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/*!
This module provides a regular expression printer for `Hir`.
*/
use core::fmt;
use crate::{
hir::{
self,
visitor::{self, Visitor},
Hir, HirKind,
},
is_meta_character,
};
/// A builder for constructing a printer.
///
/// Note that since a printer doesn't have any configuration knobs, this type
/// remains unexported.
#[derive(Clone, Debug)]
struct PrinterBuilder {
_priv: (),
}
impl Default for PrinterBuilder {
fn default() -> PrinterBuilder {
PrinterBuilder::new()
}
}
impl PrinterBuilder {
fn new() -> PrinterBuilder {
PrinterBuilder { _priv: () }
}
fn build(&self) -> Printer {
Printer { _priv: () }
}
}
/// A printer for a regular expression's high-level intermediate
/// representation.
///
/// A printer converts a high-level intermediate representation (HIR) to a
/// regular expression pattern string. This particular printer uses constant
/// stack space and heap space proportional to the size of the HIR.
///
/// Since this printer is only using the HIR, the pattern it prints will likely
/// not resemble the original pattern at all. For example, a pattern like
/// `\pL` will have its entire class written out.
///
/// The purpose of this printer is to provide a means to mutate an HIR and then
/// build a regular expression from the result of that mutation. (A regex
/// library could provide a constructor from this HIR explicitly, but that
/// creates an unnecessary public coupling between the regex library and this
/// specific HIR representation.)
#[derive(Debug)]
pub struct Printer {
_priv: (),
}
impl Printer {
/// Create a new printer.
pub fn new() -> Printer {
PrinterBuilder::new().build()
}
/// Print the given `Ast` to the given writer. The writer must implement
/// `fmt::Write`. Typical implementations of `fmt::Write` that can be used
/// here are a `fmt::Formatter` (which is available in `fmt::Display`
/// implementations) or a `&mut String`.
pub fn print<W: fmt::Write>(&mut self, hir: &Hir, wtr: W) -> fmt::Result {
visitor::visit(hir, Writer { wtr })
}
}
#[derive(Debug)]
struct Writer<W> {
wtr: W,
}
impl<W: fmt::Write> Visitor for Writer<W> {
type Output = ();
type Err = fmt::Error;
fn finish(self) -> fmt::Result {
Ok(())
}
fn visit_pre(&mut self, hir: &Hir) -> fmt::Result {
match *hir.kind() {
HirKind::Empty => {
// Technically an empty sub-expression could be "printed" by
// just ignoring it, but in practice, you could have a
// repetition operator attached to an empty expression, and you
// really need something in the concrete syntax to make that
// work as you'd expect.
self.wtr.write_str(r"(?:)")?;
}
// Repetition operators are strictly suffix oriented.
HirKind::Repetition(_) => {}
HirKind::Literal(hir::Literal(ref bytes)) => {
// See the comment on the 'Concat' and 'Alternation' case below
// for why we put parens here. Literals are, conceptually,
// a special case of concatenation where each element is a
// character. The HIR flattens this into a Box<[u8]>, but we
// still need to treat it like a concatenation for correct
// printing. As a special case, we don't write parens if there
// is only one character. One character means there is no
// concat so we don't need parens. Adding parens would still be
// correct, but we drop them here because it tends to create
// rather noisy regexes even in simple cases.
let result = core::str::from_utf8(bytes);
let len = result.map_or(bytes.len(), |s| s.chars().count());
if len > 1 {
self.wtr.write_str(r"(?:")?;
}
match result {
Ok(string) => {
for c in string.chars() {
self.write_literal_char(c)?;
}
}
Err(_) => {
for &b in bytes.iter() {
self.write_literal_byte(b)?;
}
}
}
if len > 1 {
self.wtr.write_str(r")")?;
}
}
HirKind::Class(hir::Class::Unicode(ref cls)) => {
if cls.ranges().is_empty() {
return self.wtr.write_str("[a&&b]");
}
self.wtr.write_str("[")?;
for range in cls.iter() {
if range.start() == range.end() {
self.write_literal_char(range.start())?;
} else if u32::from(range.start()) + 1
== u32::from(range.end())
{
self.write_literal_char(range.start())?;
self.write_literal_char(range.end())?;
} else {
self.write_literal_char(range.start())?;
self.wtr.write_str("-")?;
self.write_literal_char(range.end())?;
}
}
self.wtr.write_str("]")?;
}
HirKind::Class(hir::Class::Bytes(ref cls)) => {
if cls.ranges().is_empty() {
return self.wtr.write_str("[a&&b]");
}
self.wtr.write_str("(?-u:[")?;
for range in cls.iter() {
if range.start() == range.end() {
self.write_literal_class_byte(range.start())?;
} else if range.start() + 1 == range.end() {
self.write_literal_class_byte(range.start())?;
self.write_literal_class_byte(range.end())?;
} else {
self.write_literal_class_byte(range.start())?;
self.wtr.write_str("-")?;
self.write_literal_class_byte(range.end())?;
}
}
self.wtr.write_str("])")?;
}
HirKind::Look(ref look) => match *look {
hir::Look::Start => {
self.wtr.write_str(r"\A")?;
}
hir::Look::End => {
self.wtr.write_str(r"\z")?;
}
hir::Look::StartLF => {
self.wtr.write_str("(?m:^)")?;
}
hir::Look::EndLF => {
self.wtr.write_str("(?m:$)")?;
}
hir::Look::StartCRLF => {
self.wtr.write_str("(?mR:^)")?;
}
hir::Look::EndCRLF => {
self.wtr.write_str("(?mR:$)")?;
}
hir::Look::WordAscii => {
self.wtr.write_str(r"(?-u:\b)")?;
}
hir::Look::WordAsciiNegate => {
self.wtr.write_str(r"(?-u:\B)")?;
}
hir::Look::WordUnicode => {
self.wtr.write_str(r"\b")?;
}
hir::Look::WordUnicodeNegate => {
self.wtr.write_str(r"\B")?;
}
},
HirKind::Capture(hir::Capture { ref name, .. }) => {
self.wtr.write_str("(")?;
if let Some(ref name) = *name {
write!(self.wtr, "?P<{}>", name)?;
}
}
// Why do this? Wrapping concats and alts in non-capturing groups
// is not *always* necessary, but is sometimes necessary. For
// example, 'concat(a, alt(b, c))' should be written as 'a(?:b|c)'
// and not 'ab|c'. The former is clearly the intended meaning, but
// the latter is actually 'alt(concat(a, b), c)'.
//
// It would be possible to only group these things in cases where
// it's strictly necessary, but it requires knowing the parent
// expression. And since this technique is simpler and always
// correct, we take this route. More to the point, it is a non-goal
// of an HIR printer to show a nice easy-to-read regex. Indeed,
// its construction forbids it from doing so. Therefore, inserting
// extra groups where they aren't necessary is perfectly okay.
HirKind::Concat(_) | HirKind::Alternation(_) => {
self.wtr.write_str(r"(?:")?;
}
}
Ok(())
}
fn visit_post(&mut self, hir: &Hir) -> fmt::Result {
match *hir.kind() {
// Handled during visit_pre
HirKind::Empty
| HirKind::Literal(_)
| HirKind::Class(_)
| HirKind::Look(_) => {}
HirKind::Repetition(ref x) => {
match (x.min, x.max) {
(0, Some(1)) => {
self.wtr.write_str("?")?;
}
(0, None) => {
self.wtr.write_str("*")?;
}
(1, None) => {
self.wtr.write_str("+")?;
}
(1, Some(1)) => {
// 'a{1}' and 'a{1}?' are exactly equivalent to 'a'.
return Ok(());
}
(m, None) => {
write!(self.wtr, "{{{},}}", m)?;
}
(m, Some(n)) if m == n => {
write!(self.wtr, "{{{}}}", m)?;
// a{m} and a{m}? are always exactly equivalent.
return Ok(());
}
(m, Some(n)) => {
write!(self.wtr, "{{{},{}}}", m, n)?;
}
}
if !x.greedy {
self.wtr.write_str("?")?;
}
}
HirKind::Capture(_)
| HirKind::Concat(_)
| HirKind::Alternation(_) => {
self.wtr.write_str(r")")?;
}
}
Ok(())
}
fn visit_alternation_in(&mut self) -> fmt::Result {
self.wtr.write_str("|")
}
}
impl<W: fmt::Write> Writer<W> {
fn write_literal_char(&mut self, c: char) -> fmt::Result {
if is_meta_character(c) {
self.wtr.write_str("\\")?;
}
self.wtr.write_char(c)
}
fn write_literal_byte(&mut self, b: u8) -> fmt::Result {
if b <= 0x7F && !b.is_ascii_control() && !b.is_ascii_whitespace() {
self.write_literal_char(char::try_from(b).unwrap())
} else {
write!(self.wtr, "(?-u:\\x{:02X})", b)
}
}
fn write_literal_class_byte(&mut self, b: u8) -> fmt::Result {
if b <= 0x7F && !b.is_ascii_control() && !b.is_ascii_whitespace() {
self.write_literal_char(char::try_from(b).unwrap())
} else {
write!(self.wtr, "\\x{:02X}", b)
}
}
}
#[cfg(test)]
mod tests {
use alloc::{
boxed::Box,
string::{String, ToString},
};
use crate::ParserBuilder;
use super::*;
fn roundtrip(given: &str, expected: &str) {
roundtrip_with(|b| b, given, expected);
}
fn roundtrip_bytes(given: &str, expected: &str) {
roundtrip_with(|b| b.utf8(false), given, expected);
}
fn roundtrip_with<F>(mut f: F, given: &str, expected: &str)
where
F: FnMut(&mut ParserBuilder) -> &mut ParserBuilder,
{
let mut builder = ParserBuilder::new();
f(&mut builder);
let hir = builder.build().parse(given).unwrap();
let mut printer = Printer::new();
let mut dst = String::new();
printer.print(&hir, &mut dst).unwrap();
// Check that the result is actually valid.
builder.build().parse(&dst).unwrap();
assert_eq!(expected, dst);
}
#[test]
fn print_literal() {
roundtrip("a", "a");
roundtrip(r"\xff", "\u{FF}");
roundtrip_bytes(r"\xff", "\u{FF}");
roundtrip_bytes(r"(?-u)\xff", r"(?-u:\xFF)");
roundtrip("☃", "☃");
}
#[test]
fn print_class() {
roundtrip(r"[a]", r"a");
roundtrip(r"[ab]", r"[ab]");
roundtrip(r"[a-z]", r"[a-z]");
roundtrip(r"[a-z--b-c--x-y]", r"[ad-wz]");
roundtrip(r"[^\x01-\u{10FFFF}]", "\u{0}");
roundtrip(r"[-]", r"\-");
roundtrip(r"[☃-⛄]", r"[☃-⛄]");
roundtrip(r"(?-u)[a]", r"a");
roundtrip(r"(?-u)[ab]", r"(?-u:[ab])");
roundtrip(r"(?-u)[a-z]", r"(?-u:[a-z])");
roundtrip_bytes(r"(?-u)[a-\xFF]", r"(?-u:[a-\xFF])");
// The following test that the printer escapes meta characters
// in character classes.
roundtrip(r"[\[]", r"\[");
roundtrip(r"[Z-_]", r"[Z-_]");
roundtrip(r"[Z-_--Z]", r"[\[-_]");
// The following test that the printer escapes meta characters
// in byte oriented character classes.
roundtrip_bytes(r"(?-u)[\[]", r"\[");
roundtrip_bytes(r"(?-u)[Z-_]", r"(?-u:[Z-_])");
roundtrip_bytes(r"(?-u)[Z-_--Z]", r"(?-u:[\[-_])");
// This tests that an empty character class is correctly roundtripped.
#[cfg(feature = "unicode-gencat")]
roundtrip(r"\P{any}", r"[a&&b]");
roundtrip_bytes(r"(?-u)[^\x00-\xFF]", r"[a&&b]");
}
#[test]
fn print_anchor() {
roundtrip(r"^", r"\A");
roundtrip(r"$", r"\z");
roundtrip(r"(?m)^", r"(?m:^)");
roundtrip(r"(?m)$", r"(?m:$)");
}
#[test]
fn print_word_boundary() {
roundtrip(r"\b", r"\b");
roundtrip(r"\B", r"\B");
roundtrip(r"(?-u)\b", r"(?-u:\b)");
roundtrip_bytes(r"(?-u)\B", r"(?-u:\B)");
}
#[test]
fn print_repetition() {
roundtrip("a?", "a?");
roundtrip("a??", "a??");
roundtrip("(?U)a?", "a??");
roundtrip("a*", "a*");
roundtrip("a*?", "a*?");
roundtrip("(?U)a*", "a*?");
roundtrip("a+", "a+");
roundtrip("a+?", "a+?");
roundtrip("(?U)a+", "a+?");
roundtrip("a{1}", "a");
roundtrip("a{2}", "a{2}");
roundtrip("a{1,}", "a+");
roundtrip("a{1,5}", "a{1,5}");
roundtrip("a{1}?", "a");
roundtrip("a{2}?", "a{2}");
roundtrip("a{1,}?", "a+?");
roundtrip("a{1,5}?", "a{1,5}?");
roundtrip("(?U)a{1}", "a");
roundtrip("(?U)a{2}", "a{2}");
roundtrip("(?U)a{1,}", "a+?");
roundtrip("(?U)a{1,5}", "a{1,5}?");
// Test that various zero-length repetitions always translate to an
// empty regex. This is more a property of HIR's smart constructors
// than the printer though.
roundtrip("a{0}", "(?:)");
roundtrip("(?:ab){0}", "(?:)");
#[cfg(feature = "unicode-gencat")]
{
roundtrip(r"\p{any}{0}", "(?:)");
roundtrip(r"\P{any}{0}", "(?:)");
}
}
#[test]
fn print_group() {
roundtrip("()", "((?:))");
roundtrip("(?P<foo>)", "(?P<foo>(?:))");
roundtrip("(?:)", "(?:)");
roundtrip("(a)", "(a)");
roundtrip("(?P<foo>a)", "(?P<foo>a)");
roundtrip("(?:a)", "a");
roundtrip("((((a))))", "((((a))))");
}
#[test]
fn print_alternation() {
roundtrip("|", "(?:(?:)|(?:))");
roundtrip("||", "(?:(?:)|(?:)|(?:))");
roundtrip("a|b", "[ab]");
roundtrip("ab|cd", "(?:(?:ab)|(?:cd))");
roundtrip("a|b|c", "[a-c]");
roundtrip("ab|cd|ef", "(?:(?:ab)|(?:cd)|(?:ef))");
roundtrip("foo|bar|quux", "(?:(?:foo)|(?:bar)|(?:quux))");
}
// This is a regression test that stresses a peculiarity of how the HIR
// is both constructed and printed. Namely, it is legal for a repetition
// to directly contain a concatenation. This particular construct isn't
// really possible to build from the concrete syntax directly, since you'd
// be forced to put the concatenation into (at least) a non-capturing
// group. Concurrently, the printer doesn't consider this case and just
// kind of naively prints the child expression and tacks on the repetition
// operator.
//
// As a result, if you attached '+' to a 'concat(a, b)', the printer gives
// you 'ab+', but clearly it really should be '(?:ab)+'.
//
// This bug isn't easy to surface because most ways of building an HIR
// come directly from the concrete syntax, and as mentioned above, it just
// isn't possible to build this kind of HIR from the concrete syntax.
// Nevertheless, this is definitely a bug.
//
#[test]
fn regression_repetition_concat() {
let expr = Hir::concat(alloc::vec![
Hir::literal("x".as_bytes()),
Hir::repetition(hir::Repetition {
min: 1,
max: None,
greedy: true,
sub: Box::new(Hir::literal("ab".as_bytes())),
}),
Hir::literal("y".as_bytes()),
]);
assert_eq!(r"(?:x(?:ab)+y)", expr.to_string());
let expr = Hir::concat(alloc::vec![
Hir::look(hir::Look::Start),
Hir::repetition(hir::Repetition {
min: 1,
max: None,
greedy: true,
sub: Box::new(Hir::concat(alloc::vec![
Hir::look(hir::Look::Start),
Hir::look(hir::Look::End),
])),
}),
Hir::look(hir::Look::End),
]);
assert_eq!(r"(?:\A\A\z\z)", expr.to_string());
}
// Just like regression_repetition_concat, but with the repetition using
// an alternation as a child expression instead.
//
#[test]
fn regression_repetition_alternation() {
let expr = Hir::concat(alloc::vec![
Hir::literal("ab".as_bytes()),
Hir::repetition(hir::Repetition {
min: 1,
max: None,
greedy: true,
sub: Box::new(Hir::alternation(alloc::vec![
Hir::literal("cd".as_bytes()),
Hir::literal("ef".as_bytes()),
])),
}),
Hir::literal("gh".as_bytes()),
]);
assert_eq!(r"(?:(?:ab)(?:(?:cd)|(?:ef))+(?:gh))", expr.to_string());
let expr = Hir::concat(alloc::vec![
Hir::look(hir::Look::Start),
Hir::repetition(hir::Repetition {
min: 1,
max: None,
greedy: true,
sub: Box::new(Hir::alternation(alloc::vec![
Hir::look(hir::Look::Start),
Hir::look(hir::Look::End),
])),
}),
Hir::look(hir::Look::End),
]);
assert_eq!(r"(?:\A(?:\A|\z)\z)", expr.to_string());
}
// This regression test is very similar in flavor to
// regression_repetition_concat in that the root of the issue lies in a
// peculiarity of how the HIR is represented and how the printer writes it
// out. Like the other regression, this one is also rooted in the fact that
// you can't produce the peculiar HIR from the concrete syntax. Namely, you
// just can't have a 'concat(a, alt(b, c))' because the 'alt' will normally
// be in (at least) a non-capturing group. Why? Because the '|' has very
// low precedence (lower that concatenation), and so something like 'ab|c'
// is actually 'alt(ab, c)'.
//
#[test]
fn regression_alternation_concat() {
let expr = Hir::concat(alloc::vec![
Hir::literal("ab".as_bytes()),
Hir::alternation(alloc::vec![
Hir::literal("mn".as_bytes()),
Hir::literal("xy".as_bytes()),
]),
]);
assert_eq!(r"(?:(?:ab)(?:(?:mn)|(?:xy)))", expr.to_string());
let expr = Hir::concat(alloc::vec![
Hir::look(hir::Look::Start),
Hir::alternation(alloc::vec![
Hir::look(hir::Look::Start),
Hir::look(hir::Look::End),
]),
]);
assert_eq!(r"(?:\A(?:\A|\z))", expr.to_string());
}
}