From 1a138779293823177613238591768077d781de05 Mon Sep 17 00:00:00 2001
From: Jonas Maier <jonas@x77.dev>
Date: Tue, 2 Jun 2026 12:39:41 +0200
Subject: regex: parse lookahead and lookbehind

---
 src/parse/regex/mod.rs | 53 ++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 49 insertions(+), 4 deletions(-)

(limited to 'src/parse/regex/mod.rs')

diff --git a/src/parse/regex/mod.rs b/src/parse/regex/mod.rs
index 51527ae..10d332e 100644
--- a/src/parse/regex/mod.rs
+++ b/src/parse/regex/mod.rs
@@ -6,6 +6,18 @@ mod byte_range;
 mod dfa;
 mod enfa;
 
+#[derive(PartialEq, Eq, Debug, Clone, Copy)]
+pub enum LookDirection {
+    Ahead,
+    Behind,
+}
+
+#[derive(PartialEq, Eq, Debug, Clone, Copy)]
+pub enum LookPolarity {
+    Positive,
+    Negative,
+}
+
 #[derive(PartialEq, Debug, Clone)]
 pub enum Pattern {
     Byte(u8),
@@ -13,6 +25,7 @@ pub enum Pattern {
     Alt(Vec<Pattern>),
     Concat(Vec<Pattern>),
     Rep(Box<Pattern>, u32, Option<u32>),
+    Assertion(LookDirection, LookPolarity, Box<Pattern>),
     Nothing,
 }
 
@@ -166,6 +179,22 @@ fn parse_atom(s: &mut super::Cursor<'_>) -> Result<Pattern> {
         }
         b'(' => {
             s.adv();
+
+            let mut assertion = None;
+            if s.buf.starts_with(b"?=") {
+                s.advance(2);
+                assertion = Some((LookDirection::Ahead, LookPolarity::Positive));
+            } else if s.buf.starts_with(b"?!") {
+                s.advance(2);
+                assertion = Some((LookDirection::Ahead, LookPolarity::Negative));
+            } else if s.buf.starts_with(b"?<=") {
+                s.advance(3);
+                assertion = Some((LookDirection::Behind, LookPolarity::Positive));
+            } else if s.buf.starts_with(b"?<!") {
+                s.advance(3);
+                assertion = Some((LookDirection::Behind, LookPolarity::Negative));
+            }
+
             s.highlight_from(begin, OtherHighlights::RegexSymbol);
             let inner = parse0(s)?;
             if !s.has() {
@@ -176,7 +205,12 @@ fn parse_atom(s: &mut super::Cursor<'_>) -> Result<Pattern> {
                 return Err(ParseError::Expected(')'));
             }
             s.highlight_from(begin, OtherHighlights::RegexSymbol);
-            Ok(inner)
+
+            if let Some((dir, pol)) = assertion {
+                Ok(Pattern::Assertion(dir, pol, Box::new(inner)))
+            } else {
+                Ok(inner)
+            }
         }
         b'.' => {
             s.adv();
@@ -217,11 +251,22 @@ impl std::fmt::Debug for CompiledPattern {
     }
 }
 
+#[derive(Debug)]
+pub enum CompilationError {
+    Enfa(enfa::EnfaTranslationError),
+}
+
+impl From<enfa::EnfaTranslationError> for CompilationError {
+    fn from(value: enfa::EnfaTranslationError) -> Self {
+        Self::Enfa(value)
+    }
+}
+
 impl Pattern {
-    pub fn compile(self) -> CompiledPattern {
-        let enfa = enfa::ENFA::from(self);
+    pub fn try_compile(self) -> std::result::Result<CompiledPattern, CompilationError> {
+        let enfa = enfa::ENFA::try_from(self)?;
         let dfa = dfa::DFA::from(enfa);
-        CompiledPattern { dfa }
+        Ok(CompiledPattern { dfa })
     }
 }
 
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