From a99b33314fc22d50f23f5eafe64161bcb7d023f2 Mon Sep 17 00:00:00 2001
From: Lily Lin <lily_lin2@apple.com>
Date: Thu, 11 Aug 2022 11:02:19 -0700
Subject: [PATCH] Backreferences do not guarantee forward progress

---
 Sources/_StringProcessing/ByteCodeGen.swift | 2 ++
 Tests/RegexTests/MatchTests.swift           | 4 ++++
 2 files changed, 6 insertions(+)

diff --git a/Sources/_StringProcessing/ByteCodeGen.swift b/Sources/_StringProcessing/ByteCodeGen.swift
index 66fefc49e..44eb75402 100644
--- a/Sources/_StringProcessing/ByteCodeGen.swift
+++ b/Sources/_StringProcessing/ByteCodeGen.swift
@@ -1019,6 +1019,8 @@ extension DSLTree.Node {
     case .atom(let atom):
       switch atom {
       case .changeMatchingOptions, .assertion: return false
+      // Captures may be nil so backreferences may be zero length matches
+      case .backreference: return false
       default: return true
       }
     case .trivia, .empty:
diff --git a/Tests/RegexTests/MatchTests.swift b/Tests/RegexTests/MatchTests.swift
index 794e57b16..66e7a3791 100644
--- a/Tests/RegexTests/MatchTests.swift
+++ b/Tests/RegexTests/MatchTests.swift
@@ -2549,4 +2549,8 @@ extension RegexTests {
     expectProgram(for: "a{\(maxStorable-1),\(maxStorable*2)}", doesNotContain: [.quantify])
     expectProgram(for: "a{\(maxStorable),\(maxStorable*2+1)}", doesNotContain: [.quantify])
   }
+  
+  func testFuzzerArtifacts() throws {
+    expectCompletion(regex: #"(b?)\1*"#, in: "a")
+  }
 }