /// The approximate column number, starting at `1`.
        pub column: usize,
    }

    impl fmt::Debug for Position {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(
                f,
                "Position(o: {:?}, l: {:?}, c: {:?})",
                self.offset, self.line, self.column)
        }
    }

    impl Ord for Position {
        fn cmp(&self, other: &Position) -> Ordering {
            self.offset.cmp(&other.offset)
        }
    }

    impl PartialOrd for Position {
        fn partial_cmp(&self, other: &Position) -> Option<Ordering> {
            Some(self.cmp(other))
        }
    }

    impl Span {
        /// Create a new span with the given positions.
        pub fn new(start: Position, end: Position) -> Span {
            Span { start: start, end: end }
        }

        /// Create a new span using the given position as the start and end.
        pub fn splat(pos: Position) -> Span {
            Span::new(pos, pos)
        }

        /// Create a new span by replacing the starting the position with the one
        /// given.
        pub fn with_start(self, pos: Position) -> Span {
            Span { start: pos, ..self }
        }

        /// Create a new span by replacing the ending the position with the one
        /// given.
        pub fn with_end(self, pos: Position) -> Span {
            Span { end: pos, ..self }
        }

        /// Returns true if and only if this span occurs on a single line.
        pub fn is_one_line(&self) -> bool {
            self.start.line == self.end.line
        }

        /// 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.
        pub fn is_empty(&self) -> bool {
            self.start.offset == self.end.offset
        }
    }

    impl Position {
        /// 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`.
        pub fn new(offset: usize, line: usize, column: usize) -> Position {
            Position { offset: offset, line: line, column: column }
        }
    }

    /// 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.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct WithComments {
        /// The actual ast.
        pub ast: Ast,
        /// All comments found in the original regular expression.
        pub comments: Vec<Comment>,
    }

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

    /// An abstract syntax tree for a single regular expression.
    ///
    /// An `Ast`'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`.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum Ast {
        /// An empty regex that matches everything.
        Empty(Span),
        /// A set of flags, e.g., `(?is)`.
        Flags(SetFlags),
        /// A single character literal, which includes escape sequences.
        Literal(Literal),
        /// The "any character" class.
        Dot(Span),
        /// A single zero-width assertion.
        Assertion(Assertion),
        /// A single character class. This includes all forms of character classes
        /// except for `.`. e.g., `\d`, `\pN`, `[a-z]` and `[[:alpha:]]`.
        Class(Class),
        /// A repetition operator applied to an arbitrary regular expression.
        Repetition(Repetition),
        /// A grouped regular expression.
        Group(Group),
        /// An alternation of regular expressions.
        Alternation(Alternation),
        /// A concatenation of regular expressions.
        Concat(Concat),
    }

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

        /// Return true if and only if this Ast is empty.
        pub fn is_empty(&self) -> bool {
            match *self {
                Ast::Empty(_) => true,
                _ => false,
            }
        }

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

    /// 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`.
    impl fmt::Display for Ast {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            use ast::print::Printer;
            Printer::new().print(self, f)
        }
    }

    /// An alternation of regular expressions.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct Alternation {
        /// The span of this alternation.
        pub span: Span,
        /// The alternate regular expressions.
        pub asts: Vec<Ast>,
    }

    impl Alternation {
        /// 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.
        pub fn into_ast(mut self) -> Ast {
            match self.asts.len() {
                0 => Ast::Empty(self.span),
                1 => self.asts.pop().unwrap(),
                _ => Ast::Alternation(self),
            }
        }
    }

    /// A concatenation of regular expressions.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct Concat {
        /// The span of this concatenation.
        pub span: Span,
        /// The concatenation regular expressions.
        pub asts: Vec<Ast>,
    }

    impl Concat {
        /// 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.
        pub fn into_ast(mut self) -> Ast {
            match self.asts.len() {
                0 => Ast::Empty(self.span),
                1 => self.asts.pop().unwrap(),
                _ => Ast::Concat(self),
            }
        }
    }

    /// 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`.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct Literal {
        /// The span of this literal.
        pub span: Span,
        /// The kind of this literal.
        pub kind: LiteralKind,
        /// The Unicode scalar value corresponding to this literal.
        pub c: char,
    }

    impl Literal {
        /// If this literal was written as a `\x` hex escape, then this returns
        /// the corresponding byte value. Otherwise, this returns `None`.
        pub fn byte(&self) -> Option<u8> {
            let short_hex = LiteralKind::HexFixed(HexLiteralKind::X);
            if self.c as u32 <= 255 && self.kind == short_hex {
                Some(self.c as u8)
            } else {
                None
            }
        }
    }

    /// The kind of a single literal expression.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum LiteralKind {
        /// The literal is written verbatim, e.g., `a` or `☃`.
        Verbatim,
        /// The literal is written as an escape because it is punctuation, e.g.,
        /// `\*` or `\[`.
        Punctuation,
        /// The literal is written as an octal escape, e.g., `\141`.
        Octal,
        /// 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`.
        HexFixed(HexLiteralKind),
        /// 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.
        HexBrace(HexLiteralKind),
        /// The literal is written as a specially recognized escape, e.g., `\f`
        /// or `\n`.
        Special(SpecialLiteralKind),
    }

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

    /// 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.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum HexLiteralKind {
        /// A `\x` prefix. When used without brackets, this form is limited to
        /// two digits.
        X,
        /// A `\u` prefix. When used without brackets, this form is limited to
        /// four digits.
        UnicodeShort,
        /// A `\U` prefix. When used without brackets, this form is limited to
        /// eight digits.
        UnicodeLong,
    }

    impl HexLiteralKind {
        /// 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.
        pub fn digits(&self) -> u32 {
            match *self {
                HexLiteralKind::X => 2,
                HexLiteralKind::UnicodeShort => 4,
                HexLiteralKind::UnicodeLong => 8,
            }
        }
    }

    /// A single character class expression.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum Class {
        /// A Unicode character class, e.g., `\pL` or `\p{Greek}`.
        Unicode(ClassUnicode),
        /// A perl character class, e.g., `\d` or `\W`.
        Perl(ClassPerl),
        /// 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]`.
        Bracketed(ClassBracketed),
    }

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

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

    /// The available Perl character classes.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum ClassPerlKind {
        /// Decimal numbers.
        Digit,
        /// Whitespace.
        Space,
        /// Word characters.
        Word,
    }

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

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

    impl ClassAsciiKind {
        /// 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.
        pub fn from_name(name: &str) -> Option<ClassAsciiKind> {
            use self::ClassAsciiKind::*;
            match name {
                "alnum" => Some(Alnum),
                "alpha" => Some(Alpha),
                "ascii" => Some(Ascii),
                "blank" => Some(Blank),
                "cntrl" => Some(Cntrl),
                "digit" => Some(Digit),
                "graph" => Some(Graph),
                "lower" => Some(Lower),
                "print" => Some(Print),
                "punct" => Some(Punct),
                "space" => Some(Space),
                "upper" => Some(Upper),
                "word" => Some(Word),
                "xdigit" => Some(Xdigit),
                _ => None,
            }
        }
    }

    /// A Unicode character class.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct ClassUnicode {
        /// The span of this class.
        pub span: Span,
        /// 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.
        pub negated: bool,
        /// The kind of Unicode class.
        pub kind: ClassUnicodeKind,
    }

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

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

    /// The type of op used in a Unicode character class.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum ClassUnicodeOpKind {
        /// A property set to a specific value, e.g., `\p{scx=Katakana}`.
        Equal,
        /// A property set to a specific value using a colon, e.g.,
        /// `\p{scx:Katakana}`.
        Colon,
        /// A property that isn't a particular value, e.g., `\p{scx!=Katakana}`.
        NotEqual,
    }

    impl ClassUnicodeOpKind {
        /// Whether the op is an equality op or not.
        pub fn is_equal(&self) -> bool {
            match *self {
                ClassUnicodeOpKind::Equal|ClassUnicodeOpKind::Colon => true,
                _ => false,
            }
        }
    }

    /// A bracketed character class, e.g., `[a-z0-9]`.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct ClassBracketed {
        /// The span of this class.
        pub span: Span,
        /// Whether this class is negated or not. e.g., `[a]` is not negated but
        /// `[^a]` is.
        pub negated: bool,
        /// 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]`.
        pub kind: ClassSet,
    }

    /// 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.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum ClassSet {
        /// An item, which can be a single literal, range, nested character class
        /// or a union of items.
        Item(ClassSetItem),
        /// A single binary operation (i.e., &&, -- or ~~).
        BinaryOp(ClassSetBinaryOp),
    }

    impl ClassSet {
        /// Build a set from a union.
        pub fn union(ast: ClassSetUnion) -> ClassSet {
            ClassSet::Item(ClassSetItem::Union(ast))
        }

        /// Return the span of this character class set.
        pub fn span(&self) -> &Span {
            match *self {
                ClassSet::Item(ref x) => x.span(),
                ClassSet::BinaryOp(ref x) => &x.span,
            }
        }

        /// Return true if and only if this class set is empty.
        fn is_empty(&self) -> bool {
            match *self {
                ClassSet::Item(ClassSetItem::Empty(_)) => true,
                _ => false,
            }
        }
    }

    /// A single component of a character class set.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum ClassSetItem {
        /// 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, `[&&]` is the intersection of two empty classes.
        Empty(Span),
        /// A single literal.
        Literal(Literal),
        /// A range between two literals.
        Range(ClassSetRange),
        /// An ASCII character class, e.g., `[:alnum:]` or `[:punct:]`.
        Ascii(ClassAscii),
        /// A Unicode character class, e.g., `\pL` or `\p{Greek}`.
        Unicode(ClassUnicode),
        /// A perl character class, e.g., `\d` or `\W`.
        Perl(ClassPerl),
        /// 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]`.
        Bracketed(Box<ClassBracketed>),
        /// A union of items.
        Union(ClassSetUnion),
    }

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

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

    impl ClassSetRange {
        /// 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.
        pub fn is_valid(&self) -> bool {
            self.start.c <= self.end.c
        }
    }

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

    impl ClassSetUnion {
        /// Push a new item in this union.
        ///
        /// The ending position of this union'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.
        pub fn push(&mut self, item: ClassSetItem) {
            if self.items.is_empty() {
                self.span.start = item.span().start;
            }
            self.span.end = item.span().end;
            self.items.push(item);
        }

        /// 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.
        pub fn into_item(mut self) -> ClassSetItem {
            match self.items.len() {
                0 => ClassSetItem::Empty(self.span),
                1 => self.items.pop().unwrap(),
                _ => ClassSetItem::Union(self),
            }
        }
    }

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

    /// The type of a Unicode character class set operation.
    ///
    /// Note that this doesn't explicitly represent union since there is no
    /// explicit union operator. Concatenation inside a character class corresponds
    /// to the union operation.
    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
    pub enum ClassSetBinaryOpKind {
        /// The intersection of two sets, e.g., `\pN&&[a-z]`.
        Intersection,
        /// The difference of two sets, e.g., `\pN--[0-9]`.
        Difference,
        /// 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:]]`.
        SymmetricDifference,
    }

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

    /// An assertion kind.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum AssertionKind {
        /// `^`
        StartLine,
        /// `$`
        EndLine,
        /// `\A`
        StartText,
        /// `\z`
        EndText,
        /// `\b`
        WordBoundary,
        /// `\B`
        NotWordBoundary,
    }

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

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

    /// The kind of a repetition operator.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum RepetitionKind {
        /// `?`
        ZeroOrOne,
        /// `*`
        ZeroOrMore,
        /// `+`
        OneOrMore,
        /// `{m,n}`
        Range(RepetitionRange),
    }

    /// A range repetition operator.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum RepetitionRange {
        /// `{m}`
        Exactly(u32),
        /// `{m,}`
        AtLeast(u32),
        /// `{m,n}`
        Bounded(u32, u32),
    }

    impl RepetitionRange {
        /// 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.
        pub fn is_valid(&self) -> bool {
            match *self {
                RepetitionRange::Bounded(s, e) if s > e => false,
                _ => true,
            }
        }
    }

    /// 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<name>a)`, `(?:a)` and
    /// `(?is:a)`.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub struct Group {
        /// The span of this group.
        pub span: Span,
        /// The kind of this group.
        pub kind: GroupKind,
        /// The regular expression in this group.
        pub ast: Box<Ast>,
    }

    impl Group {
        /// If this group is non-capturing, then this returns the (possibly empty)
        /// set of flags. Otherwise, `None` is returned.
        pub fn flags(&self) -> Option<&Flags> {
            match self.kind {
                GroupKind::NonCapturing(ref flags) => Some(flags),
                _ => None,
            }
        }

        /// Returns true if and only if this group is capturing.
        pub fn is_capturing(&self) -> bool {
            match self.kind {
                GroupKind::CaptureIndex(_) | GroupKind::CaptureName(_) => true,
                GroupKind::NonCapturing(_) => false,
            }
        }

        /// Returns the capture index of this group, if this is a capturing group.
        ///
        /// This returns a capture index precisely when `is_capturing` is `true`.
        pub fn capture_index(&self) -> Option<u32> {
            match self.kind {
                GroupKind::CaptureIndex(i) => Some(i),
                GroupKind::CaptureName(ref x) => Some(x.index),
                GroupKind::NonCapturing(_) => None,
            }
        }
    }

    /// The kind of a group.
    #[derive(Clone, Debug, Eq, PartialEq)]
    pub enum GroupKind {
        /// `(a)`
        CaptureIndex(u32),
        /// `(?P<name>a)`
        CaptureName(CaptureName),
        /// `(?:a)` and `(?i:a)`
        NonCapturing(Flags),
    }

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

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

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

    impl Flags {
        /// 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`.
        pub fn add_item(&mut self, item: FlagsItem) -> Option<usize> {
            for (i, x) in self.items.iter().enumerate() {
                if x.kind == item.kind {
                    return Some(i);
                }
            }
            self.items.push(item);
            None
        }

        /// 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.
        pub fn flag_state(&self, flag: Flag) -> Option<bool> {
            let mut negated = false;
            for x in &self.items {
                match x.kind {
                    FlagsItemKind::Negation => {
                        negated = true;
                    }
                    FlagsItemKind::Flag(ref xflag) if xflag == &flag => {
                        return Some(!negated);
                    }
                    _ => {}
                }
            }
            None
        }
    }

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

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

    impl FlagsItemKind {
        /// Returns true if and only if this item is a negation operator.
        pub fn is_negation(&self) -> bool {
            match *self {
                FlagsItemKind::Negation => true,
                _ => false,
            }
        }
    }

    /// A single flag.
    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
    pub enum Flag {
        /// `i`
        CaseInsensitive,
        /// `m`
        MultiLine,
        /// `s`
        DotMatchesNewLine,
        /// `U`
        SwapGreed,
        /// `u`
        Unicode,
        /// `x`
        IgnoreWhitespace,
    }

    /// 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`.
    impl Drop for Ast {
        fn drop(&mut self) {
            use std::mem;

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

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

    /// 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`.
    impl Drop for ClassSet {
        fn drop(&mut self) {
            use std::mem;

            match *self {
                ClassSet::Item(ref item) => {
                    match *item {
                        ClassSetItem::Empty(_)
                        | ClassSetItem::Literal(_)
                        | ClassSetItem::Range(_)
                        | ClassSetItem::Ascii(_)
                        | ClassSetItem::Unicode(_)
                        | ClassSetItem::Perl(_) => return,
                        ClassSetItem::Bracketed(ref x) => {
                            if x.kind.is_empty() {
                                return;
                            }
                        }
                        ClassSetItem::Union(ref x) => {
                            if x.items.is_empty() {
                                return;
                            }
                        }
                    }
                }
                ClassSet::BinaryOp(ref op) => {
                    if op.lhs.is_empty() && op.rhs.is_empty() {
                        return;
                    }
                }
            }

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

    #[cfg(test)]
    mod tests {
        use super::*;

        // 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.
        #[test]
        #[cfg(any(unix, windows))]
        fn no_stack_overflow_on_drop() {
            use std::thread;

            let run = || {
                let span = || Span::splat(Position::new(0, 0, 0));
                let mut ast = Ast::Empty(span());
                for i in 0..200 {
                    ast = Ast::Group(Group {
                        span: span(),
                        kind: GroupKind::CaptureIndex(i),
                        ast: Box::new(ast),
                    });
                }
                assert!(!ast.is_empty());
            };

            // We run our test on a thread with a small stack size so we can
            // force the issue more easily.
            thread::Builder::new()
                .stack_size(1<<10)
                .spawn(run)
                .unwrap()
                .join()
                .unwrap();
        }
    }

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