Limit auto-possessification to single atom quantifiers

Author: natecook1000

Sources/_StringProcessing/Optimizations/AutoPossessification.swift

@@ -24,7 +24,6 @@ extension DSLList {
       switch atom {
       case .changeMatchingOptions(let seq):
         // Exit early if an atom changes the matching options.
-        // TODO: Allow some/all options changes.
         if allowOptionsChanges {
           options.apply(seq.ast)
           return nil
@@ -143,15 +142,17 @@ extension DSLList {
       let quantPosition = position
       position += 1
 
-      // Do a search within this quantification's contents
-      // FIXME: How to handle an inner quantification surfacing here?
-      var innerPosition = position
-      _ = autoPossessifyNextQuantification(&innerPosition, options: &options)
-      
-      switch _requiredAtomImpl(&position, options: &options, allowOptionsChanges: false) {
-      case .some(let atom?):
+      // Limit auto-possessification to a single quantified atom, to avoid
+      // issues of overlapped matches.
+      guard position < nodes.count else {
+        return nil
+      }
+      switch nodes[position] {
+      case .atom(let atom) where atom.isMatchable:
         return (quantPosition, atom)
-      case .none, .some(.none):
+      default:
+        var innerPosition = position
+        _ = autoPossessifyNextQuantification(&innerPosition, options: &options)
         return nil
       }
 

Tests/RegexTests/OptimizationTests.swift

@@ -31,7 +31,7 @@ import Testing
   }
 
   @available(macOS 9999, *)
-  @Test(arguments: [#/a+b/#, #/a*b/#, #/\w+\s/#, #/(?:a+b|b+a)/#, #/(?:(?:a+b)+b)/#, #/\d+a/#, #/a+A/#])
+  @Test(arguments: [#/a+b/#, #/a*b/#, #/\w+\s/#, #/(?:a+b|b+a)/#, #/\d+a/#, #/a+A/#])
   func autoPossessify(pattern: Regex<Substring>) throws {
     var list = DSLList(tree: pattern.program.tree)
     list.autoPossessify()
@@ -49,7 +49,8 @@ import Testing
   @available(macOS 9999, *)
   @Test(arguments: [
     #/a?/#, #/a+a/#, #/a+(?:b|c)/#, #/(?:a+|b+)/#, #/[a]/#, #/a?a/#,
-    #/(?i)a+A/#, #/(?i:a+A)/#  // case insensitivity when checking exclusion
+    #/(?i)a+A/#, #/(?i:a+A)/#,  // case insensitivity when checking exclusion
+    #/(?:(?:ab)+b)/#,           // single atom quantifications only
   ])
   func noAutoPossessify(pattern: Regex<Substring>) throws {
     var list = DSLList(tree: pattern.program.tree)