// Copyright 2018 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 a translator that converts an `Ast` to an `Hir`.
*/

use std::cell::{Cell, RefCell};
use std::result;

use ast::{self, Ast, Span, Visitor};
use hir::{self, Error, ErrorKind, Hir};
use unicode::{self, ClassQuery};

type Result<T> = result::Result<T, Error>;

/// A builder for constructing an AST->HIR translator.
#[derive(Clone, Debug)]
pub struct TranslatorBuilder {
    allow_invalid_utf8: bool,
    flags: Flags,
}

impl Default for TranslatorBuilder {
    fn default() -> TranslatorBuilder {
        TranslatorBuilder::new()
    }
}

impl TranslatorBuilder {
    /// Create a new translator builder with a default c onfiguration.
    pub fn new() -> TranslatorBuilder {
        TranslatorBuilder {
            allow_invalid_utf8: false,
            flags: Flags::default(),
        }
    }

    /// Build a translator using the current configuration.
    pub fn build(&self) -> Translator {
        Translator {
            stack: RefCell::new(vec![]),
            flags: Cell::new(self.flags),
            allow_invalid_utf8: self.allow_invalid_utf8,
        }
    }

    /// When enabled, translation will permit the construction of a regular
    /// expression that may match invalid UTF-8.
    ///
    /// When disabled (the default), the translator is guaranteed to produce
    /// an expression that will only ever match valid UTF-8 (otherwise, the
    /// translator will return an error).
    ///
    /// Perhaps surprisingly, when invalid UTF-8 isn't allowed, a negated ASCII
    /// word boundary (uttered as `(?-u:\B)` in the concrete syntax) will cause
    /// the parser to return an error. Namely, a negated ASCII word boundary
    /// can result in matching positions that aren't valid UTF-8 boundaries.
    pub fn allow_invalid_utf8(
        &mut self,
        yes: bool,
    ) -> &mut TranslatorBuilder {
        self.allow_invalid_utf8 = yes;
        self
    }

    /// Enable or disable the case insensitive flag (`i`) by default.
    pub fn case_insensitive(&mut self, yes: bool) -> &mut TranslatorBuilder {
        self.flags.case_insensitive = if yes { Some(true) } else { None };
        self
    }

    /// Enable or disable the multi-line matching flag (`m`) by default.
    pub fn multi_line(&mut self, yes: bool) -> &mut TranslatorBuilder {
        self.flags.multi_line = if yes { Some(true) } else { None };
        self
    }

    /// Enable or disable the "dot matches any character" flag (`s`) by
    /// default.
    pub fn dot_matches_new_line(
        &mut self,
        yes: bool,
    ) -> &mut TranslatorBuilder {
        self.flags.dot_matches_new_line = if yes { Some(true) } else { None };
        self
    }

    /// Enable or disable the "swap greed" flag (`U`) by default.
    pub fn swap_greed(&mut self, yes: bool) -> &mut TranslatorBuilder {
        self.flags.swap_greed = if yes { Some(true) } else { None };
        self
    }

    /// Enable or disable the Unicode flag (`u`) by default.
    pub fn unicode(&mut self, yes: bool) -> &mut TranslatorBuilder {
        self.flags.unicode = if yes { None } else { Some(false) };
        self
    }
}

/// A translator maps abstract syntax to a high level intermediate
/// representation.
///
/// A translator may be benefit from reuse. That is, a translator can translate
/// many abstract syntax trees.
///
/// A `Translator` can be configured in more detail via a
/// [`TranslatorBuilder`](struct.TranslatorBuilder.html).
#[derive(Clone, Debug)]
pub struct Translator {
    /// Our call stack, but on the heap.
    stack: RefCell<Vec<HirFrame>>,
    /// The current flag settings.
    flags: Cell<Flags>,
    /// Whether we're allowed to produce HIR that can match arbitrary bytes.
    allow_invalid_utf8: bool,
}

impl Translator {
    /// Create a new translator using the default configuration.
    pub fn new() -> Translator {
        TranslatorBuilder::new().build()
    }

    /// Translate the given abstract syntax tree (AST) into a high level
    /// intermediate representation (HIR).
    ///
    /// If there was a problem doing the translation, then an HIR-specific
    /// error is returned.
    ///
    /// The original pattern string used to produce the `Ast` *must* also be
    /// provided. The translator does not use the pattern string during any
    /// correct translation, but is used for error reporting.
    pub fn translate(&mut self, pattern: &str, ast: &Ast) -> Result<Hir> {
        ast::visit(ast, TranslatorI::new(self, pattern))
    }
}

/// An HirFrame is a single stack frame, represented explicitly, which is
/// created for each item in the Ast that we traverse.
///
/// Note that technically, this type doesn't represent our entire stack
/// frame. In particular, the Ast visitor represents any state associated with
/// traversing the Ast itself.
#[derive(Clone, Debug)]
enum HirFrame {
    /// An arbitrary HIR expression. These get pushed whenever we hit a base
    /// case in the Ast. They get popped after an inductive (i.e., recursive)
    /// step is complete.
    Expr(Hir),
    /// A Unicode character class. This frame is mutated as we descend into
    /// the Ast of a character class (which is itself its own mini recursive
    /// structure).
    ClassUnicode(hir::ClassUnicode),
    /// A byte-oriented character class. This frame is mutated as we descend
    /// into the Ast of a character class (which is itself its own mini
    /// recursive structure).
    ///
    /// Byte character classes are created when Unicode mode (`u`) is disabled.
    /// If `allow_invalid_utf8` is disabled (the default), then a byte
    /// character is only permitted to match ASCII text.
    ClassBytes(hir::ClassBytes),
    /// This is pushed on to the stack upon first seeing any kind of group,
    /// indicated by parentheses (including non-capturing groups). It is popped
    /// upon leaving a group.
    Group {
        /// The old active flags, if any, when this group was opened.
        ///
        /// If this group sets flags, then the new active flags are set to the
        /// result of merging the old flags with the flags introduced by this
        /// group.
        ///
        /// When this group is popped, the active flags should be restored to
        /// the flags set here.
        ///
        /// The "active" flags correspond to whatever flags are set in the
        /// Translator.
        old_flags: Option<Flags>,
    },
    /// This is pushed whenever a concatenation is observed. After visiting
    /// every sub-expression in the concatenation, the translator's stack is
    /// popped until it sees a Concat frame.
    Concat,
    /// This is pushed whenever an alternation is observed. After visiting
    /// every sub-expression in the alternation, the translator's stack is
    /// popped until it sees an Alternation frame.
    Alternation,
}

impl HirFrame {
    /// Assert that the current stack frame is an Hir expression and return it.
    fn unwrap_expr(self) -> Hir {
        match self {
            HirFrame::Expr(expr) => expr,
            _ => panic!("tried to unwrap expr from HirFrame, got: {:?}", self)
        }
    }

    /// Assert that the current stack frame is a Unicode class expression and
    /// return it.
    fn unwrap_class_unicode(self) -> hir::ClassUnicode {
        match self {
            HirFrame::ClassUnicode(cls) => cls,
            _ => panic!("tried to unwrap Unicode class \
                         from HirFrame, got: {:?}", self)
        }
    }

    /// Assert that the current stack frame is a byte class expression and
    /// return it.
    fn unwrap_class_bytes(self) -> hir::ClassBytes {
        match self {
            HirFrame::ClassBytes(cls) => cls,
            _ => panic!("tried to unwrap byte class \
                         from HirFrame, got: {:?}", self)
        }
    }

    /// Assert that the current stack frame is a group indicator and return
    /// its corresponding flags (the flags that were active at the time the
    /// group was entered) if they exist.
    fn unwrap_group(self) -> Option<Flags> {
        match self {
            HirFrame::Group { old_flags } => old_flags,
            _ => panic!("tried to unwrap group from HirFrame, got: {:?}", self)
        }
    }
}

impl<'t, 'p> Visitor for TranslatorI<'t, 'p> {
    type Output = Hir;
    type Err = Error;

    fn finish(self) -> Result<Hir> {
        // ... otherwise, we should have exactly one HIR on the stack.
        assert_eq!(self.trans().stack.borrow().len(), 1);
        Ok(self.pop().unwrap().unwrap_expr())
    }

    fn visit_pre(&mut self, ast: &Ast) -> Result<()> {
        match *ast {
            Ast::Class(ast::Class::Bracketed(_)) => {
                if self.flags().unicode() {
                    let cls = hir::ClassUnicode::empty();
                    self.push(HirFrame::ClassUnicode(cls));
                } else {
                    let cls = hir::ClassBytes::empty();
                    self.push(HirFrame::ClassBytes(cls));
                }
            }
            Ast::Group(ref x) => {
                let old_flags = x.flags().map(|ast| self.set_flags(ast));
                self.push(HirFrame::Group {
                    old_flags: old_flags,
                });
            }
            Ast::Concat(ref x) if x.asts.is_empty() => {}
            Ast::Concat(_) => {
                self.push(HirFrame::Concat);
            }
            Ast::Alternation(ref x) if x.asts.is_empty() => {}
            Ast::Alternation(_) => {
                self.push(HirFrame::Alternation);
            }
            _ => {}
        }
        Ok(())
    }

    fn visit_post(&mut self, ast: &Ast) -> Result<()> {
        match *ast {
            Ast::Empty(_) => {
                self.push(HirFrame::Expr(Hir::empty()));
            }
            Ast::Flags(ref x) => {
                self.set_flags(&x.flags);
                // Flags in the AST are generally considered directives and
                // not actual sub-expressions. However, they can be used in
                // the concrete syntax like `((?i))`, and we need some kind of
                // indication of an expression there, and Empty is the correct
                // choice.
                //
                // There can also be things like `(?i)+`, but we rule those out
                // in the parser. In the future, we might allow them for
                // consistency sake.
                self.push(HirFrame::Expr(Hir::empty()));
            }
            Ast::Literal(ref x) => {
                self.push(HirFrame::Expr(self.hir_literal(x)?));
            }
            Ast::Dot(span) => {
                self.push(HirFrame::Expr(self.hir_dot(span)?));
            }
            Ast::Assertion(ref x) => {
                self.push(HirFrame::Expr(self.hir_assertion(x)?));
            }
            Ast::Class(ast::Class::Perl(ref x)) => {
                if self.flags().unicode() {
                    let cls = self.hir_perl_unicode_class(x);
                    let hcls = hir::Class::Unicode(cls);
                    self.push(HirFrame::Expr(Hir::class(hcls)));
                } else {
                    let cls = self.hir_perl_byte_class(x);
                    let hcls = hir::Class::Bytes(cls);
                    self.push(HirFrame::Expr(Hir::class(hcls)));
                }
            }
            Ast::Class(ast::Class::Unicode(ref x)) => {
                let cls = hir::Class::Unicode(self.hir_unicode_class(x)?);
                self.push(HirFrame::Expr(Hir::class(cls)));
            }
            Ast::Class(ast::Class::Bracketed(ref ast)) => {
                if self.flags().unicode() {
                    let mut cls = self.pop().unwrap().unwrap_class_unicode();
                    self.unicode_fold_and_negate(ast.negated, &mut cls);
                    if cls.iter().next().is_none() {
                        return Err(self.error(
                            ast.span, ErrorKind::EmptyClassNotAllowed));
                    }
                    let expr = Hir::class(hir::Class::Unicode(cls));
                    self.push(HirFrame::Expr(expr));
                } else {
                    let mut cls = self.pop().unwrap().unwrap_class_bytes();
                    self.bytes_fold_and_negate(
                        &ast.span, ast.negated, &mut cls)?;
                    if cls.iter().next().is_none() {
                        return Err(self.error(
                            ast.span, ErrorKind::EmptyClassNotAllowed));
                    }

                    let expr = Hir::class(hir::Class::Bytes(cls));
                    self.push(HirFrame::Expr(expr));
                }
            }
            Ast::Repetition(ref x) => {
                let expr = self.pop().unwrap().unwrap_expr();
                self.push(HirFrame::Expr(self.hir_repetition(x, expr)));
            }
            Ast::Group(ref x) => {
                let expr = self.pop().unwrap().unwrap_expr();
                if let Some(flags) = self.pop().unwrap().unwrap_group() {
                    self.trans().flags.set(flags);
                }
                self.push(HirFrame::Expr(self.hir_group(x, expr)));
            }
            Ast::Concat(_) => {
                let mut exprs = vec![];
                while let Some(HirFrame::Expr(expr)) = self.pop() {
                    if !expr.kind().is_empty() {
                        exprs.push(expr);
                    }
                }
                exprs.reverse();
                self.push(HirFrame::Expr(Hir::concat(exprs)));
            }
            Ast::Alternation(_) => {
                let mut exprs = vec![];
                while let Some(HirFrame::Expr(expr)) = self.pop() {
                    exprs.push(expr);
                }
                exprs.reverse();
                self.push(HirFrame::Expr(Hir::alternation(exprs)));
            }
        }
        Ok(())
    }

    fn visit_class_set_item_pre(
        &mut self,
        ast: &ast::ClassSetItem,
    ) -> Result<()> {
        match *ast {
            ast::ClassSetItem::Bracketed(_) => {
                if self.flags().unicode() {
                    let cls = hir::ClassUnicode::empty();
                    self.push(HirFrame::ClassUnicode(cls));
                } else {
                    let cls = hir::ClassBytes::empty();
                    self.push(HirFrame::ClassBytes(cls));
                }
            }
            // We needn't handle the Union case here since the visitor will
            // do it for us.
            _ => {}
        }
        Ok(())
    }

    fn visit_class_set_item_post(
        &mut self,
        ast: &ast::ClassSetItem,
    ) -> Result<()> {
        match *ast {
            ast::ClassSetItem::Empty(_) => {}
            ast::ClassSetItem::Literal(ref x) => {
                if self.flags().unicode() {
                    let mut cls = self.pop().unwrap().unwrap_class_unicode();
                    cls.push(hir::ClassUnicodeRange::new(x.c, x.c));
                    self.push(HirFrame::ClassUnicode(cls));
                } else {
                    let mut cls = self.pop().unwrap().unwrap_class_bytes();
                    let byte = self.class_literal_byte(x)?;
                    cls.push(hir::ClassBytesRange::new(byte, byte));
                    self.push(HirFrame::ClassBytes(cls));
                }
            }
            ast::ClassSetItem::Range(ref x) => {
                if self.flags().unicode() {
                    let mut cls = self.pop().unwrap().unwrap_class_unicode();
                    cls.push(hir::ClassUnicodeRange::new(x.start.c, x.end.c));
                    self.push(HirFrame::ClassUnicode(cls));
                } else {
                    let mut cls = self.pop().unwrap().unwrap_class_bytes();
                    let start = self.class_literal_byte(&x.start)?;
                    let end = self.class_literal_byte(&x.end)?;
                    cls.push(hir::ClassBytesRange::new(start, end));
                    self.push(HirFrame::ClassBytes(cls));
                }
            }
            ast::ClassSetItem::Ascii(ref x) => {
                if self.flags().unicode() {
                    let mut cls = self.pop().unwrap().unwrap_class_unicode();
                    for &(s, e) in ascii_class(&x.kind) {
                        cls.push(hir::ClassUnicodeRange::new(s, e));
                    }
                    self.unicode_fold_and_negate(x.negated, &mut cls);
                    self.push(HirFrame::ClassUnicode(cls));
                } else {
                    let mut cls = self.pop().unwrap().unwrap_class_bytes();
                    for &(s, e) in ascii_class(&x.kind) {
                        cls.push(hir::ClassBytesRange::new(s as u8, e as u8));
                    }
                    self.bytes_fold_and_negate(
                        &x.span, x.negated, &mut cls)?;
                    self.push(HirFrame::ClassBytes(cls));
                }
            }
            ast::ClassSetItem::Unicode(ref x) => {
                let xcls = self.hir_unicode_class(x)?;
                let mut cls = self.pop().unwrap().unwrap_class_unicode();
                cls.union(&xcls);
                self.push(HirFrame::ClassUnicode(cls));
            }
            ast::ClassSetItem::Perl(ref x) => {
                if self.flags().unicode() {
                    let xcls = self.hir_perl_unicode_class(x);
                    let mut cls = self.pop().unwrap().unwrap_class_unicode();
                    cls.union(&xcls);
                    self.push(HirFrame::ClassUnicode(cls));
                } else {
                    let xcls = self.hir_perl_byte_class(x);
                    let mut cls = self.pop().unwrap().unwrap_class_bytes();
                    cls.union(&xcls);
                    self.push(HirFrame::ClassBytes(cls));
                }
            }
            ast::ClassSetItem::Bracketed(ref ast) => {
                if self.flags().unicode() {
                    let mut cls1 = self.pop().unwrap().unwrap_class_unicode();
                    self.unicode_fold_and_negate(ast.negated, &mut cls1);

                    let mut cls2 = self.pop().unwrap().unwrap_class_unicode();
                    cls2.union(&cls1);
                    self.push(HirFrame::ClassUnicode(cls2));
                } else {
                    let mut cls1 = self.pop().unwrap().unwrap_class_bytes();
                    self.bytes_fold_and_negate(
                        &ast.span, ast.negated, &mut cls1)?;

                    let mut cls2 = self.pop().unwrap().unwrap_class_bytes();
                    cls2.union(&cls1);
                    self.push(HirFrame::ClassBytes(cls2));
                }
            }
            // This is handled automatically by the visitor.
            ast::ClassSetItem::Union(_) => {}
        }
        Ok(())
    }

    fn visit_class_set_binary_op_pre(
        &mut self,
        _op: &ast::ClassSetBinaryOp,
    ) -> Result<()> {
        if self.flags().unicode() {
            let cls = hir::ClassUnicode::empty();
            self.push(HirFrame::ClassUnicode(cls));
        } else {
            let cls = hir::ClassBytes::empty();
            self.push(HirFrame::ClassBytes(cls));
        }
        Ok(())
    }

    fn visit_class_set_binary_op_in(
        &mut self,
        _op: &ast::ClassSetBinaryOp,
    ) -> Result<()> {
        if self.flags().unicode() {
            let cls = hir::ClassUnicode::empty();
            self.push(HirFrame::ClassUnicode(cls));
        } else {
            let cls = hir::ClassBytes::empty();
            self.push(HirFrame::ClassBytes(cls));
        }
        Ok(())
    }

    fn visit_class_set_binary_op_post(
        &mut self,
        op: &ast::ClassSetBinaryOp,
    ) -> Result<()> {
        use ast::ClassSetBinaryOpKind::*;

        if self.flags().unicode() {
            let mut rhs = self.pop().unwrap().unwrap_class_unicode();
            let mut lhs = self.pop().unwrap().unwrap_class_unicode();
            let mut cls = self.pop().unwrap().unwrap_class_unicode();
            if self.flags().case_insensitive() {
                rhs.case_fold_simple();
                lhs.case_fold_simple();
            }
            match op.kind {
                Intersection => lhs.intersect(&rhs),
                Difference => lhs.difference(&rhs),
                SymmetricDifference => lhs.symmetric_difference(&rhs),
            }
            cls.union(&lhs);
            self.push(HirFrame::ClassUnicode(cls));
        } else {
            let mut rhs = self.pop().unwrap().unwrap_class_bytes();
            let mut lhs = self.pop().unwrap().unwrap_class_bytes();
            let mut cls = self.pop().unwrap().unwrap_class_bytes();
            if self.flags().case_insensitive() {
                rhs.case_fold_simple();
                lhs.case_fold_simple();
            }
            match op.kind {
                Intersection => lhs.intersect(&rhs),
                Difference => lhs.difference(&rhs),
                SymmetricDifference => lhs.symmetric_difference(&rhs),
            }
            cls.union(&lhs);
            self.push(HirFrame::ClassBytes(cls));
        }
        Ok(())
    }
}

/// The internal implementation of a translator.
///
/// This type is responsible for carrying around the original pattern string,
/// which is not tied to the internal state of a translator.
///
/// A TranslatorI exists for the time it takes to translate a single Ast.
#[derive(Clone, Debug)]
struct TranslatorI<'t, 'p> {
    trans: &'t Translator,
    pattern: &'p str,
}

impl<'t, 'p> TranslatorI<'t, 'p> {
    /// Build a new internal translator.
    fn new(trans: &'t Translator, pattern: &'p str) -> TranslatorI<'t, 'p> {
        TranslatorI { trans: trans, pattern: pattern }
    }

    /// Return a reference to the underlying translator.
    fn trans(&self) -> &Translator {
        &self.trans
    }

    /// Push the given frame on to the call stack.
    fn push(&self, frame: HirFrame) {
        self.trans().stack.borrow_mut().push(frame);
    }

    /// Pop the top of the call stack. If the call stack is empty, return None.
    fn pop(&self) -> Option<HirFrame> {
        self.trans().stack.borrow_mut().pop()
    }

    /// Create a new error with the given span and error type.
    fn error(&self, span: Span, kind: ErrorKind) -> Error {
        Error { kind: kind, pattern: self.pattern.to_string(), span: span }
    }

    /// Return a copy of the active flags.
    fn flags(&self) -> Flags {
        self.trans().flags.get()
    }

    /// Set the flags of this translator from the flags set in the given AST.
    /// Then, return the old flags.
    fn set_flags(&self, ast_flags: &ast::Flags) -> Flags {
        let old_flags = self.flags();
        let mut new_flags = Flags::from_ast(ast_flags);
        new_flags.merge(&old_flags);
        self.trans().flags.set(new_flags);
        old_flags
    }

    fn hir_literal(&self, lit: &ast::Literal) -> Result<Hir> {
        let ch = match self.literal_to_char(lit)? {
            byte @ hir::Literal::Byte(_) => return Ok(Hir::literal(byte)),
            hir::Literal::Unicode(ch) => ch,
        };
        if self.flags().case_insensitive() {
            self.hir_from_char_case_insensitive(lit.span, ch)
        } else {
            self.hir_from_char(lit.span, ch)
        }
    }

    /// Convert an Ast literal to its scalar representation.
    ///
    /// When Unicode mode is enabled, then this always succeeds and returns a
    /// `char` (Unicode scalar value).
    ///
    /// When Unicode mode is disabled, then a raw byte is returned. If that
    /// byte is not ASCII and invalid UTF-8 is not allowed, then this returns
    /// an error.
    fn literal_to_char(&self, lit: &ast::Literal) -> Result<hir::Literal> {
        if self.flags().unicode() {
            return Ok(hir::Literal::Unicode(lit.c));
        }
        let byte = match lit.byte() {
            None => return Ok(hir::Literal::Unicode(lit.c)),
            Some(byte) => byte,
        };
        if byte <= 0x7F {
            return Ok(hir::Literal::Unicode(byte as char));
        }
        if !self.trans().allow_invalid_utf8 {
            return Err(self.error(lit.span, ErrorKind::InvalidUtf8));
        }
        Ok(hir::Literal::Byte(byte))
    }

    fn hir_from_char(&self, span: Span, c: char) -> Result<Hir> {
        if !self.flags().unicode() && c.len_utf8() > 1 {
            return Err(self.error(span, ErrorKind::UnicodeNotAllowed));
        }
        Ok(Hir::literal(hir::Literal::Unicode(c)))
    }

    fn hir_from_char_case_insensitive(
        &self,
        span: Span,
        c: char,
    ) -> Result<Hir> {
        // If case folding won't do anything, then don't bother trying.
        if !unicode::contains_simple_case_mapping(c, c) {
            return self.hir_from_char(span, c);
        }
        if self.flags().unicode() {
            let mut cls = hir::ClassUnicode::new(vec![
                hir::ClassUnicodeRange::new(c, c),
            ]);
            cls.case_fold_simple();
            Ok(Hir::class(hir::Class::Unicode(cls)))
        } else {
            if c.len_utf8() > 1 {
                return Err(self.error(span, ErrorKind::UnicodeNotAllowed));
            }
            let mut cls = hir::ClassBytes::new(vec![
                hir::ClassBytesRange::new(c as u8, c as u8),
            ]);
            cls.case_fold_simple();
            Ok(Hir::class(hir::Class::Bytes(cls)))
        }
    }

    fn hir_dot(&self, span: Span) -> Result<Hir> {
        let unicode = self.flags().unicode();
        if !unicode && !self.trans().allow_invalid_utf8 {
            return Err(self.error(span, ErrorKind::InvalidUtf8));
        }
        Ok(if self.flags().dot_matches_new_line() {
            Hir::any(!unicode)
        } else {
            Hir::dot(!unicode)
        })
    }

    fn hir_assertion(&self, asst: &ast::Assertion) -> Result<Hir> {
        let unicode = self.flags().unicode();
        let multi_line = self.flags().multi_line();
        Ok(match asst.kind {
            ast::AssertionKind::StartLine => {
                Hir::anchor(if multi_line {
                    hir::Anchor::StartLine
                } else {
                    hir::Anchor::StartText
                })
            }
            ast::AssertionKind::EndLine => {
                Hir::anchor(if multi_line {
                    hir::Anchor::EndLine
                } else {
                    hir::Anchor::EndText
                })
            }
            ast::AssertionKind::StartText => {
                Hir::anchor(hir::Anchor::StartText)
            }
            ast::AssertionKind::EndText => {
                Hir::anchor(hir::Anchor::EndText)
            }
            ast::AssertionKind::WordBoundary => {
                Hir::word_boundary(if unicode {
                    hir::WordBoundary::Unicode
                } else {
                    hir::WordBoundary::Ascii
                })
            }
            ast::AssertionKind::NotWordBoundary => {
                Hir::word_boundary(if unicode {
                    hir::WordBoundary::UnicodeNegate
                } else {
                    // It is possible for negated ASCII word boundaries to
                    // match at invalid UTF-8 boundaries, even when searching
                    // valid UTF-8.
                    if !self.trans().allow_invalid_utf8 {
                        return Err(self.error(
                            asst.span, ErrorKind::InvalidUtf8));
                    }
                    hir::WordBoundary::AsciiNegate
                })
            }
        })
    }

    fn hir_group(&self, group: &ast::Group, expr: Hir) -> Hir {
        let kind = match group.kind {
            ast::GroupKind::CaptureIndex(idx) => {
                hir::GroupKind::CaptureIndex(idx)
            }
            ast::GroupKind::CaptureName(ref capname) => {
                hir::GroupKind::CaptureName {
                    name: capname.name.clone(),
                    index: capname.index,
                }
            }
            ast::GroupKind::NonCapturing(_) => hir::GroupKind::NonCapturing,
        };
        Hir::group(hir::Group {
            kind: kind,
            hir: Box::new(expr),
        })
    }

    fn hir_repetition(&self, rep: &ast::Repetition, expr: Hir) -> Hir {
        let kind = match rep.op.kind {
            ast::RepetitionKind::ZeroOrOne => hir::RepetitionKind::ZeroOrOne,
            ast::RepetitionKind::ZeroOrMore => hir::RepetitionKind::ZeroOrMore,
            ast::RepetitionKind::OneOrMore => hir::RepetitionKind::OneOrMore,
            ast::RepetitionKind::Range(ast::RepetitionRange::Exactly(m)) => {
                hir::RepetitionKind::Range(hir::RepetitionRange::Exactly(m))
            }
            ast::RepetitionKind::Range(ast::RepetitionRange::AtLeast(m)) => {
                hir::RepetitionKind::Range(hir::RepetitionRange::AtLeast(m))
            }
            ast::RepetitionKind::Range(ast::RepetitionRange::Bounded(m,n)) => {
                hir::RepetitionKind::Range(hir::RepetitionRange::Bounded(m, n))
            }
        };
        let greedy =
            if self.flags().swap_greed() {
                !rep.greedy
            } else {
                rep.greedy
            };
        Hir::repetition(hir::Repetition {
            kind: kind,
            greedy: greedy,
            hir: Box::new(expr),
        })
    }

    fn hir_unicode_class(
        &self,
        ast_class: &ast::ClassUnicode,
    ) -> Result<hir::ClassUnicode> {
        use ast::ClassUnicodeKind::*;

        if !self.flags().unicode() {
            return Err(self.error(
                ast_class.span,
                ErrorKind::UnicodeNotAllowed,
            ));
        }
        let query = match ast_class.kind {
            OneLetter(name) => ClassQuery::OneLetter(name),
            Named(ref name) => ClassQuery::Binary(name),
            NamedValue { ref name, ref value, .. } => {
                ClassQuery::ByValue {
                    property_name: name,
                    property_value: value,
                }
            }
        };
        match unicode::class(query) {
            Ok(mut class) => {
                self.unicode_fold_and_negate(ast_class.negated, &mut class);
                Ok(class)
            }
            Err(unicode::Error::PropertyNotFound) => {
                Err(self.error(
                    ast_class.span,
                    ErrorKind::UnicodePropertyNotFound,
                ))
            }
            Err(unicode::Error::PropertyValueNotFound) => {
                Err(self.error(
                    ast_class.span,
                    ErrorKind::UnicodePropertyValueNotFound,
                ))
            }
        }
    }

    fn hir_perl_unicode_class(
        &self,
        ast_class: &ast::ClassPerl,
    ) -> hir::ClassUnicode {
        use ast::ClassPerlKind::*;
        use unicode_tables::perl_word::PERL_WORD;

        assert!(self.flags().unicode());
        let mut class = match ast_class.kind {
            Digit => {
                let query = ClassQuery::Binary("Decimal_Number");
                unicode::class(query).unwrap()
            }
            Space => {
                let query = ClassQuery::Binary("Whitespace");
                unicode::class(query).unwrap()
            }
            Word => unicode::hir_class(PERL_WORD),
        };
        // We needn't apply case folding here because the Perl Unicode classes
        // are already closed under Unicode simple case folding.
        if ast_class.negated {
            class.negate();
        }
        class
    }

    fn hir_perl_byte_class(
        &self,
        ast_class: &ast::ClassPerl,
    ) -> hir::ClassBytes {
        use ast::ClassPerlKind::*;

        assert!(!self.flags().unicode());
        let mut class = match ast_class.kind {
            Digit => hir_ascii_class_bytes(&ast::ClassAsciiKind::Digit),
            Space => hir_ascii_class_bytes(&ast::ClassAsciiKind::Space),
            Word => hir_ascii_class_bytes(&ast::ClassAsciiKind::Word),
        };
        // We needn't apply case folding here because the Perl ASCII classes
        // are already closed (under ASCII case folding).
        if ast_class.negated {
            class.negate();
        }
        class
    }

    fn unicode_fold_and_negate(
        &self,
        negated: bool,
        class: &mut hir::ClassUnicode,
    ) {
        // Note that we must apply case folding before negation!
        // Consider `(?i)[^x]`. If we applied negation field, then
        // the result would be the character class that matched any
        // Unicode scalar value.
        if self.flags().case_insensitive() {
            class.case_fold_simple();
        }
        if negated {
            class.negate();
        }
    }

    fn bytes_fold_and_negate(
        &self,
        span: &Span,
        negated: bool,
        class: &mut hir::ClassBytes,
    ) -> Result<()> {
        // Note that we must apply case folding before negation!
        // Consider `(?i)[^x]`. If we applied negation field, then
        // the result would be the character class that matched any
        // Unicode scalar value.
        if self.flags().case_insensitive() {
            class.case_fold_simple();
        }
        if negated {
            class.negate();
        }
        if !self.trans().allow_invalid_utf8 && !class.is_all_ascii() {
            return Err(self.error(span.clone(), ErrorKind::InvalidUtf8));
        }
        Ok(())
    }

    /// Return a scalar byte value suitable for use as a literal in a byte
    /// character class.
    fn class_literal_byte(&self, ast: &ast::Literal) -> Result<u8> {
        match self.literal_to_char(ast)? {
            hir::Literal::Byte(byte) => Ok(byte),
            hir::Literal::Unicode(ch) => {
                if ch <= 0x7F as char {
                    Ok(ch as u8)
                } else {
                    // We can't feasibly support Unicode in
                    // byte oriented classes. Byte classes don't
                    // do Unicode case folding.
                    Err(self.error(ast.span, ErrorKind::UnicodeNotAllowed))
                }
            }
        }
    }
}

/// A translator's representation of a regular expression's flags at any given
/// moment in time.
///
/// Each flag can be in one of three states: absent, present but disabled or
/// present but enabled.
#[derive(Clone, Copy, Debug, Default)]
struct Flags {
    case_insensitive: Option<bool>,
    multi_line: Option<bool>,
    dot_matches_new_line: Option<bool>,
    swap_greed: Option<bool>,
    unicode: Option<bool>,
    // Note that `ignore_whitespace` is omitted here because it is handled
    // entirely in the parser.
}

impl Flags {
    fn from_ast(ast: &ast::Flags) -> Flags {
        let mut flags = Flags::default();
        let mut enable = true;
        for item in &ast.items {
            match item.kind {
                ast::FlagsItemKind::Negation => {
                    enable = false;
                }
                ast::FlagsItemKind::Flag(ast::Flag::CaseInsensitive) => {
                    flags.case_insensitive = Some(enable);
                }
                ast::FlagsItemKind::Flag(ast::Flag::MultiLine) => {
                    flags.multi_line = Some(enable);
                }
                ast::FlagsItemKind::Flag(ast::Flag::DotMatchesNewLine) => {
                    flags.dot_matches_new_line = Some(enable);
                }
                ast::FlagsItemKind::Flag(ast::Flag::SwapGreed) => {
                    flags.swap_greed = Some(enable);
                }
                ast::FlagsItemKind::Flag(ast::Flag::Unicode) => {
                    flags.unicode = Some(enable);
                }
                ast::FlagsItemKind::Flag(ast::Flag::IgnoreWhitespace) => {}
            }
        }
        flags
    }

    fn merge(&mut self, previous: &Flags) {
        if self.case_insensitive.is_none() {
            self.case_insensitive = previous.case_insensitive;
        }
        if self.multi_line.is_none() {
            self.multi_line = previous.multi_line;
        }
        if self.dot_matches_new_line.is_none() {
            self.dot_matches_new_line = previous.dot_matches_new_line;
        }
        if self.swap_greed.is_none() {
            self.swap_greed = previous.swap_greed;
        }
        if self.unicode.is_none() {
            self.unicode = previous.unicode;
        }
    }

    fn case_insensitive(&self) -> bool {
        self.case_insensitive.unwrap_or(false)
    }

    fn multi_line(&self) -> bool {
        self.multi_line.unwrap_or(false)
    }

    fn dot_matches_new_line(&self) -> bool {
        self.dot_matches_new_line.unwrap_or(false)
    }

    fn swap_greed(&self) -> bool {
        self.swap_greed.unwrap_or(false)
    }

    fn unicode(&self) -> bool {
        self.unicode.unwrap_or(true)
    }
}

fn hir_ascii_class_bytes(kind: &ast::ClassAsciiKind) -> hir::ClassBytes {
    let ranges: Vec<_> = ascii_class(kind).iter().cloned().map(|(s, e)| {
        hir::ClassBytesRange::new(s as u8, e as u8)
    }).collect();
    hir::ClassBytes::new(ranges)
}

fn ascii_class(kind: &ast::ClassAsciiKind) -> &'static [(char, char)] {
    use ast::ClassAsciiKind::*;

    // The contortions below with `const` appear necessary for older versions
    // of Rust.
    type T = &'static [(char, char)];
    match *kind {
        Alnum => {
            const X: T = &[('0', '9'), ('A', 'Z'), ('a', 'z')];
            X
        }
        Alpha => {
            const X: T = &[('A', 'Z'), ('a', 'z')];
            X
        }
        Ascii => {
            const X: T = &[('\x00', '\x7F')];
            X
        }
        Blank => {
            const X: T = &[('\t', '\t'), (' ', ' ')];
            X
        }
        Cntrl => {
            const X: T = &[('\x00', '\x1F'), ('\x7F', '\x7F')];
            X
        }
        Digit => {
            const X: T = &[('0', '9')];
            X
        }
        Graph => {
            const X: T = &[('!', '~')];
            X
        }
        Lower => {
            const X: T = &[('a', 'z')];
            X
        }
        Print => {
            const X: T = &[(' ', '~')];
            X
        }
        Punct => {
            const X: T = &[('!', '/'), (':', '@'), ('[', '`'), ('{', '~')];
            X
        }
        Space => {
            const X: T = &[
                ('\t', '\t'), ('\n', '\n'), ('\x0B', '\x0B'), ('\x0C', '\x0C'),
                ('\r', '\r'), (' ', ' '),
            ];
            X
        }
        Upper => {
            const X: T = &[('A', 'Z')];
            X
        }
        Word => {
            const X: T = &[('0', '9'), ('A', 'Z'), ('_', '_'), ('a', 'z')];
            X
        }
        Xdigit => {
            const X: T = &[('0', '9'), ('A', 'F'), ('a', 'f')];
            X
        }
    }
}

#[cfg(test)]
mod tests {
    use ast::{self, Ast, Position, Span};
    use ast::parse::ParserBuilder;
    use hir::{self, Hir, HirKind};
    use unicode::{self, ClassQuery};

    use super::{TranslatorBuilder, ascii_class};

    // We create these errors to compare with real hir::Errors in the tests.
    // We define equality between TestError and hir::Error to disregard the
    // pattern string in hir::Error, which is annoying to provide in tests.
    #[derive(Clone, Debug)]
    struct TestError {
        span: Span,
        kind: hir::ErrorKind,
    }

    impl PartialEq<hir::Error> for TestError {
        fn eq(&self, other: &hir::Error) -> bool {
            self.span == other.span && self.kind == other.kind
        }
    }

    impl PartialEq<TestError> for hir::Error {
        fn eq(&self, other: &TestError) -> bool {
            self.span == other.span && self.kind == other.kind
        }
    }

    fn parse(pattern: &str) -> Ast {
        ParserBuilder::new().octal(true).build().parse(pattern).unwrap()
    }

    fn t(pattern: &str) -> Hir {
        TranslatorBuilder::new()
            .allow_invalid_utf8(false)
            .build()
            .translate(pattern, &parse(pattern))
            .unwrap()
    }

    fn t_err(pattern: &str) -> hir::Error {
        TranslatorBuilder::new()
            .allow_invalid_utf8(false)
            .build()
            .translate(pattern, &parse(pattern))
            .unwrap_err()
    }

    fn t_bytes(pattern: &str) -> Hir {
        TranslatorBuilder::new()
            .allow_invalid_utf8(true)
            .build()
            .translate(pattern, &parse(pattern))
            .unwrap()
    }

    fn hir_lit(s: &str) -> Hir {
        match s.len() {
            0 => Hir::empty(),
            _ => {
                let lits = s
                    .chars()
                    .map(hir::Literal::Unicode)
                    .map(Hir::literal)
                    .collect();
                Hir::concat(lits)
            }
        }
    }

    fn hir_blit(s: &[u8]) -> Hir {
        match s.len() {
            0 => Hir::empty(),
            1 => Hir::literal(hir::Literal::Byte(s[0])),
            _ => {
                let lits = s
                    .iter()
                    .cloned()
                    .map(hir::Literal::Byte)
                    .map(Hir::literal)
                    .collect();
                Hir::concat(lits)
            }
        }
    }

    fn hir_group(i: u32, expr: Hir)  -> Hir {
        Hir::group(hir::Group {
            kind: hir::GroupKind::CaptureIndex(i),
            hir: Box::new(expr),
        })
    }

    fn hir_group_name(i: u32, name: &str, expr: Hir)  -> Hir {
        Hir::group(hir::Group {
            kind: hir::GroupKind::CaptureName {
                name: name.to_string(),
                index: i,
            },
            hir: Box::new(expr),
        })
    }

    fn hir_group_nocap(expr: Hir)  -> Hir {
        Hir::group(hir::Group {
            kind: hir::GroupKind::NonCapturing,
            hir: Box::new(expr),
        })
    }

    fn hir_quest(greedy: bool, expr: Hir) -> Hir {
        Hir::repetition(hir::Repetition {
            kind: hir::RepetitionKind::ZeroOrOne,
            greedy: greedy,
            hir: Box::new(expr),
        })
    }

    fn hir_star(greedy: bool, expr: Hir) -> Hir {
        Hir::repetition(hir::Repetition {
            kind: hir::RepetitionKind::ZeroOrMore,
            greedy: greedy,
            hir: Box::new(expr),
        })
    }

    fn hir_plus(greedy: bool, expr: Hir) -> Hir {
        Hir::repetition(hir::Repetition {
            kind: hir::RepetitionKind::OneOrMore,
            greedy: greedy,
            hir: Box::new(expr),
        })
    }

    fn hir_range(greedy: bool, range: hir::RepetitionRange, expr: Hir) -> Hir {
        Hir::repetition(hir::Repetition {
            kind: hir::RepetitionKind::Range(range),
            greedy: greedy,
            hir: Box::new(expr),
        })
    }

    fn hir_alt(alts: Vec<Hir>) -> Hir {
        Hir::alternation(alts)
    }

    fn hir_cat(exprs: Vec<Hir>) -> Hir {
        Hir::concat(exprs)
    }

    fn hir_uclass_query(query: ClassQuery) -> Hir {
        Hir::class(hir::Class::Unicode(unicode::class(query).unwrap()))
    }

    fn hir_uclass_perl_word() -> Hir {
        use unicode_tables::perl_word::PERL_WORD;
        Hir::class(hir::Class::Unicode(unicode::hir_class(PERL_WORD)))
    }

    fn hir_uclass(ranges: &[(char, char)]) -> Hir {
        let ranges: Vec<hir::ClassUnicodeRange> = ranges
            .iter()
            .map(|&(s, e)| hir::ClassUnicodeRange::new(s, e))
            .collect();
        Hir::class(hir::Class::Unicode(hir::ClassUnicode::new(ranges)))
    }

    fn hir_bclass(ranges: &[(u8, u8)]) -> Hir {
        let ranges: Vec<hir::ClassBytesRange> = ranges
            .iter()
            .map(|&(s, e)| hir::ClassBytesRange::new(s, e))
            .collect();
        Hir::class(hir::Class::Bytes(hir::ClassBytes::new(ranges)))
    }

    fn hir_bclass_from_char(ranges: &[(char, char)]) -> Hir {
        let ranges: Vec<hir::ClassBytesRange> = ranges
            .iter()
            .map(|&(s, e)| {
                assert!(s as u32 <= 0x7F);
                assert!(e as u32 <= 0x7F);
                hir::ClassBytesRange::new(s as u8, e as u8)
            })
            .collect();
        Hir::class(hir::Class::Bytes(hir::ClassBytes::new(ranges)))
    }

    fn hir_case_fold(expr: Hir) -> Hir {
        match expr.into_kind() {
            HirKind::Class(mut cls) => {
                cls.case_fold_simple();
                Hir::class(cls)
            }
            _ => panic!("cannot case fold non-class Hir expr"),
        }
    }

    fn hir_negate(expr: Hir) -> Hir {
        match expr.into_kind() {
            HirKind::Class(mut cls) => {
                cls.negate();
                Hir::class(cls)
            }
            _ => panic!("cannot negate non-class Hir expr"),
        }
    }

    fn hir_union(expr1: Hir, expr2: Hir) -> Hir {
        use hir::Class::{Bytes, Unicode};

        match (expr1.into_kind(), expr2.into_kind()) {
            (
                HirKind::Class(Unicode(mut c1)),
                HirKind::Class(Unicode(c2)),
            ) => {
                c1.union(&c2);
                Hir::class(hir::Class::Unicode(c1))
            }
            (
                HirKind::Class(Bytes(mut c1)),
                HirKind::Class(Bytes(c2)),
            ) => {
                c1.union(&c2);
                Hir::class(hir::Class::Bytes(c1))
            }
            _ => panic!("cannot union non-class Hir exprs"),
        }
    }

    fn hir_difference(expr1: Hir, expr2: Hir) -> Hir {
        use hir::Class::{Bytes, Unicode};

        match (expr1.into_kind(), expr2.into_kind()) {
            (
                HirKind::Class(Unicode(mut c1)),
                HirKind::Class(Unicode(c2)),
            ) => {
                c1.difference(&c2);
                Hir::class(hir::Class::Unicode(c1))
            }
            (
                HirKind::Class(Bytes(mut c1)),
                HirKind::Class(Bytes(c2)),
            ) => {
                c1.difference(&c2);
                Hir::class(hir::Class::Bytes(c1))
            }
            _ => panic!("cannot difference non-class Hir exprs"),
        }
    }

    fn hir_anchor(anchor: hir::Anchor) -> Hir {
        Hir::anchor(anchor)
    }

    fn hir_word(wb: hir::WordBoundary) -> Hir {
        Hir::word_boundary(wb)
    }

    #[test]
    fn empty() {
        assert_eq!(t(""), Hir::empty());
        assert_eq!(t("(?i)"), Hir::empty());
        assert_eq!(t("()"), hir_group(1, Hir::empty()));
        assert_eq!(t("(?:)"), hir_group_nocap(Hir::empty()));
        assert_eq!(t("(?P<wat>)"), hir_group_name(1, "wat", Hir::empty()));
        assert_eq!(t("|"), hir_alt(vec![Hir::empty(), Hir::empty()]));
        assert_eq!(t("()|()"), hir_alt(vec![
            hir_group(1, Hir::empty()),
            hir_group(2, Hir::empty()),
        ]));
        assert_eq!(t("(|b)"), hir_group(1, hir_alt(vec![
            Hir::empty(),
            hir_lit("b"),
        ])));
        assert_eq!(t("(a|)"), hir_group(1, hir_alt(vec![
            hir_lit("a"),
            Hir::empty(),
        ])));
        assert_eq!(t("(a||c)"), hir_group(1, hir_alt(vec![
            hir_lit("a"),
            Hir::empty(),
            hir_lit("c"),
        ])));
        assert_eq!(t("(||)"), hir_group(1, hir_alt(vec![
            Hir::empty(),
            Hir::empty(),
            Hir::empty(),
        ])));
    }

    #[test]
    fn literal() {
        assert_eq!(t("a"), hir_lit("a"));
        assert_eq!(t("(?-u)a"), hir_lit("a"));
        assert_eq!(t("☃"), hir_lit("☃"));
        assert_eq!(t("abcd"), hir_lit("abcd"));

        assert_eq!(t_bytes("(?-u)a"), hir_lit("a"));
        assert_eq!(t_bytes("(?-u)\x61"), hir_lit("a"));
        assert_eq!(t_bytes(r"(?-u)\x61"), hir_lit("a"));
        assert_eq!(t_bytes(r"(?-u)\xFF"), hir_blit(b"\xFF"));

        assert_eq!(t_err("(?-u)☃"), TestError {
            kind: hir::ErrorKind::UnicodeNotAllowed,
            span: Span::new(Position::new(5, 1, 6), Position::new(8, 1, 7)),
        });
        assert_eq!(t_err(r"(?-u)\xFF"), TestError {
            kind: hir::ErrorKind::InvalidUtf8,
            span: Span::new(Position::new(5, 1, 6), Position::new(9, 1, 10)),
        });
    }

    #[test]
    fn literal_case_insensitive() {
        assert_eq!(t("(?i)a"), hir_uclass(&[
            ('A', 'A'), ('a', 'a'),
        ]));
        assert_eq!(t("(?i:a)"), hir_group_nocap(hir_uclass(&[
            ('A', 'A'), ('a', 'a')],
        )));
        assert_eq!(t("a(?i)a(?-i)a"), hir_cat(vec![
            hir_lit("a"),
            hir_uclass(&[('A', 'A'), ('a', 'a')]),
            hir_lit("a"),
        ]));
        assert_eq!(t("(?i)ab@c"), hir_cat(vec![
            hir_uclass(&[('A', 'A'), ('a', 'a')]),
            hir_uclass(&[('B', 'B'), ('b', 'b')]),
            hir_lit("@"),
            hir_uclass(&[('C', 'C'), ('c', 'c')]),
        ]));
        assert_eq!(t("(?i)β"), hir_uclass(&[
            ('Β', 'Β'), ('β', 'β'), ('ϐ', 'ϐ'),
        ]));

        assert_eq!(t("(?i-u)a"), hir_bclass(&[
            (b'A', b'A'), (b'a', b'a'),
        ]));
        assert_eq!(t("(?-u)a(?i)a(?-i)a"), hir_cat(vec![
            hir_lit("a"),
            hir_bclass(&[(b'A', b'A'), (b'a', b'a')]),
            hir_lit("a"),
        ]));
        assert_eq!(t("(?i-u)ab@c"), hir_cat(vec![
            hir_bclass(&[(b'A', b'A'), (b'a', b'a')]),
            hir_bclass(&[(b'B', b'B'), (b'b', b'b')]),
            hir_lit("@"),
            hir_bclass(&[(b'C', b'C'), (b'c', b'c')]),
        ]));

        assert_eq!(t_bytes("(?i-u)a"), hir_bclass(&[
            (b'A', b'A'), (b'a', b'a'),
        ]));
        assert_eq!(t_bytes("(?i-u)\x61"), hir_bclass(&[
            (b'A', b'A'), (b'a', b'a'),
        ]));
        assert_eq!(t_bytes(r"(?i-u)\x61"), hir_bclass(&[
            (b'A', b'A'), (b'a', b'a'),
        ]));
        assert_eq!(t_bytes(r"(?i-u)\xFF"), hir_blit(b"\xFF"));

        assert_eq!(t_err("(?i-u)β"), TestError {
            kind: hir::ErrorKind::UnicodeNotAllowed,
            span: Span::new(
                Position::new(6, 1, 7),
                Position::new(8, 1, 8),
            ),
        });
    }

    #[test]
    fn dot() {
        assert_eq!(t("."), hir_uclass(&[
            ('\0', '\t'),
            ('\x0B', '\u{10FFFF}'),
        ]));
        assert_eq!(t("(?s)."), hir_uclass(&[
            ('\0', '\u{10FFFF}'),
        ]));
        assert_eq!(t_bytes("(?-u)."), hir_bclass(&[
            (b'\0', b'\t'),
            (b'\x0B', b'\xFF'),
        ]));
        assert_eq!(t_bytes("(?s-u)."), hir_bclass(&[
            (b'\0', b'\xFF'),
        ]));

        // If invalid UTF-8 isn't allowed, then non-Unicode `.` isn't allowed.
        assert_eq!(t_err("(?-u)."), TestError {
            kind: hir::ErrorKind::InvalidUtf8,
            span: Span::new(Position::new(5, 1, 6), Position::new(6, 1, 7)),
        });
        assert_eq!(t_err("(?s-u)."), TestError {
            kind: hir::ErrorKind::InvalidUtf8,
            span: Span::new(Position::new(6, 1, 7), Position::new(7, 1, 8)),
        });
    }

    #[test]
    fn assertions() {
        assert_eq!(t("^"), hir_anchor(hir::Anchor::StartText));
        assert_eq!(t("$"), hir_anchor(hir::Anchor::EndText));
        assert_eq!(t(r"\A"), hir_anchor(hir::Anchor::StartText));
        assert_eq!(t(r"\z"), hir_anchor(hir::Anchor::EndText));
        assert_eq!(t("(?m)^"), hir_anchor(hir::Anchor::StartLine));
        assert_eq!(t("(?m)$"), hir_anchor(hir::Anchor::EndLine));
        assert_eq!(t(r"(?m)\A"), hir_anchor(hir::Anchor::StartText));
        assert_eq!(t(r"(?m)\z"), hir_anchor(hir::Anchor::EndText));

        assert_eq!(t(r"\b"), hir_word(hir::WordBoundary::Unicode));
        assert_eq!(t(r"\B"), hir_word(hir::WordBoundary::UnicodeNegate));
        assert_eq!(t(r"(?-u)\b"), hir_word(hir::WordBoundary::Ascii));
        assert_eq!(
            t_bytes(r"(?-u)\B"),
            hir_word(hir::WordBoundary::AsciiNegate));

        assert_eq!(t_err(r"(?-u)\B"), TestError {
            kind: hir::ErrorKind::InvalidUtf8,
            span: Span::new(Position::new(5, 1, 6), Position::new(7, 1, 8)),
        });
    }

    #[test]
    fn group() {
        assert_eq!(t("(a)"), hir_group(1, hir_lit("a")));
        assert_eq!(t("(a)(b)"), hir_cat(vec![
            hir_group(1, hir_lit("a")),
            hir_group(2, hir_lit("b")),
        ]));
        assert_eq!(t("(a)|(b)"), hir_alt(vec![
            hir_group(1, hir_lit("a")),
            hir_group(2, hir_lit("b")),
        ]));
        assert_eq!(t("(?P<foo>)"), hir_group_name(1, "foo", Hir::empty()));
        assert_eq!(t("(?P<foo>a)"), hir_group_name(1, "foo", hir_lit("a")));
        assert_eq!(t("(?P<foo>a)(?P<bar>b)"), hir_cat(vec![
            hir_group_name(1, "foo", hir_lit("a")),
            hir_group_name(2, "bar", hir_lit("b")),
        ]));
        assert_eq!(t("(?:)"), hir_group_nocap(Hir::empty()));
        assert_eq!(t("(?:a)"), hir_group_nocap(hir_lit("a")));
        assert_eq!(t("(?:a)(b)"), hir_cat(vec![
            hir_group_nocap(hir_lit("a")),
            hir_group(1, hir_lit("b")),
        ]));
        assert_eq!(t("(a)(?:b)(c)"), hir_cat(vec![
            hir_group(1, hir_lit("a")),
            hir_group_nocap(hir_lit("b")),
            hir_group(2, hir_lit("c")),
        ]));
        assert_eq!(t("(a)(?P<foo>b)(c)"), hir_cat(vec![
            hir_group(1, hir_lit("a")),
            hir_group_name(2, "foo", hir_lit("b")),
            hir_group(3, hir_lit("c")),
        ]));
        assert_eq!(t("()"), hir_group(1, Hir::empty()));
        assert_eq!(t("((?i))"), hir_group(1, Hir::empty()));
        assert_eq!(t("((?x))"), hir_group(1, Hir::empty()));
        assert_eq!(t("(((?x)))"), hir_group(1, hir_group(2, Hir::empty())));
    }

    #[test]
    fn flags() {
        assert_eq!(t("(?i:a)a"), hir_cat(vec![
            hir_group_nocap(hir_uclass(&[('A', 'A'), ('a', 'a')])),
            hir_lit("a"),
        ]));
        assert_eq!(t("(?i-u:a)β"), hir_cat(vec![
            hir_group_nocap(hir_bclass(&[(b'A', b'A'), (b'a', b'a')])),
            hir_lit("β"),
        ]));
        assert_eq!(t("(?i)(?-i:a)a"), hir_cat(vec![
            hir_group_nocap(hir_lit("a")),
            hir_uclass(&[('A', 'A'), ('a', 'a')]),
        ]));
        assert_eq!(t("(?im)a^"), hir_cat(vec![
            hir_uclass(&[('A', 'A'), ('a', 'a')]),
            hir_anchor(hir::Anchor::StartLine),
        ]));
        assert_eq!(t("(?im)a^(?i-m)a^"), hir_cat(vec![
            hir_uclass(&[('A', 'A'), ('a', 'a')]),
            hir_anchor(hir::Anchor::StartLine),
            hir_uclass(&[('A', 'A'), ('a', 'a')]),
            hir_anchor(hir::Anchor::StartText),
        ]));
        assert_eq!(t("(?U)a*a*?(?-U)a*a*?"), hir_cat(vec![
            hir_star(false, hir_lit("a")),
            hir_star(true, hir_lit("a")),
            hir_star(true, hir_lit("a")),
            hir_star(false, hir_lit("a")),
        ]));
        assert_eq!(t("(?:a(?i)a)a"), hir_cat(vec![
            hir_group_nocap(hir_cat(vec![
                hir_lit("a"),
                hir_uclass(&[('A', 'A'), ('a', 'a')]),
            ])),
            hir_lit("a"),
        ]));
        assert_eq!(t("(?i)(?:a(?-i)a)a"), hir_cat(vec![
            hir_group_nocap(hir_cat(vec![
                hir_uclass(&[('A', 'A'), ('a', 'a')]),
                hir_lit("a"),
            ])),
            hir_uclass(&[('A', 'A'), ('a', 'a')]),
        ]));
    }

    #[test]
    fn escape() {
        assert_eq!(
            t(r"\\\.\+\*\?\(\)\|\[\]\{\}\^\$\#"),
            hir_lit(r"\.+*?()|[]{}^$#"));
    }

    #[test]
    fn repetition() {
        assert_eq!(t("a?"), hir_quest(true, hir_lit("a")));
        assert_eq!(t("a*"), hir_star(true, hir_lit("a")));
        assert_eq!(t("a+"), hir_plus(true, hir_lit("a")));
        assert_eq!(t("a??"), hir_quest(false, hir_lit("a")));
        assert_eq!(t("a*?"), hir_star(false, hir_lit("a")));
        assert_eq!(t("a+?"), hir_plus(false, hir_lit("a")));

        assert_eq!(
            t("a{1}"),
            hir_range(
                true,
                hir::RepetitionRange::Exactly(1),
                hir_lit("a"),
            ));
        assert_eq!(
            t("a{1,}"),
            hir_range(
                true,
                hir::RepetitionRange::AtLeast(1),
                hir_lit("a"),
            ));
        assert_eq!(
            t("a{1,2}"),
            hir_range(
                true,
                hir::RepetitionRange::Bounded(1, 2),
                hir_lit("a"),
            ));
        assert_eq!(
            t("a{1}?"),
            hir_range(
                false,
                hir::RepetitionRange::Exactly(1),
                hir_lit("a"),
            ));
        assert_eq!(
            t("a{1,}?"),
            hir_range(
                false,
                hir::RepetitionRange::AtLeast(1),
                hir_lit("a"),
            ));
        assert_eq!(
            t("a{1,2}?"),
            hir_range(
                false,
                hir::RepetitionRange::Bounded(1, 2),
                hir_lit("a"),
            ));

        assert_eq!(t("ab?"), hir_cat(vec![
            hir_lit("a"),
            hir_quest(true, hir_lit("b")),
        ]));
        assert_eq!(t("(ab)?"), hir_quest(true, hir_group(1, hir_cat(vec![
            hir_lit("a"),
            hir_lit("b"),
        ]))));
        assert_eq!(t("a|b?"), hir_alt(vec![
            hir_lit("a"),
            hir_quest(true, hir_lit("b")),
        ]));
    }

    #[test]
    fn cat_alt() {
        assert_eq!(t("(ab)"), hir_group(1, hir_cat(vec![
            hir_lit("a"),
            hir_lit("b"),
        ])));
        assert_eq!(t("a|b"), hir_alt(vec![
            hir_lit("a"),
            hir_lit("b"),
        ]));
        assert_eq!(t("a|b|c"), hir_alt(vec![
            hir_lit("a"),
            hir_lit("b"),
            hir_lit("c"),
        ]));
        assert_eq!(t("ab|bc|cd"), hir_alt(vec![
            hir_lit("ab"),
            hir_lit("bc"),
            hir_lit("cd"),
        ]));
        assert_eq!(t("(a|b)"), hir_group(1, hir_alt(vec![
            hir_lit("a"),
            hir_lit("b"),
        ])));
        assert_eq!(t("(a|b|c)"), hir_group(1, hir_alt(vec![
            hir_lit("a"),
            hir_lit("b"),
            hir_lit("c"),
        ])));
        assert_eq!(t("(ab|bc|cd)"), hir_group(1, hir_alt(vec![
            hir_lit("ab"),
            hir_lit("bc"),
            hir_lit("cd"),
        ])));
        assert_eq!(t("(ab|(bc|(cd)))"), hir_group(1, hir_alt(vec![
            hir_lit("ab"),
            hir_group(2, hir_alt(vec![
                hir_lit("bc"),
                hir_group(3, hir_lit("cd")),
            ])),
        ])));
    }

    #[test]
    fn class_ascii() {
        assert_eq!(
            t("[[:alnum:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Alnum)));
        assert_eq!(
            t("[[:alpha:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Alpha)));
        assert_eq!(
            t("[[:ascii:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Ascii)));
        assert_eq!(
            t("[[:blank:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Blank)));
        assert_eq!(
            t("[[:cntrl:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Cntrl)));
        assert_eq!(
            t("[[:digit:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Digit)));
        assert_eq!(
            t("[[:graph:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Graph)));
        assert_eq!(
            t("[[:lower:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Lower)));
        assert_eq!(
            t("[[:print:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Print)));
        assert_eq!(
            t("[[:punct:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Punct)));
        assert_eq!(
            t("[[:space:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Space)));
        assert_eq!(
            t("[[:upper:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Upper)));
        assert_eq!(
            t("[[:word:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Word)));
        assert_eq!(
            t("[[:xdigit:]]"),
            hir_uclass(ascii_class(&ast::ClassAsciiKind::Xdigit)));

        assert_eq!(
            t("[[:^lower:]]"),
            hir_negate(hir_uclass(ascii_class(&ast::ClassAsciiKind::Lower))));
        assert_eq!(
            t("(?i)[[:lower:]]"),
            hir_uclass(&[
                ('A', 'Z'), ('a', 'z'),
                ('\u{17F}', '\u{17F}'),
                ('\u{212A}', '\u{212A}'),
            ]));

        assert_eq!(
            t("(?-u)[[:lower:]]"),
            hir_bclass_from_char(ascii_class(&ast::ClassAsciiKind::Lower)));
        assert_eq!(
            t("(?i-u)[[:lower:]]"),
            hir_case_fold(hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Lower))));

        assert_eq!(t_err("(?-u)[[:^lower:]]"), TestError {
            kind: hir::ErrorKind::InvalidUtf8,
            span: Span::new(Position::new(6, 1, 7), Position::new(16, 1, 17)),
        });
        assert_eq!(t_err("(?i-u)[[:^lower:]]"), TestError {
            kind: hir::ErrorKind::InvalidUtf8,
            span: Span::new(Position::new(7, 1, 8), Position::new(17, 1, 18)),
        });
    }

    #[test]
    fn class_perl() {
        // Unicode
        assert_eq!(
            t(r"\d"),
            hir_uclass_query(ClassQuery::Binary("digit")));
        assert_eq!(
            t(r"\s"),
            hir_uclass_query(ClassQuery::Binary("space")));
        assert_eq!(
            t(r"\w"),
            hir_uclass_perl_word());
        assert_eq!(
            t(r"(?i)\d"),
            hir_uclass_query(ClassQuery::Binary("digit")));
        assert_eq!(
            t(r"(?i)\s"),
            hir_uclass_query(ClassQuery::Binary("space")));
        assert_eq!(
            t(r"(?i)\w"),
            hir_uclass_perl_word());

        // Unicode, negated
        assert_eq!(
            t(r"\D"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("digit"))));
        assert_eq!(
            t(r"\S"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("space"))));
        assert_eq!(
            t(r"\W"),
            hir_negate(hir_uclass_perl_word()));
        assert_eq!(
            t(r"(?i)\D"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("digit"))));
        assert_eq!(
            t(r"(?i)\S"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("space"))));
        assert_eq!(
            t(r"(?i)\W"),
            hir_negate(hir_uclass_perl_word()));

        // ASCII only
        assert_eq!(
            t(r"(?-u)\d"),
            hir_bclass_from_char(ascii_class(&ast::ClassAsciiKind::Digit)));
        assert_eq!(
            t(r"(?-u)\s"),
            hir_bclass_from_char(ascii_class(&ast::ClassAsciiKind::Space)));
        assert_eq!(
            t(r"(?-u)\w"),
            hir_bclass_from_char(ascii_class(&ast::ClassAsciiKind::Word)));
        assert_eq!(
            t(r"(?i-u)\d"),
            hir_bclass_from_char(ascii_class(&ast::ClassAsciiKind::Digit)));
        assert_eq!(
            t(r"(?i-u)\s"),
            hir_bclass_from_char(ascii_class(&ast::ClassAsciiKind::Space)));
        assert_eq!(
            t(r"(?i-u)\w"),
            hir_bclass_from_char(ascii_class(&ast::ClassAsciiKind::Word)));

        // ASCII only, negated
        assert_eq!(
            t(r"(?-u)\D"),
            hir_negate(hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Digit))));
        assert_eq!(
            t(r"(?-u)\S"),
            hir_negate(hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Space))));
        assert_eq!(
            t(r"(?-u)\W"),
            hir_negate(hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Word))));
        assert_eq!(
            t(r"(?i-u)\D"),
            hir_negate(hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Digit))));
        assert_eq!(
            t(r"(?i-u)\S"),
            hir_negate(hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Space))));
        assert_eq!(
            t(r"(?i-u)\W"),
            hir_negate(hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Word))));
    }

    #[test]
    fn class_unicode() {
        assert_eq!(
            t(r"\pZ"),
            hir_uclass_query(ClassQuery::Binary("Z")));
        assert_eq!(
            t(r"\pz"),
            hir_uclass_query(ClassQuery::Binary("Z")));
        assert_eq!(
            t(r"\p{Separator}"),
            hir_uclass_query(ClassQuery::Binary("Z")));
        assert_eq!(
            t(r"\p{se      PaRa ToR}"),
            hir_uclass_query(ClassQuery::Binary("Z")));
        assert_eq!(
            t(r"\p{gc:Separator}"),
            hir_uclass_query(ClassQuery::Binary("Z")));
        assert_eq!(
            t(r"\p{gc=Separator}"),
            hir_uclass_query(ClassQuery::Binary("Z")));
        assert_eq!(
            t(r"\p{Other}"),
            hir_uclass_query(ClassQuery::Binary("Other")));
        assert_eq!(
            t(r"\pC"),
            hir_uclass_query(ClassQuery::Binary("Other")));

        assert_eq!(
            t(r"\PZ"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("Z"))));
        assert_eq!(
            t(r"\P{separator}"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("Z"))));
        assert_eq!(
            t(r"\P{gc!=separator}"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("Z"))));

        assert_eq!(
            t(r"\p{Greek}"),
            hir_uclass_query(ClassQuery::Binary("Greek")));
        assert_eq!(
            t(r"(?i)\p{Greek}"),
            hir_case_fold(hir_uclass_query(ClassQuery::Binary("Greek"))));
        assert_eq!(
            t(r"(?i)\P{Greek}"),
            hir_negate(hir_case_fold(hir_uclass_query(
                ClassQuery::Binary("Greek")))));

        assert_eq!(
            t(r"\p{any}"),
            hir_uclass_query(ClassQuery::Binary("Any")));
        assert_eq!(
            t(r"\p{assigned}"),
            hir_uclass_query(ClassQuery::Binary("Assigned")));
        assert_eq!(
            t(r"\p{ascii}"),
            hir_uclass_query(ClassQuery::Binary("ASCII")));
        assert_eq!(
            t(r"\p{gc:any}"),
            hir_uclass_query(ClassQuery::Binary("Any")));
        assert_eq!(
            t(r"\p{gc:assigned}"),
            hir_uclass_query(ClassQuery::Binary("Assigned")));
        assert_eq!(
            t(r"\p{gc:ascii}"),
            hir_uclass_query(ClassQuery::Binary("ASCII")));

        assert_eq!(t_err(r"(?-u)\pZ"), TestError {
            kind: hir::ErrorKind::UnicodeNotAllowed,
            span: Span::new(Position::new(5, 1, 6), Position::new(8, 1, 9)),
        });
        assert_eq!(t_err(r"(?-u)\p{Separator}"), TestError {
            kind: hir::ErrorKind::UnicodeNotAllowed,
            span: Span::new(Position::new(5, 1, 6), Position::new(18, 1, 19)),
        });
        assert_eq!(t_err(r"\pE"), TestError {
            kind: hir::ErrorKind::UnicodePropertyNotFound,
            span: Span::new(Position::new(0, 1, 1), Position::new(3, 1, 4)),
        });
        assert_eq!(t_err(r"\p{Foo}"), TestError {
            kind: hir::ErrorKind::UnicodePropertyNotFound,
            span: Span::new(Position::new(0, 1, 1), Position::new(7, 1, 8)),
        });
        assert_eq!(t_err(r"\p{gc:Foo}"), TestError {
            kind: hir::ErrorKind::UnicodePropertyValueNotFound,
            span: Span::new(Position::new(0, 1, 1), Position::new(10, 1, 11)),
        });
        assert_eq!(t_err(r"\p{sc:Foo}"), TestError {
            kind: hir::ErrorKind::UnicodePropertyValueNotFound,
            span: Span::new(Position::new(0, 1, 1), Position::new(10, 1, 11)),
        });
        assert_eq!(t_err(r"\p{scx:Foo}"), TestError {
            kind: hir::ErrorKind::UnicodePropertyValueNotFound,
            span: Span::new(Position::new(0, 1, 1), Position::new(11, 1, 12)),
        });
        assert_eq!(t_err(r"\p{age:Foo}"), TestError {
            kind: hir::ErrorKind::UnicodePropertyValueNotFound,
            span: Span::new(Position::new(0, 1, 1), Position::new(11, 1, 12)),
        });
    }

    #[test]
    fn class_bracketed() {
        assert_eq!(t("[a]"), hir_uclass(&[('a', 'a')]));
        assert_eq!(t("[^[a]]"), hir_negate(hir_uclass(&[('a', 'a')])));
        assert_eq!(t("[a-z]"), hir_uclass(&[('a', 'z')]));
        assert_eq!(t("[a-fd-h]"), hir_uclass(&[('a', 'h')]));
        assert_eq!(t("[a-fg-m]"), hir_uclass(&[('a', 'm')]));
        assert_eq!(t(r"[\x00]"), hir_uclass(&[('\0', '\0')]));
        assert_eq!(t(r"[\n]"), hir_uclass(&[('\n', '\n')]));
        assert_eq!(t("[\n]"), hir_uclass(&[('\n', '\n')]));
        assert_eq!(
            t(r"[\d]"),
            hir_uclass_query(ClassQuery::Binary("digit")));
        assert_eq!(
            t(r"[\pZ]"),
            hir_uclass_query(ClassQuery::Binary("separator")));
        assert_eq!(
            t(r"[\p{separator}]"),
            hir_uclass_query(ClassQuery::Binary("separator")));
        assert_eq!(
            t(r"[^\D]"),
            hir_uclass_query(ClassQuery::Binary("digit")));
        assert_eq!(
            t(r"[^\PZ]"),
            hir_uclass_query(ClassQuery::Binary("separator")));
        assert_eq!(
            t(r"[^\P{separator}]"),
            hir_uclass_query(ClassQuery::Binary("separator")));
        assert_eq!(
            t(r"(?i)[^\D]"),
            hir_uclass_query(ClassQuery::Binary("digit")));
        assert_eq!(
            t(r"(?i)[^\P{greek}]"),
            hir_case_fold(hir_uclass_query(ClassQuery::Binary("greek"))));

        assert_eq!(t("(?-u)[a]"), hir_bclass(&[(b'a', b'a')]));
        assert_eq!(t(r"(?-u)[\x00]"), hir_bclass(&[(b'\0', b'\0')]));
        assert_eq!(t_bytes(r"(?-u)[\xFF]"), hir_bclass(&[(b'\xFF', b'\xFF')]));

        assert_eq!(t("(?i)[a]"), hir_uclass(&[('A', 'A'), ('a', 'a')]));
        assert_eq!(t("(?i)[k]"), hir_uclass(&[
            ('K', 'K'), ('k', 'k'), ('\u{212A}', '\u{212A}'),
        ]));
        assert_eq!(t("(?i)[β]"), hir_uclass(&[
            ('Β', 'Β'), ('β', 'β'), ('ϐ', 'ϐ'),
        ]));
        assert_eq!(t("(?i-u)[k]"), hir_bclass(&[
            (b'K', b'K'), (b'k', b'k'),
        ]));

        assert_eq!(t("[^a]"), hir_negate(hir_uclass(&[('a', 'a')])));
        assert_eq!(t(r"[^\x00]"), hir_negate(hir_uclass(&[('\0', '\0')])));
        assert_eq!(
            t_bytes("(?-u)[^a]"),
            hir_negate(hir_bclass(&[(b'a', b'a')])));
        assert_eq!(
            t(r"[^\d]"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("digit"))));
        assert_eq!(
            t(r"[^\pZ]"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("separator"))));
        assert_eq!(
            t(r"[^\p{separator}]"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("separator"))));
        assert_eq!(
            t(r"(?i)[^\p{greek}]"),
            hir_negate(hir_case_fold(hir_uclass_query(
                ClassQuery::Binary("greek")))));
        assert_eq!(
            t(r"(?i)[\P{greek}]"),
            hir_negate(hir_case_fold(hir_uclass_query(
                ClassQuery::Binary("greek")))));

        // Test some weird cases.
        assert_eq!(t(r"[\[]"), hir_uclass(&[('[', '[')]));

        assert_eq!(t(r"[&]"), hir_uclass(&[('&', '&')]));
        assert_eq!(t(r"[\&]"), hir_uclass(&[('&', '&')]));
        assert_eq!(t(r"[\&\&]"), hir_uclass(&[('&', '&')]));
        assert_eq!(t(r"[\x00-&]"), hir_uclass(&[('\0', '&')]));
        assert_eq!(t(r"[&-\xFF]"), hir_uclass(&[('&', '\u{FF}')]));

        assert_eq!(t(r"[~]"), hir_uclass(&[('~', '~')]));
        assert_eq!(t(r"[\~]"), hir_uclass(&[('~', '~')]));
        assert_eq!(t(r"[\~\~]"), hir_uclass(&[('~', '~')]));
        assert_eq!(t(r"[\x00-~]"), hir_uclass(&[('\0', '~')]));
        assert_eq!(t(r"[~-\xFF]"), hir_uclass(&[('~', '\u{FF}')]));

        assert_eq!(t(r"[-]"), hir_uclass(&[('-', '-')]));
        assert_eq!(t(r"[\-]"), hir_uclass(&[('-', '-')]));
        assert_eq!(t(r"[\-\-]"), hir_uclass(&[('-', '-')]));
        assert_eq!(t(r"[\x00-\-]"), hir_uclass(&[('\0', '-')]));
        assert_eq!(t(r"[\--\xFF]"), hir_uclass(&[('-', '\u{FF}')]));

        assert_eq!(t_err("(?-u)[^a]"), TestError {
            kind: hir::ErrorKind::InvalidUtf8,
            span: Span::new(Position::new(5, 1, 6), Position::new(9, 1, 10)),
        });
        assert_eq!(t_err(r"[^\s\S]"), TestError {
            kind: hir::ErrorKind::EmptyClassNotAllowed,
            span: Span::new(Position::new(0, 1, 1), Position::new(7, 1, 8)),
        });
        assert_eq!(t_err(r"(?-u)[^\s\S]"), TestError {
            kind: hir::ErrorKind::EmptyClassNotAllowed,
            span: Span::new(Position::new(5, 1, 6), Position::new(12, 1, 13)),
        });
    }

    #[test]
    fn class_bracketed_union() {
        assert_eq!(
            t("[a-zA-Z]"),
            hir_uclass(&[('A', 'Z'), ('a', 'z')]));
        assert_eq!(
            t(r"[a\pZb]"),
            hir_union(
                hir_uclass(&[('a', 'b')]),
                hir_uclass_query(ClassQuery::Binary("separator"))));
        assert_eq!(
            t(r"[\pZ\p{Greek}]"),
            hir_union(
                hir_uclass_query(ClassQuery::Binary("greek")),
                hir_uclass_query(ClassQuery::Binary("separator"))));
        assert_eq!(
            t(r"[\p{age:3.0}\pZ\p{Greek}]"),
            hir_union(
                hir_uclass_query(ClassQuery::ByValue {
                    property_name: "age",
                    property_value: "3.0",
                }),
                hir_union(
                    hir_uclass_query(ClassQuery::Binary("greek")),
                    hir_uclass_query(ClassQuery::Binary("separator")))));
        assert_eq!(
            t(r"[[[\p{age:3.0}\pZ]\p{Greek}][\p{Cyrillic}]]"),
            hir_union(
                hir_uclass_query(ClassQuery::ByValue {
                    property_name: "age",
                    property_value: "3.0",
                }),
                hir_union(
                    hir_uclass_query(ClassQuery::Binary("cyrillic")),
                    hir_union(
                        hir_uclass_query(ClassQuery::Binary("greek")),
                        hir_uclass_query(ClassQuery::Binary("separator"))))));

        assert_eq!(
            t(r"(?i)[\p{age:3.0}\pZ\p{Greek}]"),
            hir_case_fold(hir_union(
                hir_uclass_query(ClassQuery::ByValue {
                    property_name: "age",
                    property_value: "3.0",
                }),
                hir_union(
                    hir_uclass_query(ClassQuery::Binary("greek")),
                    hir_uclass_query(ClassQuery::Binary("separator"))))));
        assert_eq!(
            t(r"[^\p{age:3.0}\pZ\p{Greek}]"),
            hir_negate(hir_union(
                hir_uclass_query(ClassQuery::ByValue {
                    property_name: "age",
                    property_value: "3.0",
                }),
                hir_union(
                    hir_uclass_query(ClassQuery::Binary("greek")),
                    hir_uclass_query(ClassQuery::Binary("separator"))))));
        assert_eq!(
            t(r"(?i)[^\p{age:3.0}\pZ\p{Greek}]"),
            hir_negate(hir_case_fold(hir_union(
                hir_uclass_query(ClassQuery::ByValue {
                    property_name: "age",
                    property_value: "3.0",
                }),
                hir_union(
                    hir_uclass_query(ClassQuery::Binary("greek")),
                    hir_uclass_query(ClassQuery::Binary("separator")))))));
    }

    #[test]
    fn class_bracketed_nested() {
        assert_eq!(
            t(r"[a[^c]]"),
            hir_negate(hir_uclass(&[('c', 'c')])));
        assert_eq!(
            t(r"[a-b[^c]]"),
            hir_negate(hir_uclass(&[('c', 'c')])));
        assert_eq!(
            t(r"[a-c[^c]]"),
            hir_negate(hir_uclass(&[])));

        assert_eq!(
            t(r"[^a[^c]]"),
            hir_uclass(&[('c', 'c')]));
        assert_eq!(
            t(r"[^a-b[^c]]"),
            hir_uclass(&[('c', 'c')]));

        assert_eq!(
            t(r"(?i)[a[^c]]"),
            hir_negate(hir_case_fold(hir_uclass(&[('c', 'c')]))));
        assert_eq!(
            t(r"(?i)[a-b[^c]]"),
            hir_negate(hir_case_fold(hir_uclass(&[('c', 'c')]))));

        assert_eq!(
            t(r"(?i)[^a[^c]]"),
            hir_uclass(&[('C', 'C'), ('c', 'c')]));
        assert_eq!(
            t(r"(?i)[^a-b[^c]]"),
            hir_uclass(&[('C', 'C'), ('c', 'c')]));

        assert_eq!(t_err(r"[^a-c[^c]]"), TestError {
            kind: hir::ErrorKind::EmptyClassNotAllowed,
            span: Span::new(Position::new(0, 1, 1), Position::new(10, 1, 11)),
        });
        assert_eq!(t_err(r"(?i)[^a-c[^c]]"), TestError {
            kind: hir::ErrorKind::EmptyClassNotAllowed,
            span: Span::new(Position::new(4, 1, 5), Position::new(14, 1, 15)),
        });
    }

    #[test]
    fn class_bracketed_intersect() {
        assert_eq!(t("[abc&&b-c]"), hir_uclass(&[('b', 'c')]));
        assert_eq!(t("[abc&&[b-c]]"), hir_uclass(&[('b', 'c')]));
        assert_eq!(t("[[abc]&&[b-c]]"), hir_uclass(&[('b', 'c')]));
        assert_eq!(t("[a-z&&b-y&&c-x]"), hir_uclass(&[('c', 'x')]));
        assert_eq!(t("[c-da-b&&a-d]"), hir_uclass(&[('a', 'd')]));
        assert_eq!(t("[a-d&&c-da-b]"), hir_uclass(&[('a', 'd')]));
        assert_eq!(t(r"[a-z&&a-c]"), hir_uclass(&[('a', 'c')]));
        assert_eq!(t(r"[[a-z&&a-c]]"), hir_uclass(&[('a', 'c')]));
        assert_eq!(t(r"[^[a-z&&a-c]]"), hir_negate(hir_uclass(&[('a', 'c')])));

        assert_eq!(t("(?-u)[abc&&b-c]"), hir_bclass(&[(b'b', b'c')]));
        assert_eq!(t("(?-u)[abc&&[b-c]]"), hir_bclass(&[(b'b', b'c')]));
        assert_eq!(t("(?-u)[[abc]&&[b-c]]"), hir_bclass(&[(b'b', b'c')]));
        assert_eq!(t("(?-u)[a-z&&b-y&&c-x]"), hir_bclass(&[(b'c', b'x')]));
        assert_eq!(t("(?-u)[c-da-b&&a-d]"), hir_bclass(&[(b'a', b'd')]));
        assert_eq!(t("(?-u)[a-d&&c-da-b]"), hir_bclass(&[(b'a', b'd')]));

        assert_eq!(
            t("(?i)[abc&&b-c]"),
            hir_case_fold(hir_uclass(&[('b', 'c')])));
        assert_eq!(
            t("(?i)[abc&&[b-c]]"),
            hir_case_fold(hir_uclass(&[('b', 'c')])));
        assert_eq!(
            t("(?i)[[abc]&&[b-c]]"),
            hir_case_fold(hir_uclass(&[('b', 'c')])));
        assert_eq!(
            t("(?i)[a-z&&b-y&&c-x]"),
            hir_case_fold(hir_uclass(&[('c', 'x')])));
        assert_eq!(
            t("(?i)[c-da-b&&a-d]"),
            hir_case_fold(hir_uclass(&[('a', 'd')])));
        assert_eq!(
            t("(?i)[a-d&&c-da-b]"),
            hir_case_fold(hir_uclass(&[('a', 'd')])));

        assert_eq!(
            t("(?i-u)[abc&&b-c]"),
            hir_case_fold(hir_bclass(&[(b'b', b'c')])));
        assert_eq!(
            t("(?i-u)[abc&&[b-c]]"),
            hir_case_fold(hir_bclass(&[(b'b', b'c')])));
        assert_eq!(
            t("(?i-u)[[abc]&&[b-c]]"),
            hir_case_fold(hir_bclass(&[(b'b', b'c')])));
        assert_eq!(
            t("(?i-u)[a-z&&b-y&&c-x]"),
            hir_case_fold(hir_bclass(&[(b'c', b'x')])));
        assert_eq!(
            t("(?i-u)[c-da-b&&a-d]"),
            hir_case_fold(hir_bclass(&[(b'a', b'd')])));
        assert_eq!(
            t("(?i-u)[a-d&&c-da-b]"),
            hir_case_fold(hir_bclass(&[(b'a', b'd')])));

        // In `[a^]`, `^` does not need to be escaped, so it makes sense that
        // `^` is also allowed to be unescaped after `&&`.
        assert_eq!(t(r"[\^&&^]"), hir_uclass(&[('^', '^')]));
        // `]` needs to be escaped after `&&` since it's not at start of class.
        assert_eq!(t(r"[]&&\]]"), hir_uclass(&[(']', ']')]));
        assert_eq!(t(r"[-&&-]"), hir_uclass(&[('-', '-')]));
        assert_eq!(t(r"[\&&&&]"), hir_uclass(&[('&', '&')]));
        assert_eq!(t(r"[\&&&\&]"), hir_uclass(&[('&', '&')]));
        // Test precedence.
        assert_eq!(
            t(r"[a-w&&[^c-g]z]"),
            hir_uclass(&[('a', 'b'), ('h', 'w')]));
    }

    #[test]
    fn class_bracketed_intersect_negate() {
        assert_eq!(
            t(r"[^\w&&\d]"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("digit"))));
        assert_eq!(
            t(r"[^[a-z&&a-c]]"),
            hir_negate(hir_uclass(&[('a', 'c')])));
        assert_eq!(
            t(r"[^[\w&&\d]]"),
            hir_negate(hir_uclass_query(ClassQuery::Binary("digit"))));
        assert_eq!(
            t(r"[^[^\w&&\d]]"),
            hir_uclass_query(ClassQuery::Binary("digit")));
        assert_eq!(
            t(r"[[[^\w]&&[^\d]]]"),
            hir_negate(hir_uclass_perl_word()));

        assert_eq!(
            t_bytes(r"(?-u)[^\w&&\d]"),
            hir_negate(hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Digit))));
        assert_eq!(
            t_bytes(r"(?-u)[^[a-z&&a-c]]"),
            hir_negate(hir_bclass(&[(b'a', b'c')])));
        assert_eq!(
            t_bytes(r"(?-u)[^[\w&&\d]]"),
            hir_negate(hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Digit))));
        assert_eq!(
            t_bytes(r"(?-u)[^[^\w&&\d]]"),
            hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Digit)));
        assert_eq!(
            t_bytes(r"(?-u)[[[^\w]&&[^\d]]]"),
            hir_negate(hir_bclass_from_char(ascii_class(
                &ast::ClassAsciiKind::Word))));
    }

    #[test]
    fn class_bracketed_difference() {
        assert_eq!(
            t(r"[\pL--[:ascii:]]"),
            hir_difference(
                hir_uclass_query(ClassQuery::Binary("letter")),
                hir_uclass(&[('\0', '\x7F')])));

        assert_eq!(
            t(r"(?-u)[[:alpha:]--[:lower:]]"),
            hir_bclass(&[(b'A', b'Z')]));
    }

    #[test]
    fn class_bracketed_symmetric_difference() {
        assert_eq!(
            t(r"[\p{sc:Greek}~~\p{scx:Greek}]"),
            hir_uclass(&[
                ('\u{0342}', '\u{0342}'),
                ('\u{0345}', '\u{0345}'),
                ('\u{1DC0}', '\u{1DC1}'),
            ]));
        assert_eq!(
            t(r"[a-g~~c-j]"),
            hir_uclass(&[('a', 'b'), ('h', 'j')]));

        assert_eq!(
            t(r"(?-u)[a-g~~c-j]"),
            hir_bclass(&[(b'a', b'b'), (b'h', b'j')]));
    }

    #[test]
    fn ignore_whitespace() {
        assert_eq!(t(r"(?x)\12 3"), hir_lit("\n3"));
        assert_eq!(t(r"(?x)\x { 53 }"), hir_lit("S"));
        assert_eq!(t(r"(?x)\x # comment
{ # comment
    53 # comment
} #comment"), hir_lit("S"));

        assert_eq!(t(r"(?x)\x 53"), hir_lit("S"));
        assert_eq!(t(r"(?x)\x # comment
        53 # comment"), hir_lit("S"));
        assert_eq!(t(r"(?x)\x5 3"), hir_lit("S"));

        assert_eq!(t(r"(?x)\p # comment
{ # comment
    Separator # comment
} # comment"), hir_uclass_query(ClassQuery::Binary("separator")));

        assert_eq!(t(r"(?x)a # comment
{ # comment
    5 # comment
    , # comment
    10 # comment
} # comment"),
            hir_range(
                true, hir::RepetitionRange::Bounded(5, 10), hir_lit("a")));

        assert_eq!(t(r"(?x)a\  # hi there"), hir_lit("a "));
    }

    #[test]
    fn analysis_is_always_utf8() {
        // Positive examples.
        assert!(t_bytes(r"a").is_always_utf8());
        assert!(t_bytes(r"ab").is_always_utf8());
        assert!(t_bytes(r"(?-u)a").is_always_utf8());
        assert!(t_bytes(r"(?-u)ab").is_always_utf8());
        assert!(t_bytes(r"\xFF").is_always_utf8());
        assert!(t_bytes(r"\xFF\xFF").is_always_utf8());
        assert!(t_bytes(r"[^a]").is_always_utf8());
        assert!(t_bytes(r"[^a][^a]").is_always_utf8());
        assert!(t_bytes(r"\b").is_always_utf8());
        assert!(t_bytes(r"\B").is_always_utf8());
        assert!(t_bytes(r"(?-u)\b").is_always_utf8());

        // Negative examples.
        assert!(!t_bytes(r"(?-u)\xFF").is_always_utf8());
        assert!(!t_bytes(r"(?-u)\xFF\xFF").is_always_utf8());
        assert!(!t_bytes(r"(?-u)[^a]").is_always_utf8());
        assert!(!t_bytes(r"(?-u)[^a][^a]").is_always_utf8());
        assert!(!t_bytes(r"(?-u)\B").is_always_utf8());
    }

    #[test]
    fn analysis_is_all_assertions() {
        // Positive examples.
        assert!(t(r"\b").is_all_assertions());
        assert!(t(r"\B").is_all_assertions());
        assert!(t(r"^").is_all_assertions());
        assert!(t(r"$").is_all_assertions());
        assert!(t(r"\A").is_all_assertions());
        assert!(t(r"\z").is_all_assertions());
        assert!(t(r"$^\z\A\b\B").is_all_assertions());
        assert!(t(r"$|^|\z|\A|\b|\B").is_all_assertions());
        assert!(t(r"^$|$^").is_all_assertions());
        assert!(t(r"((\b)+())*^").is_all_assertions());

        // Negative examples.
        assert!(!t(r"^a").is_all_assertions());
    }

    #[test]
    fn analysis_is_anchored() {
        // Positive examples.
        assert!(t(r"^").is_anchored_start());
        assert!(t(r"$").is_anchored_end());
        assert!(t(r"^").is_line_anchored_start());
        assert!(t(r"$").is_line_anchored_end());

        assert!(t(r"^^").is_anchored_start());
        assert!(t(r"$$").is_anchored_end());
        assert!(t(r"^^").is_line_anchored_start());
        assert!(t(r"$$").is_line_anchored_end());

        assert!(t(r"^$").is_anchored_start());
        assert!(t(r"^$").is_anchored_end());
        assert!(t(r"^$").is_line_anchored_start());
        assert!(t(r"^$").is_line_anchored_end());

        assert!(t(r"^foo").is_anchored_start());
        assert!(t(r"foo$").is_anchored_end());
        assert!(t(r"^foo").is_line_anchored_start());
        assert!(t(r"foo$").is_line_anchored_end());

        assert!(t(r"^foo|^bar").is_anchored_start());
        assert!(t(r"foo$|bar$").is_anchored_end());
        assert!(t(r"^foo|^bar").is_line_anchored_start());
        assert!(t(r"foo$|bar$").is_line_anchored_end());

        assert!(t(r"^(foo|bar)").is_anchored_start());
        assert!(t(r"(foo|bar)$").is_anchored_end());
        assert!(t(r"^(foo|bar)").is_line_anchored_start());
        assert!(t(r"(foo|bar)$").is_line_anchored_end());

        assert!(t(r"^+").is_anchored_start());
        assert!(t(r"$+").is_anchored_end());
        assert!(t(r"^+").is_line_anchored_start());
        assert!(t(r"$+").is_line_anchored_end());
        assert!(t(r"^++").is_anchored_start());
        assert!(t(r"$++").is_anchored_end());
        assert!(t(r"^++").is_line_anchored_start());
        assert!(t(r"$++").is_line_anchored_end());
        assert!(t(r"(^)+").is_anchored_start());
        assert!(t(r"($)+").is_anchored_end());
        assert!(t(r"(^)+").is_line_anchored_start());
        assert!(t(r"($)+").is_line_anchored_end());

        assert!(t(r"$^").is_anchored_start());
        assert!(t(r"$^").is_anchored_start());
        assert!(t(r"$^").is_line_anchored_end());
        assert!(t(r"$^").is_line_anchored_end());
        assert!(t(r"$^|^$").is_anchored_start());
        assert!(t(r"$^|^$").is_anchored_end());
        assert!(t(r"$^|^$").is_line_anchored_start());
        assert!(t(r"$^|^$").is_line_anchored_end());

        assert!(t(r"\b^").is_anchored_start());
        assert!(t(r"$\b").is_anchored_end());
        assert!(t(r"\b^").is_line_anchored_start());
        assert!(t(r"$\b").is_line_anchored_end());
        assert!(t(r"^(?m:^)").is_anchored_start());
        assert!(t(r"(?m:$)$").is_anchored_end());
        assert!(t(r"^(?m:^)").is_line_anchored_start());
        assert!(t(r"(?m:$)$").is_line_anchored_end());
        assert!(t(r"(?m:^)^").is_anchored_start());
        assert!(t(r"$(?m:$)").is_anchored_end());
        assert!(t(r"(?m:^)^").is_line_anchored_start());
        assert!(t(r"$(?m:$)").is_line_anchored_end());

        // Negative examples.
        assert!(!t(r"(?m)^").is_anchored_start());
        assert!(!t(r"(?m)$").is_anchored_end());
        assert!(!t(r"(?m:^$)|$^").is_anchored_start());
        assert!(!t(r"(?m:^$)|$^").is_anchored_end());
        assert!(!t(r"$^|(?m:^$)").is_anchored_start());
        assert!(!t(r"$^|(?m:^$)").is_anchored_end());

        assert!(!t(r"a^").is_anchored_start());
        assert!(!t(r"$a").is_anchored_start());
        assert!(!t(r"a^").is_line_anchored_start());
        assert!(!t(r"$a").is_line_anchored_start());

        assert!(!t(r"a^").is_anchored_end());
        assert!(!t(r"$a").is_anchored_end());
        assert!(!t(r"a^").is_line_anchored_end());
        assert!(!t(r"$a").is_line_anchored_end());

        assert!(!t(r"^foo|bar").is_anchored_start());
        assert!(!t(r"foo|bar$").is_anchored_end());
        assert!(!t(r"^foo|bar").is_line_anchored_start());
        assert!(!t(r"foo|bar$").is_line_anchored_end());

        assert!(!t(r"^*").is_anchored_start());
        assert!(!t(r"$*").is_anchored_end());
        assert!(!t(r"^*").is_line_anchored_start());
        assert!(!t(r"$*").is_line_anchored_end());
        assert!(!t(r"^*+").is_anchored_start());
        assert!(!t(r"$*+").is_anchored_end());
        assert!(!t(r"^*+").is_line_anchored_start());
        assert!(!t(r"$*+").is_line_anchored_end());
        assert!(!t(r"^+*").is_anchored_start());
        assert!(!t(r"$+*").is_anchored_end());
        assert!(!t(r"^+*").is_line_anchored_start());
        assert!(!t(r"$+*").is_line_anchored_end());
        assert!(!t(r"(^)*").is_anchored_start());
        assert!(!t(r"($)*").is_anchored_end());
        assert!(!t(r"(^)*").is_line_anchored_start());
        assert!(!t(r"($)*").is_line_anchored_end());
    }

    #[test]
    fn analysis_is_line_anchored() {
        assert!(t(r"(?m)^(foo|bar)").is_line_anchored_start());
        assert!(t(r"(?m)(foo|bar)$").is_line_anchored_end());

        assert!(t(r"(?m)^foo|^bar").is_line_anchored_start());
        assert!(t(r"(?m)foo$|bar$").is_line_anchored_end());

        assert!(t(r"(?m)^").is_line_anchored_start());
        assert!(t(r"(?m)$").is_line_anchored_end());

        assert!(t(r"(?m:^$)|$^").is_line_anchored_start());
        assert!(t(r"(?m:^$)|$^").is_line_anchored_end());

        assert!(t(r"$^|(?m:^$)").is_line_anchored_start());
        assert!(t(r"$^|(?m:^$)").is_line_anchored_end());
    }

    #[test]
    fn analysis_is_any_anchored() {
        // Positive examples.
        assert!(t(r"^").is_any_anchored_start());
        assert!(t(r"$").is_any_anchored_end());
        assert!(t(r"\A").is_any_anchored_start());
        assert!(t(r"\z").is_any_anchored_end());

        // Negative examples.
        assert!(!t(r"(?m)^").is_any_anchored_start());
        assert!(!t(r"(?m)$").is_any_anchored_end());
        assert!(!t(r"$").is_any_anchored_start());
        assert!(!t(r"^").is_any_anchored_end());
    }

    #[test]
    fn analysis_is_match_empty() {
        // Positive examples.
        assert!(t(r"").is_match_empty());
        assert!(t(r"()").is_match_empty());
        assert!(t(r"()*").is_match_empty());
        assert!(t(r"()+").is_match_empty());
        assert!(t(r"()?").is_match_empty());
        assert!(t(r"a*").is_match_empty());
        assert!(t(r"a?").is_match_empty());
        assert!(t(r"a{0}").is_match_empty());
        assert!(t(r"a{0,}").is_match_empty());
        assert!(t(r"a{0,1}").is_match_empty());
        assert!(t(r"a{0,10}").is_match_empty());
        assert!(t(r"\pL*").is_match_empty());
        assert!(t(r"a*|b").is_match_empty());
        assert!(t(r"b|a*").is_match_empty());
        assert!(t(r"a*a?(abcd)*").is_match_empty());
        assert!(t(r"^").is_match_empty());
        assert!(t(r"$").is_match_empty());
        assert!(t(r"(?m)^").is_match_empty());
        assert!(t(r"(?m)$").is_match_empty());
        assert!(t(r"\A").is_match_empty());
        assert!(t(r"\z").is_match_empty());
        assert!(t(r"\B").is_match_empty());
        assert!(t_bytes(r"(?-u)\B").is_match_empty());

        // Negative examples.
        assert!(!t(r"a+").is_match_empty());
        assert!(!t(r"a{1}").is_match_empty());
        assert!(!t(r"a{1,}").is_match_empty());
        assert!(!t(r"a{1,2}").is_match_empty());
        assert!(!t(r"a{1,10}").is_match_empty());
        assert!(!t(r"b|a").is_match_empty());
        assert!(!t(r"a*a+(abcd)*").is_match_empty());
        assert!(!t(r"\b").is_match_empty());
        assert!(!t(r"(?-u)\b").is_match_empty());
    }

    #[test]
    fn analysis_is_literal() {
        // Positive examples.
        assert!(t(r"").is_literal());
        assert!(t(r"a").is_literal());
        assert!(t(r"ab").is_literal());
        assert!(t(r"abc").is_literal());
        assert!(t(r"(?m)abc").is_literal());

        // Negative examples.
        assert!(!t(r"^").is_literal());
        assert!(!t(r"a|b").is_literal());
        assert!(!t(r"(a)").is_literal());
        assert!(!t(r"a+").is_literal());
        assert!(!t(r"foo(a)").is_literal());
        assert!(!t(r"(a)foo").is_literal());
        assert!(!t(r"[a]").is_literal());
    }

    #[test]
    fn analysis_is_alternation_literal() {
        // Positive examples.
        assert!(t(r"").is_alternation_literal());
        assert!(t(r"a").is_alternation_literal());
        assert!(t(r"ab").is_alternation_literal());
        assert!(t(r"abc").is_alternation_literal());
        assert!(t(r"(?m)abc").is_alternation_literal());
        assert!(t(r"a|b").is_alternation_literal());
        assert!(t(r"a|b|c").is_alternation_literal());
        assert!(t(r"foo|bar").is_alternation_literal());
        assert!(t(r"foo|bar|baz").is_alternation_literal());

        // Negative examples.
        assert!(!t(r"^").is_alternation_literal());
        assert!(!t(r"(a)").is_alternation_literal());
        assert!(!t(r"a+").is_alternation_literal());
        assert!(!t(r"foo(a)").is_alternation_literal());
        assert!(!t(r"(a)foo").is_alternation_literal());
        assert!(!t(r"[a]").is_alternation_literal());
        assert!(!t(r"[a]|b").is_alternation_literal());
        assert!(!t(r"a|[b]").is_alternation_literal());
        assert!(!t(r"(a)|b").is_alternation_literal());
        assert!(!t(r"a|(b)").is_alternation_literal());
    }
}