Start of line in negative lookahead optimization fix

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)

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)
   }
 }

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 +2883,12 @@ extension RegexTests {
       ("\r\n", "\r\n")
     )
   }
+
+  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"))
+  }
 
   func testNSRECompatibility() throws {
     // NSRE-compatibility includes scalar matching, so `[\r\n]` should match