4 files changed, 141 insertions, 0 deletions
diff --git a/src/lib.rs b/src/lib.rs
index b93cf3f..ecefcb8 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,5 @@
+mod syntax;
+
 pub fn add(left: u64, right: u64) -> u64 {
     left + right
 }
diff --git a/src/syntax.rs b/src/syntax.rs
new file mode 100644
index 0000000..abf7309
--- /dev/null
+++ b/src/syntax.rs
@@ -0,0 +1,27 @@
+pub mod ast;
+pub mod parse;
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parse_valid() {
+        assert!(parse::parse("").is_ok());
+        assert!(parse::parse("a").is_ok());
+        assert!(parse::parse("ab").is_ok());
+        assert!(parse::parse("abc").is_ok());
+        assert!(parse::parse("a|b").is_ok());
+        assert!(parse::parse("a|b*").is_ok());
+        assert!(parse::parse("(a|b)*").is_ok());
+        assert!(parse::parse("(((a|b)))*").is_ok());
+        assert!(parse::parse("a*b*").is_ok());
+    }
+
+    #[test]
+    fn parse_invalid() {
+        assert!(parse::parse("(").is_err());
+        assert!(parse::parse("()))").is_err());
+        assert!(parse::parse("(((())").is_err());
+    }
+}
diff --git a/src/syntax/ast.rs b/src/syntax/ast.rs
new file mode 100644
index 0000000..bb12e81
--- /dev/null
+++ b/src/syntax/ast.rs
@@ -0,0 +1,11 @@
+pub struct Regex {
+    pub root: Box<Pattern>,
+}
+
+pub enum Pattern {
+    Empty,
+    Literal(u8),
+    Concat(Box<Pattern>, Box<Pattern>),
+    Alt(Box<Pattern>, Box<Pattern>),
+    Star(Box<Pattern>),
+}
diff --git a/src/syntax/parse.rs b/src/syntax/parse.rs
new file mode 100644
index 0000000..d7933ce
--- /dev/null
+++ b/src/syntax/parse.rs
@@ -0,0 +1,101 @@
+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)),
+        }
+    }
+}