Fix a concatenation coalescing issue

Sources/_StringProcessing/Regex/DSLList.swift

@@ -125,7 +125,7 @@ extension DSLList {
     case .concatenation(let count):
       var position = position + 1
       if findLast {
-        for _ in 0..<(count - 1) {
+        for _ in (0..<count).dropLast() {
           skipNode(&position)
           position += 1
         }
@@ -165,17 +165,21 @@ extension DSLList {
     // Remove the postfix node and fix up any parent concatenations
     other.nodes.remove(at: postfixIndex)
     var i = postfixIndex - 1
-  Loop:
+
     while i >= 0 {
       switch other.nodes[i] {
       case .concatenation(let count):
-        other.nodes[i] = .concatenation(count - 1)
-        break Loop
+        // Omit a concatenation entirely if it would have zero children.
+        if count == 1 {
+          other.nodes[i] = .empty
+        } else {
+          other.nodes[i] = .concatenation(count - 1)
+        }
       case .limitCaptureNesting, .ignoreCapturesInTypedOutput:
         other.nodes.remove(at: i)
         i -= 1
       default:
-        break Loop
+        return
       }
     }
   }

Tests/RegexBuilderTests/RegexDSLTests.swift

@@ -1673,6 +1673,29 @@ class RegexDSLTests: XCTestCase {
       """)
   }
 
+  func testManyConcatenatedComponents() throws {
+    let r = Regex {
+      #/a\n/#
+      Regex<Substring>(verbatim: "/some/path")
+      #/\n/#
+      #/b\n/#
+      Regex<Substring>(verbatim: "/some/path")
+      #/\n/#
+      #/c\n/#
+      Regex<Substring>(verbatim: "/some/path")
+      #/\n/#
+      #/d\n/#
+      Regex<Substring>(verbatim: "/some/path")
+      #/\n/#
+      #/e\n/#
+      Regex<Substring>(verbatim: "/some/path")
+      #/\n/#
+      #/f/#
+    }
+    let input = "a\n/some/path\nb\n/some/path\nc\n/some/path\nd\n/some/path\ne\n/some/path\nf"
+    XCTAssertNotNil(try r.wholeMatch(in: input))
+  }
+
   func testRegexComponentBuilderResultType() {
     // Test that the user can declare a closure or computed property marked with
     // `@RegexComponentBuilder` with `Regex` as the result type.