From 3656fd84cd1231c42928ef5333f4888b02e6658d Mon Sep 17 00:00:00 2001
From: Nate Cook <natecook@apple.com>
Date: Tue, 23 Sep 2025 11:12:12 -0500
Subject: [PATCH 1/3] Start of line in negative lookahead optimization fix

Don't use the content of negative lookaheads when determining
whether a start-of-line assertion can require that match has
to start at the beginning of the subject.

Fixes swiftlang/swift#81789.
rdar://152119639
---
 Sources/_StringProcessing/Regex/DSLTree.swift |  8 ++++++++
 Tests/RegexTests/MatchTests.swift             | 15 ++++++++++++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/Sources/_StringProcessing/Regex/DSLTree.swift b/Sources/_StringProcessing/Regex/DSLTree.swift
index 8d6a5fbc..5971cd93 100644
--- a/Sources/_StringProcessing/Regex/DSLTree.swift
+++ b/Sources/_StringProcessing/Regex/DSLTree.swift
@@ -791,6 +791,10 @@ extension DSLTree.Node {
 
     // Groups (and other parent nodes) defer to the child.
     case .nonCapturingGroup(let kind, let child):
+      // Don't let a negative lookahead affect this - need to continue to next sibling
+      if kind.isNegativeLookahead {
+        return nil
+      }
       options.beginScope()
       defer { options.endScope() }
       if case .changeMatchingOptions(let sequence) = kind.ast {
@@ -902,6 +906,10 @@ extension DSLTree {
       public static var negativeLookahead: Self {
         .init(ast: .negativeLookahead)
       }
+      
+      internal var isNegativeLookahead: Bool {
+        self.ast == .negativeLookahead
+      }
     }
 
     @_spi(RegexBuilder)
diff --git a/Tests/RegexTests/MatchTests.swift b/Tests/RegexTests/MatchTests.swift
index 27302cda..307ee3f9 100644
--- a/Tests/RegexTests/MatchTests.swift
+++ b/Tests/RegexTests/MatchTests.swift
@@ -46,7 +46,7 @@ func _firstMatch(
 ) throws -> (String, [String?])? {
   var regex = try Regex(regexStr, syntax: syntax).matchingSemantics(semanticLevel)
   let result = try regex.firstMatch(in: input)
-  
+
   func validateSubstring(_ substringInput: Substring) throws {
     // Sometimes the characters we add to a substring merge with existing
     // string members. This messes up cross-validation, so skip the test.
@@ -1629,6 +1629,14 @@ extension RegexTests {
     // engines generally enforce that lookbehinds are fixed width
     firstMatchTest(
       #"\d{3}(?<!USD\d{3})"#, input: "Price: JYP100", match: "100", xfail: true)
+    
+    // Assertions inside negative lookahead
+    firstMatchTest(
+      #"(?!\b)(With)"#, input: "dispatchWithName", match: "With")
+    firstMatchTest(
+      #"(?!^)(With)"#, input: "dispatchWithName", match: "With")
+    firstMatchTest(
+      #"(?!\s)^dispatch"#, input: "dispatchWithName", match: "dispatch")
   }
 
   func testMatchAnchors() throws {
@@ -2875,6 +2883,11 @@ extension RegexTests {
       ("\r\n", "\r\n")
     )
   }
+
+  func testIssue81789() throws {
+    let matches = "dispatchWithName".matches(of: #/(?!^)(With(?!No)|For|In|At|To)(?=[A-Z])/#)
+    XCTAssert(matches[0].output == ("With", "With"))
+  }
   
   func testNSRECompatibility() throws {
     // NSRE-compatibility includes scalar matching, so `[\r\n]` should match

From 21434ae6b238359b06c410ca45d0cfd529abd55f Mon Sep 17 00:00:00 2001
From: Nate Cook <natecook@apple.com>
Date: Mon, 29 Sep 2025 12:52:46 -0500
Subject: [PATCH 2/3] Add test cases for only-match-at-start with lookaheads

---
 Tests/RegexTests/CompileTests.swift | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Tests/RegexTests/CompileTests.swift b/Tests/RegexTests/CompileTests.swift
index 7ea38490..d3129130 100644
--- a/Tests/RegexTests/CompileTests.swift
+++ b/Tests/RegexTests/CompileTests.swift
@@ -589,5 +589,9 @@ extension RegexTests {
     try expectCanOnlyMatchAtStart("(foo)?^bar", true)  // The initial group must match ""
     try expectCanOnlyMatchAtStart("(?:foo)?^bar", true)
     try expectCanOnlyMatchAtStart("(foo)+^bar", false) // This can't actually match anywhere
+    
+    // Test lookahead assertions with anchor
+    try expectCanOnlyMatchAtStart("(?=^)foo", true)
+    try expectCanOnlyMatchAtStart("(?!^)foo", false)
   }
 }

From 33adee45a6d82fd6f6fb818c1fc12a473cff9c7e Mon Sep 17 00:00:00 2001
From: Nate Cook <natecook@apple.com>
Date: Mon, 29 Sep 2025 13:03:39 -0500
Subject: [PATCH 3/3] Use correct issue number

---
 Tests/RegexTests/MatchTests.swift | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Tests/RegexTests/MatchTests.swift b/Tests/RegexTests/MatchTests.swift
index 307ee3f9..e20beeaf 100644
--- a/Tests/RegexTests/MatchTests.swift
+++ b/Tests/RegexTests/MatchTests.swift
@@ -2884,7 +2884,8 @@ extension RegexTests {
     )
   }
 
-  func testIssue81789() throws {
+  func testIssue815() throws {
+    // Original report from https://github.com/swiftlang/swift-experimental-string-processing/issues/815
     let matches = "dispatchWithName".matches(of: #/(?!^)(With(?!No)|For|In|At|To)(?=[A-Z])/#)
     XCTAssert(matches[0].output == ("With", "With"))
   }