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use crate::syntax::ast::{Pattern, Regex};
// SYNTAX
//
// regex ::= pattern
// pattern ::= alt-pattern
//
// alt-pattern ::= concat-pattern
// | concat-pattern '|' alt-pattern
// concat-pattern ::= star-pattern
// | star-pattern concat-pattern
// star-pattern ::= primary-pattern
// | primary-pattern '*'
// primary-pattern ::= <empty>
// | <non-meta-character>
// | '(' pattern ')'
pub fn parse(s: &str) -> Result<Regex, String> {
let mut parser = Parser::new(s.as_bytes());
parser.parse_regex()
}
struct Parser<'input> {
str: &'input [u8],
pos: usize,
}
impl<'input> Parser<'input> {
fn new(s: &'input [u8]) -> Self {
Self { str: s, pos: 0 }
}
fn parse_regex(&mut self) -> Result<Regex, String> {
let p = self.parse_pattern()?;
if self.pos == self.str.len() {
Ok(Regex { root: p })
} else {
Err(format!("unconsumed input: {}", self.pos))
}
}
fn parse_pattern(&mut self) -> Result<Box<Pattern>, String> {
self.parse_alt_pattern()
}
fn parse_alt_pattern(&mut self) -> Result<Box<Pattern>, String> {
let mut p1 = self.parse_concat_pattern()?;
loop {
if matches!(self.str.get(self.pos), Some(b'|')) {
self.pos += 1;
let p2 = self.parse_concat_pattern()?;
p1 = Box::new(Pattern::Alt(p1, p2));
} else {
return Ok(p1);
}
}
}
fn parse_concat_pattern(&mut self) -> Result<Box<Pattern>, String> {
let mut p1 = self.parse_star_pattern()?;
loop {
match self.str.get(self.pos) {
None | Some(b'|') | Some(b')') => return Ok(p1),
_ => {
let p2 = self.parse_star_pattern()?;
p1 = Box::new(Pattern::Concat(p1, p2));
}
}
}
}
fn parse_star_pattern(&mut self) -> Result<Box<Pattern>, String> {
let pat = self.parse_primary_pattern()?;
if matches!(self.str.get(self.pos), Some(b'*')) {
self.pos += 1;
Ok(Box::new(Pattern::Star(pat)))
} else {
Ok(pat)
}
}
fn parse_primary_pattern(&mut self) -> Result<Box<Pattern>, String> {
match self.str.get(self.pos) {
Some(b'(') => {
self.pos += 1;
let pat = self.parse_pattern()?;
if matches!(self.str.get(self.pos), Some(b')')) {
self.pos += 1;
Ok(pat)
} else {
Err("paren not balanced".into())
}
}
Some(b')') => Ok(Box::new(Pattern::Empty)),
Some(c) => {
self.pos += 1;
Ok(Box::new(Pattern::Literal(*c)))
}
None => Ok(Box::new(Pattern::Empty)),
}
}
}
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