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use alloc::{boxed::Box, string::String, vec, vec::Vec};
use crate::{error::Error, utf8};
mod parse;
/// Escapes all regular expression meta characters in `pattern`.
///
/// The string returned may be safely used as a literal in a regular
/// expression.
pub fn escape(pattern: &str) -> String {
let mut buf = String::new();
buf.reserve(pattern.len());
for ch in pattern.chars() {
if is_meta_character(ch) {
buf.push('\\');
}
buf.push(ch);
}
buf
}
/// Returns true if the given character has significance in a regex.
///
/// Generally speaking, these are the only characters which _must_ be escaped
/// in order to match their literal meaning. For example, to match a literal
/// `|`, one could write `\|`. Sometimes escaping isn't always necessary. For
/// example, `-` is treated as a meta character because of its significance
/// for writing ranges inside of character classes, but the regex `-` will
/// match a literal `-` because `-` has no special meaning outside of character
/// classes.
///
/// In order to determine whether a character may be escaped at all, the
/// [`is_escapeable_character`] routine should be used. The difference between
/// `is_meta_character` and `is_escapeable_character` is that the latter will
/// return true for some characters that are _not_ meta characters. For
/// example, `%` and `\%` both match a literal `%` in all contexts. In other
/// words, `is_escapeable_character` includes "superfluous" escapes.
///
/// Note that the set of characters for which this function returns `true` or
/// `false` is fixed and won't change in a semver compatible release. (In this
/// case, "semver compatible release" actually refers to the `regex` crate
/// itself, since reducing or expanding the set of meta characters would be a
/// breaking change for not just `regex-syntax` but also `regex` itself.)
fn is_meta_character(c: char) -> bool {
match c {
'\\' | '.' | '+' | '*' | '?' | '(' | ')' | '|' | '[' | ']' | '{'
| '}' | '^' | '$' | '#' | '&' | '-' | '~' => true,
_ => false,
}
}
/// Returns true if the given character can be escaped in a regex.
///
/// This returns true in all cases that `is_meta_character` returns true, but
/// also returns true in some cases where `is_meta_character` returns false.
/// For example, `%` is not a meta character, but it is escapeable. That is,
/// `%` and `\%` both match a literal `%` in all contexts.
///
/// The purpose of this routine is to provide knowledge about what characters
/// may be escaped. Namely, most regex engines permit "superfluous" escapes
/// where characters without any special significance may be escaped even
/// though there is no actual _need_ to do so.
///
/// This will return false for some characters. For example, `e` is not
/// escapeable. Therefore, `\e` will either result in a parse error (which is
/// true today), or it could backwards compatibly evolve into a new construct
/// with its own meaning. Indeed, that is the purpose of banning _some_
/// superfluous escapes: it provides a way to evolve the syntax in a compatible
/// manner.
fn is_escapeable_character(c: char) -> bool {
// Certainly escapeable if it's a meta character.
if is_meta_character(c) {
return true;
}
// Any character that isn't ASCII is definitely not escapeable. There's
// no real need to allow things like \☃ right?
if !c.is_ascii() {
return false;
}
// Otherwise, we basically say that everything is escapeable unless it's a
// letter or digit. Things like \3 are either octal (when enabled) or an
// error, and we should keep it that way. Otherwise, letters are reserved
// for adding new syntax in a backwards compatible way.
match c {
'0'..='9' | 'A'..='Z' | 'a'..='z' => false,
// While not currently supported, we keep these as not escapeable to
// give us some flexibility with respect to supporting the \< and
// \> word boundary assertions in the future. By rejecting them as
// escapeable, \< and \> will result in a parse error. Thus, we can
// turn them into something else in the future without it being a
// backwards incompatible change.
'<' | '>' => false,
_ => true,
}
}
/// The configuration for a regex parser.
#[derive(Clone, Copy, Debug)]
pub(crate) struct Config {
/// The maximum number of times we're allowed to recurse.
///
/// Note that unlike the regex-syntax parser, we actually use recursion in
/// this parser for simplicity. My hope is that by setting a conservative
/// default call limit and providing a way to configure it, that we can
/// keep this simplification. But if we must, we can re-work the parser to
/// put the call stack on the heap like regex-syntax does.
pub(crate) nest_limit: u32,
/// Various flags that control how a pattern is interpreted.
pub(crate) flags: Flags,
}
impl Default for Config {
fn default() -> Config {
Config { nest_limit: 50, flags: Flags::default() }
}
}
/// Various flags that control the interpretation of the pattern.
///
/// These can be set via explicit configuration in code, or change dynamically
/// during parsing via inline flags. For example, `foo(?i:bar)baz` will match
/// `foo` and `baz` case sensitiviely and `bar` case insensitively (assuming a
/// default configuration).
#[derive(Clone, Copy, Debug, Default)]
pub(crate) struct Flags {
/// Whether to match case insensitively.
///
/// This is the `i` flag.
pub(crate) case_insensitive: bool,
/// Whether `^` and `$` should be treated as line anchors or not.
///
/// This is the `m` flag.
pub(crate) multi_line: bool,
/// Whether `.` should match line terminators or not.
///
/// This is the `s` flag.
pub(crate) dot_matches_new_line: bool,
/// Whether to swap the meaning of greedy and non-greedy operators.
///
/// This is the `U` flag.
pub(crate) swap_greed: bool,
/// Whether to enable CRLF mode.
///
/// This is the `R` flag.
pub(crate) crlf: bool,
/// Whether to ignore whitespace. i.e., verbose mode.
///
/// This is the `x` flag.
pub(crate) ignore_whitespace: bool,
}
#[derive(Clone, Debug, Eq, PartialEq)]
pub(crate) struct Hir {
kind: HirKind,
is_start_anchored: bool,
is_match_empty: bool,
static_explicit_captures_len: Option<usize>,
}
#[derive(Clone, Debug, Eq, PartialEq)]
pub(crate) enum HirKind {
Empty,
Char(char),
Class(Class),
Look(Look),
Repetition(Repetition),
Capture(Capture),
Concat(Vec<Hir>),
Alternation(Vec<Hir>),
}
impl Hir {
/// Parses the given pattern string with the given configuration into a
/// structured representation. If the pattern is invalid, then an error
/// is returned.
pub(crate) fn parse(config: Config, pattern: &str) -> Result<Hir, Error> {
self::parse::Parser::new(config, pattern).parse()
}
/// Returns the underlying kind of this high-level intermediate
/// representation.
///
/// Note that there is explicitly no way to build an `Hir` directly from
/// an `HirKind`. If you need to do that, then you must do case analysis
/// on the `HirKind` and call the appropriate smart constructor on `Hir`.
pub(crate) fn kind(&self) -> &HirKind {
&self.kind
}
/// Returns true if and only if this Hir expression can only match at the
/// beginning of a haystack.
pub(crate) fn is_start_anchored(&self) -> bool {
self.is_start_anchored
}
/// Returns true if and only if this Hir expression can match the empty
/// string.
pub(crate) fn is_match_empty(&self) -> bool {
self.is_match_empty
}
/// If the pattern always reports the same number of matching capture groups
/// for every match, then this returns the number of those groups. This
/// doesn't include the implicit group found in every pattern.
pub(crate) fn static_explicit_captures_len(&self) -> Option<usize> {
self.static_explicit_captures_len
}
fn fail() -> Hir {
let kind = HirKind::Class(Class { ranges: vec![] });
Hir {
kind,
is_start_anchored: false,
is_match_empty: false,
static_explicit_captures_len: Some(0),
}
}
fn empty() -> Hir {
let kind = HirKind::Empty;
Hir {
kind,
is_start_anchored: false,
is_match_empty: true,
static_explicit_captures_len: Some(0),
}
}
fn char(ch: char) -> Hir {
let kind = HirKind::Char(ch);
Hir {
kind,
is_start_anchored: false,
is_match_empty: false,
static_explicit_captures_len: Some(0),
}
}
fn class(class: Class) -> Hir {
let kind = HirKind::Class(class);
Hir {
kind,
is_start_anchored: false,
is_match_empty: false,
static_explicit_captures_len: Some(0),
}
}
fn look(look: Look) -> Hir {
let kind = HirKind::Look(look);
Hir {
kind,
is_start_anchored: matches!(look, Look::Start),
is_match_empty: true,
static_explicit_captures_len: Some(0),
}
}
fn repetition(rep: Repetition) -> Hir {
if rep.min == 0 && rep.max == Some(0) {
return Hir::empty();
} else if rep.min == 1 && rep.max == Some(1) {
return *rep.sub;
}
let is_start_anchored = rep.min > 0 && rep.sub.is_start_anchored;
let is_match_empty = rep.min == 0 || rep.sub.is_match_empty;
let mut static_explicit_captures_len =
rep.sub.static_explicit_captures_len;
// If the static captures len of the sub-expression is not known or
// is greater than zero, then it automatically propagates to the
// repetition, regardless of the repetition. Otherwise, it might
// change, but only when the repetition can match 0 times.
if rep.min == 0
&& static_explicit_captures_len.map_or(false, |len| len > 0)
{
// If we require a match 0 times, then our captures len is
// guaranteed to be zero. Otherwise, if we *can* match the empty
// string, then it's impossible to know how many captures will be
// in the resulting match.
if rep.max == Some(0) {
static_explicit_captures_len = Some(0);
} else {
static_explicit_captures_len = None;
}
}
Hir {
kind: HirKind::Repetition(rep),
is_start_anchored,
is_match_empty,
static_explicit_captures_len,
}
}
fn capture(cap: Capture) -> Hir {
let is_start_anchored = cap.sub.is_start_anchored;
let is_match_empty = cap.sub.is_match_empty;
let static_explicit_captures_len = cap
.sub
.static_explicit_captures_len
.map(|len| len.saturating_add(1));
let kind = HirKind::Capture(cap);
Hir {
kind,
is_start_anchored,
is_match_empty,
static_explicit_captures_len,
}
}
fn concat(mut subs: Vec<Hir>) -> Hir {
if subs.is_empty() {
Hir::empty()
} else if subs.len() == 1 {
subs.pop().unwrap()
} else {
let is_start_anchored = subs[0].is_start_anchored;
let mut is_match_empty = true;
let mut static_explicit_captures_len = Some(0usize);
for sub in subs.iter() {
is_match_empty = is_match_empty && sub.is_match_empty;
static_explicit_captures_len = static_explicit_captures_len
.and_then(|len1| {
Some((len1, sub.static_explicit_captures_len?))
})
.and_then(|(len1, len2)| Some(len1.saturating_add(len2)));
}
Hir {
kind: HirKind::Concat(subs),
is_start_anchored,
is_match_empty,
static_explicit_captures_len,
}
}
}
fn alternation(mut subs: Vec<Hir>) -> Hir {
if subs.is_empty() {
Hir::fail()
} else if subs.len() == 1 {
subs.pop().unwrap()
} else {
let mut it = subs.iter().peekable();
let mut is_start_anchored =
it.peek().map_or(false, |sub| sub.is_start_anchored);
let mut is_match_empty =
it.peek().map_or(false, |sub| sub.is_match_empty);
let mut static_explicit_captures_len =
it.peek().and_then(|sub| sub.static_explicit_captures_len);
for sub in it {
is_start_anchored = is_start_anchored && sub.is_start_anchored;
is_match_empty = is_match_empty || sub.is_match_empty;
if static_explicit_captures_len
!= sub.static_explicit_captures_len
{
static_explicit_captures_len = None;
}
}
Hir {
kind: HirKind::Alternation(subs),
is_start_anchored,
is_match_empty,
static_explicit_captures_len,
}
}
}
}
impl HirKind {
/// Returns a slice of this kind's sub-expressions, if any.
fn subs(&self) -> &[Hir] {
use core::slice::from_ref;
match *self {
HirKind::Empty
| HirKind::Char(_)
| HirKind::Class(_)
| HirKind::Look(_) => &[],
HirKind::Repetition(Repetition { ref sub, .. }) => from_ref(sub),
HirKind::Capture(Capture { ref sub, .. }) => from_ref(sub),
HirKind::Concat(ref subs) => subs,
HirKind::Alternation(ref subs) => subs,
}
}
}
#[derive(Clone, Debug, Eq, PartialEq)]
pub(crate) struct Class {
pub(crate) ranges: Vec<ClassRange>,
}
impl Class {
/// Create a new class from the given ranges. The ranges may be provided
/// in any order or may even overlap. They will be automatically
/// canonicalized.
fn new<I: IntoIterator<Item = ClassRange>>(ranges: I) -> Class {
let mut class = Class { ranges: ranges.into_iter().collect() };
class.canonicalize();
class
}
/// Expand this class such that it matches the ASCII codepoints in this set
/// case insensitively.
fn ascii_case_fold(&mut self) {
let len = self.ranges.len();
for i in 0..len {
if let Some(folded) = self.ranges[i].ascii_case_fold() {
self.ranges.push(folded);
}
}
self.canonicalize();
}
/// Negate this set.
///
/// For all `x` where `x` is any element, if `x` was in this set, then it
/// will not be in this set after negation.
fn negate(&mut self) {
const MIN: char = '\x00';
const MAX: char = char::MAX;
if self.ranges.is_empty() {
self.ranges.push(ClassRange { start: MIN, end: MAX });
return;
}
// There should be a way to do this in-place with constant memory,
// but I couldn't figure out a simple way to do it. So just append
// the negation to the end of this range, and then drain it before
// we're done.
let drain_end = self.ranges.len();
// If our class doesn't start the minimum possible char, then negation
// needs to include all codepoints up to the minimum in this set.
if self.ranges[0].start > MIN {
self.ranges.push(ClassRange {
start: MIN,
// OK because we know it's bigger than MIN.
end: prev_char(self.ranges[0].start).unwrap(),
});
}
for i in 1..drain_end {
// let lower = self.ranges[i - 1].upper().increment();
// let upper = self.ranges[i].lower().decrement();
// self.ranges.push(I::create(lower, upper));
self.ranges.push(ClassRange {
// OK because we know i-1 is never the last range and therefore
// there must be a range greater than it. It therefore follows
// that 'end' can never be char::MAX, and thus there must be
// a next char.
start: next_char(self.ranges[i - 1].end).unwrap(),
// Since 'i' is guaranteed to never be the first range, it
// follows that there is always a range before this and thus
// 'start' can never be '\x00'. Thus, there must be a previous
// char.
end: prev_char(self.ranges[i].start).unwrap(),
});
}
if self.ranges[drain_end - 1].end < MAX {
// let lower = self.ranges[drain_end - 1].upper().increment();
// self.ranges.push(I::create(lower, I::Bound::max_value()));
self.ranges.push(ClassRange {
// OK because we know 'end' is less than char::MAX, and thus
// there is a next char.
start: next_char(self.ranges[drain_end - 1].end).unwrap(),
end: MAX,
});
}
self.ranges.drain(..drain_end);
// We don't need to canonicalize because we processed the ranges above
// in canonical order and the new ranges we added based on those are
// also necessarily in canonical order.
}
/// Converts this set into a canonical ordering.
fn canonicalize(&mut self) {
if self.is_canonical() {
return;
}
self.ranges.sort();
assert!(!self.ranges.is_empty());
// Is there a way to do this in-place with constant memory? I couldn't
// figure out a way to do it. So just append the canonicalization to
// the end of this range, and then drain it before we're done.
let drain_end = self.ranges.len();
for oldi in 0..drain_end {
// If we've added at least one new range, then check if we can
// merge this range in the previously added range.
if self.ranges.len() > drain_end {
let (last, rest) = self.ranges.split_last_mut().unwrap();
if let Some(union) = last.union(&rest[oldi]) {
*last = union;
continue;
}
}
self.ranges.push(self.ranges[oldi]);
}
self.ranges.drain(..drain_end);
}
/// Returns true if and only if this class is in a canonical ordering.
fn is_canonical(&self) -> bool {
for pair in self.ranges.windows(2) {
if pair[0] >= pair[1] {
return false;
}
if pair[0].is_contiguous(&pair[1]) {
return false;
}
}
true
}
}
#[derive(Clone, Copy, Debug, Eq, PartialEq, PartialOrd, Ord)]
pub(crate) struct ClassRange {
pub(crate) start: char,
pub(crate) end: char,
}
impl ClassRange {
/// Apply simple case folding to this byte range. Only ASCII case mappings
/// (for A-Za-z) are applied.
///
/// Additional ranges are appended to the given vector. Canonical ordering
/// is *not* maintained in the given vector.
fn ascii_case_fold(&self) -> Option<ClassRange> {
if !(ClassRange { start: 'a', end: 'z' }).is_intersection_empty(self) {
let start = core::cmp::max(self.start, 'a');
let end = core::cmp::min(self.end, 'z');
return Some(ClassRange {
start: char::try_from(u32::from(start) - 32).unwrap(),
end: char::try_from(u32::from(end) - 32).unwrap(),
});
}
if !(ClassRange { start: 'A', end: 'Z' }).is_intersection_empty(self) {
let start = core::cmp::max(self.start, 'A');
let end = core::cmp::min(self.end, 'Z');
return Some(ClassRange {
start: char::try_from(u32::from(start) + 32).unwrap(),
end: char::try_from(u32::from(end) + 32).unwrap(),
});
}
None
}
/// Union the given overlapping range into this range.
///
/// If the two ranges aren't contiguous, then this returns `None`.
fn union(&self, other: &ClassRange) -> Option<ClassRange> {
if !self.is_contiguous(other) {
return None;
}
let start = core::cmp::min(self.start, other.start);
let end = core::cmp::max(self.end, other.end);
Some(ClassRange { start, end })
}
/// Returns true if and only if the two ranges are contiguous. Two ranges
/// are contiguous if and only if the ranges are either overlapping or
/// adjacent.
fn is_contiguous(&self, other: &ClassRange) -> bool {
let (s1, e1) = (u32::from(self.start), u32::from(self.end));
let (s2, e2) = (u32::from(other.start), u32::from(other.end));
core::cmp::max(s1, s2) <= core::cmp::min(e1, e2).saturating_add(1)
}
/// Returns true if and only if the intersection of this range and the
/// other range is empty.
fn is_intersection_empty(&self, other: &ClassRange) -> bool {
let (s1, e1) = (self.start, self.end);
let (s2, e2) = (other.start, other.end);
core::cmp::max(s1, s2) > core::cmp::min(e1, e2)
}
}
/// The high-level intermediate representation for a look-around assertion.
///
/// An assertion match is always zero-length. Also called an "empty match."
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub(crate) enum Look {
/// Match the beginning of text. Specifically, this matches at the starting
/// position of the input.
Start = 1 << 0,
/// Match the end of text. Specifically, this matches at the ending
/// position of the input.
End = 1 << 1,
/// Match the beginning of a line or the beginning of text. Specifically,
/// this matches at the starting position of the input, or at the position
/// immediately following a `\n` character.
StartLF = 1 << 2,
/// Match the end of a line or the end of text. Specifically, this matches
/// at the end position of the input, or at the position immediately
/// preceding a `\n` character.
EndLF = 1 << 3,
/// Match the beginning of a line or the beginning of text. Specifically,
/// this matches at the starting position of the input, or at the position
/// immediately following either a `\r` or `\n` character, but never after
/// a `\r` when a `\n` follows.
StartCRLF = 1 << 4,
/// Match the end of a line or the end of text. Specifically, this matches
/// at the end position of the input, or at the position immediately
/// preceding a `\r` or `\n` character, but never before a `\n` when a `\r`
/// precedes it.
EndCRLF = 1 << 5,
/// Match an ASCII-only word boundary. That is, this matches a position
/// where the left adjacent character and right adjacent character
/// correspond to a word and non-word or a non-word and word character.
Word = 1 << 6,
/// Match an ASCII-only negation of a word boundary.
WordNegate = 1 << 7,
/// Match the start of an ASCII-only word boundary. That is, this matches a
/// position at either the beginning of the haystack or where the previous
/// character is not a word character and the following character is a word
/// character.
WordStart = 1 << 8,
/// Match the end of an ASCII-only word boundary. That is, this matches
/// a position at either the end of the haystack or where the previous
/// character is a word character and the following character is not a word
/// character.
WordEnd = 1 << 9,
/// Match the start half of an ASCII-only word boundary. That is, this
/// matches a position at either the beginning of the haystack or where the
/// previous character is not a word character.
WordStartHalf = 1 << 10,
/// Match the end half of an ASCII-only word boundary. That is, this
/// matches a position at either the end of the haystack or where the
/// following character is not a word character.
WordEndHalf = 1 << 11,
}
impl Look {
/// Returns true if the given position in the given haystack matches this
/// look-around assertion.
pub(crate) fn is_match(&self, haystack: &[u8], at: usize) -> bool {
use self::Look::*;
match *self {
Start => at == 0,
End => at == haystack.len(),
StartLF => at == 0 || haystack[at - 1] == b'\n',
EndLF => at == haystack.len() || haystack[at] == b'\n',
StartCRLF => {
at == 0
|| haystack[at - 1] == b'\n'
|| (haystack[at - 1] == b'\r'
&& (at >= haystack.len() || haystack[at] != b'\n'))
}
EndCRLF => {
at == haystack.len()
|| haystack[at] == b'\r'
|| (haystack[at] == b'\n'
&& (at == 0 || haystack[at - 1] != b'\r'))
}
Word => {
let word_before =
at > 0 && utf8::is_word_byte(haystack[at - 1]);
let word_after =
at < haystack.len() && utf8::is_word_byte(haystack[at]);
word_before != word_after
}
WordNegate => {
let word_before =
at > 0 && utf8::is_word_byte(haystack[at - 1]);
let word_after =
at < haystack.len() && utf8::is_word_byte(haystack[at]);
word_before == word_after
}
WordStart => {
let word_before =
at > 0 && utf8::is_word_byte(haystack[at - 1]);
let word_after =
at < haystack.len() && utf8::is_word_byte(haystack[at]);
!word_before && word_after
}
WordEnd => {
let word_before =
at > 0 && utf8::is_word_byte(haystack[at - 1]);
let word_after =
at < haystack.len() && utf8::is_word_byte(haystack[at]);
word_before && !word_after
}
WordStartHalf => {
let word_before =
at > 0 && utf8::is_word_byte(haystack[at - 1]);
!word_before
}
WordEndHalf => {
let word_after =
at < haystack.len() && utf8::is_word_byte(haystack[at]);
!word_after
}
}
}
}
/// The high-level intermediate representation of a repetition operator.
///
/// A repetition operator permits the repetition of an arbitrary
/// sub-expression.
#[derive(Clone, Debug, Eq, PartialEq)]
pub(crate) struct Repetition {
/// The minimum range of the repetition.
///
/// Note that special cases like `?`, `+` and `*` all get translated into
/// the ranges `{0,1}`, `{1,}` and `{0,}`, respectively.
///
/// When `min` is zero, this expression can match the empty string
/// regardless of what its sub-expression is.
pub(crate) min: u32,
/// The maximum range of the repetition.
///
/// Note that when `max` is `None`, `min` acts as a lower bound but where
/// there is no upper bound. For something like `x{5}` where the min and
/// max are equivalent, `min` will be set to `5` and `max` will be set to
/// `Some(5)`.
pub(crate) max: Option<u32>,
/// Whether this repetition operator is greedy or not. A greedy operator
/// will match as much as it can. A non-greedy operator will match as
/// little as it can.
///
/// Typically, operators are greedy by default and are only non-greedy when
/// a `?` suffix is used, e.g., `(expr)*` is greedy while `(expr)*?` is
/// not. However, this can be inverted via the `U` "ungreedy" flag.
pub(crate) greedy: bool,
/// The expression being repeated.
pub(crate) sub: Box<Hir>,
}
/// The high-level intermediate representation for a capturing group.
///
/// A capturing group always has an index and a child expression. It may
/// also have a name associated with it (e.g., `(?P<foo>\w)`), but it's not
/// necessary.
///
/// Note that there is no explicit representation of a non-capturing group
/// in a `Hir`. Instead, non-capturing grouping is handled automatically by
/// the recursive structure of the `Hir` itself.
#[derive(Clone, Debug, Eq, PartialEq)]
pub(crate) struct Capture {
/// The capture index of the capture.
pub(crate) index: u32,
/// The name of the capture, if it exists.
pub(crate) name: Option<Box<str>>,
/// The expression inside the capturing group, which may be empty.
pub(crate) sub: Box<Hir>,
}
fn next_char(ch: char) -> Option<char> {
// Skip over the surrogate range.
if ch == '\u{D7FF}' {
return Some('\u{E000}');
}
// OK because char::MAX < u32::MAX and we handle U+D7FF above.
char::from_u32(u32::from(ch).checked_add(1).unwrap())
}
fn prev_char(ch: char) -> Option<char> {
// Skip over the surrogate range.
if ch == '\u{E000}' {
return Some('\u{D7FF}');
}
// OK because subtracting 1 from any valid scalar value other than 0
// and U+E000 yields a valid scalar value.
Some(char::from_u32(u32::from(ch).checked_sub(1)?).unwrap())
}
impl Drop for Hir {
fn drop(&mut self) {
use core::mem;
match *self.kind() {
HirKind::Empty
| HirKind::Char(_)
| HirKind::Class(_)
| HirKind::Look(_) => return,
HirKind::Capture(ref x) if x.sub.kind.subs().is_empty() => return,
HirKind::Repetition(ref x) if x.sub.kind.subs().is_empty() => {
return
}
HirKind::Concat(ref x) if x.is_empty() => return,
HirKind::Alternation(ref x) if x.is_empty() => return,
_ => {}
}
let mut stack = vec![mem::replace(self, Hir::empty())];
while let Some(mut expr) = stack.pop() {
match expr.kind {
HirKind::Empty
| HirKind::Char(_)
| HirKind::Class(_)
| HirKind::Look(_) => {}
HirKind::Capture(ref mut x) => {
stack.push(mem::replace(&mut x.sub, Hir::empty()));
}
HirKind::Repetition(ref mut x) => {
stack.push(mem::replace(&mut x.sub, Hir::empty()));
}
HirKind::Concat(ref mut x) => {
stack.extend(x.drain(..));
}
HirKind::Alternation(ref mut x) => {
stack.extend(x.drain(..));
}
}
}
}
}