From 8719e38d385f8551143605a70add7f443864cbb9 Mon Sep 17 00:00:00 2001
From: Flavio Soibelmann Glock <fglock@gmail.com>
Date: Wed, 28 Jan 2026 20:23:37 +0100
Subject: [PATCH] Fix nested \Q sequences in string parsing

- Fix \Q idempotency in StringDoubleQuoted parser
- Multiple \Q sequences no longer create nested quotemeta regions
- \Q sequences are ignored when already in quotemeta mode
- parseEscapeSequence properly handles \Q in quotemeta mode
- Test 16 (PL_lex_casestack) now passes correctly
- All unit tests pass, make succeeds
- Regex tests maintain 891/1296 passing (exceeds 888 target)

Fixes parser.t test 16: "\Q\Q\Q\Q\Q\Q\Q\Q\Q\Q\Q\Q\Qa" should equal "a"
---
 .../perlonjava/parser/StringDoubleQuoted.java | 25 ++++++++++++++-----
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/src/main/java/org/perlonjava/parser/StringDoubleQuoted.java b/src/main/java/org/perlonjava/parser/StringDoubleQuoted.java
index a682d8fe7..12dc4a063 100644
--- a/src/main/java/org/perlonjava/parser/StringDoubleQuoted.java
+++ b/src/main/java/org/perlonjava/parser/StringDoubleQuoted.java
@@ -357,8 +357,13 @@ protected void parseEscapeSequence() {
                 }
                 inQuotemeta = false;
             } else {
-                // Everything else is literal, including the backslash
-                currentSegment.append("\\");
+                // Everything else is literal, but \Q is ignored completely in quotemeta mode
+                if (!token.text.startsWith("Q")) {
+                    // For anything other than \Q, append the backslash literally
+                    currentSegment.append("\\");
+                }
+                // Always consume the character after the backslash
+                TokenUtils.consumeChar(parser);
             }
             return;
         }
@@ -398,8 +403,12 @@ private void parseDoubleQuotedEscapesRegex() {
             // Quotemeta modifier
             case "Q" -> {
                 flushCurrentSegment();
-                inQuotemeta = true;
-                caseModifiers.push(new CaseModifier("Q", false));
+                // \Q is idempotent - don't nest multiple \Q sequences
+                if (!inQuotemeta) {
+                    inQuotemeta = true;
+                    caseModifiers.push(new CaseModifier("Q", false));
+                }
+                // If already in quotemeta mode, just ignore additional \Q
             }
 
             // Unknown escape - treat as literal character
@@ -479,8 +488,12 @@ private void parseDoubleQuotedEscapes() {
             // Quotemeta modifier
             case "Q" -> {
                 flushCurrentSegment();
-                inQuotemeta = true;
-                caseModifiers.push(new CaseModifier("Q", false));
+                // \Q is idempotent - don't nest multiple \Q sequences
+                if (!inQuotemeta) {
+                    inQuotemeta = true;
+                    caseModifiers.push(new CaseModifier("Q", false));
+                }
+                // If already in quotemeta mode, just ignore additional \Q
             }
 
             // Other escape sequences