From 2f402eb3609ac8f14d77cf597a2804d9c97c885c Mon Sep 17 00:00:00 2001
From: Nate Cook <natecook@apple.com>
Date: Mon, 26 Oct 2026 09:27:01 -0500
Subject: [PATCH] Match literal at end of sub-character substring

Matching was failing for substrings that have a sub-character end
index. This change fixes the boundary check for those cases, so that
the end of the subject bounds is always a boundary.

rdar://190999851
---
 Sources/_StringProcessing/Engine/Processor.swift |  4 ++--
 Tests/RegexTests/MatchTests.swift                | 12 ++++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/Sources/_StringProcessing/Engine/Processor.swift b/Sources/_StringProcessing/Engine/Processor.swift
index 049ecaf23..cdd2bd3e5 100644
--- a/Sources/_StringProcessing/Engine/Processor.swift
+++ b/Sources/_StringProcessing/Engine/Processor.swift
@@ -799,7 +799,7 @@ extension String {
     let idx = unicodeScalars.index(after: pos)
     assert(idx <= end, "Input is a substring with a sub-scalar endIndex.")
 
-    if boundaryCheck && !isOnGraphemeClusterBoundary(idx) {
+    if boundaryCheck && idx != end && !isOnGraphemeClusterBoundary(idx) {
       return nil
     }
 
@@ -820,7 +820,7 @@ extension String {
 
     guard cur <= end else { return nil }
 
-    if boundaryCheck && !isOnGraphemeClusterBoundary(cur) {
+    if boundaryCheck && cur != end && !isOnGraphemeClusterBoundary(cur) {
       return nil
     }
 
diff --git a/Tests/RegexTests/MatchTests.swift b/Tests/RegexTests/MatchTests.swift
index 2d9a6b1c2..036265ad8 100644
--- a/Tests/RegexTests/MatchTests.swift
+++ b/Tests/RegexTests/MatchTests.swift
@@ -3058,4 +3058,16 @@ extension RegexTests {
     XCTAssertNotNil("testS".wholeMatch(of: regex))
     XCTAssertNotNil("tesTs".wholeMatch(of: regex))
   }
+
+  func testLiteralAtSubCharacterEnd() throws {
+    let cafe = "cafe\u{301}"
+    let sub = cafe[..<cafe.unicodeScalars.index(before: cafe.endIndex)]
+    for regex in [#/afe/#, #/af./#] {
+      XCTAssertEqual(sub.firstMatch(of: regex)?.0, "afe")
+    }
+    XCTAssertNotNil(sub.firstRange(of: "afe"))
+    
+    // No match for a literal ending sub-character
+    XCTAssertNil(cafe.firstRange(of: Regex<Substring>(verbatim: "afe")))
+  }
 }