Model reverse as an option

Author: Jacob Hearst

Sources/_RegexParser/Regex/AST/MatchingOptions.swift

@@ -47,6 +47,9 @@ extension AST {
 
       // NSRegularExpression compatibility special-case
       case nsreCompatibleDot        // no AST representation
+
+      // Lookbehind support
+      case reverse                  // no AST representation
     }
 
     public var kind: Kind

Sources/_RegexParser/Regex/Parse/Sema.swift

@@ -143,7 +143,7 @@ extension RegexValidator {
     case .caseInsensitive, .possessiveByDefault, .reluctantByDefault,
         .singleLine, .multiline, .namedCapturesOnly, .extended, .extraExtended,
         .asciiOnlyDigit, .asciiOnlyWord, .asciiOnlySpace, .asciiOnlyPOSIXProps,
-        .nsreCompatibleDot:
+        .nsreCompatibleDot, .reverse:
       break
     }
   }

Sources/_StringProcessing/ByteCodeGen.swift

@@ -16,7 +16,9 @@ internal import _RegexParser
 
 extension Compiler {
   struct ByteCodeGen {
-    var reverse = false
+    var reverse: Bool {
+      options.reversed
+    }
     var options: MatchingOptions
     var builder = MEProgram.Builder()
     /// A Boolean indicating whether the first matchable atom has been emitted.
@@ -392,14 +394,16 @@ fileprivate extension Compiler.ByteCodeGen {
       throw Unsupported("Lookarounds with custom consumers")
     }
 
-    let previousReverse = reverse
-    reverse = !kind.forwards
+    defer { options.endScope() }
+    options.beginScope()
+    // TODO: JH - Is it okay to use .fake here?
+    options.apply(.init(adding: [.init(.reverse, location: .fake)]))
+
     if kind.positive {
       try emitPositiveLookaround(child)
     } else {
       try emitNegativeLookaround(child)
     }
-    reverse = previousReverse
   }
 
   mutating func emitAtomicNoncapturingGroup(

Sources/_StringProcessing/LiteralPrinter.swift

@@ -575,6 +575,9 @@ extension AST.MatchingOption.Kind {
     // NSRE Compatibility option; no literal representation
     case .nsreCompatibleDot: return nil
 
+    // Reverse option for lookbehinds; no literal representation
+    case .reverse: return nil
+
     #if RESILIENT_LIBRARIES
     @unknown default:
       fatalError()

Sources/_StringProcessing/MatchingOptions.swift

@@ -129,6 +129,10 @@ extension MatchingOptions {
   var usesNSRECompatibleDot: Bool {
     stack.last!.contains(.nsreCompatibleDot)
   }
+
+  var reversed: Bool {
+    stack.last!.contains(.reverse)
+  }
 }
 
 // MARK: - Implementation
@@ -152,6 +156,9 @@ extension MatchingOptions {
     case withoutAnchoringBounds
     case nsreCompatibleDot
 
+    // Not available via regex literal flags
+    case reverse
+
     // Oniguruma options
     case asciiOnlyDigit
     case asciiOnlyPOSIXProps
@@ -217,6 +224,8 @@ extension MatchingOptions {
         self = .extended
       case .extraExtended:
         self = .extraExtended
+      case .reverse:
+        self = .reverse
       #if RESILIENT_LIBRARIES
       @unknown default:
         fatalError()