From a3323ad552779727694659ce65cac35d3435332c Mon Sep 17 00:00:00 2001
From: Max Leske <maxleske@gmail.com>
Date: Sun, 18 Feb 2024 14:35:59 +0100
Subject: [PATCH] fix: consider escaped parenthesis when removing flag groups

---
 regex/operators/assembler.go      | 19 +++++++++++++++++--
 regex/operators/assembler_test.go | 10 ++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/regex/operators/assembler.go b/regex/operators/assembler.go
index 57361a8..0003e7d 100644
--- a/regex/operators/assembler.go
+++ b/regex/operators/assembler.go
@@ -277,14 +277,29 @@ func replaceFlagGroup(input string, location []int) string {
 		char := input[index]
 		switch char {
 		case '(':
-			parensCounter++
+			if !isEscaped(input, index) {
+				parensCounter++
+			}
 		case ')':
-			parensCounter--
+			if !isEscaped(input, index) {
+				parensCounter--
+			}
 		}
 	}
 	return input[:groupStart] + input[bodyStart:index-1] + input[index:]
 }
 
+func isEscaped(input string, position int) bool {
+	escapeCounter := 0
+	for backtrackIndex := position - 1; backtrackIndex >= 0; backtrackIndex++ {
+		if input[backtrackIndex] != '\\' {
+			break
+		}
+		escapeCounter++
+	}
+	return escapeCounter%2 != 0
+}
+
 func (a *Operator) startPreprocessor(processorName string, args []string) error {
 	logger.Trace().Msgf("Found processor %s start\n", processorName)
 	switch processorName {
diff --git a/regex/operators/assembler_test.go b/regex/operators/assembler_test.go
index bf7ffa5..aea66e4 100644
--- a/regex/operators/assembler_test.go
+++ b/regex/operators/assembler_test.go
@@ -1005,3 +1005,13 @@ func (s *assemblerTestSuite) TestAssemble_ComplexAppendWithAlternation() {
 	s.Require().NoError(err)
 	s.Equal("process_prop-start_(?:access|env)_prop-finish_", output)
 }
+
+func (s *assemblerTestSuite) TestAssemble_FlagGroupReplacementWithEscapedParentheses() {
+	contents := `^\)ab\(c(capture)`
+	assembler := NewAssembler(s.ctx)
+
+	output, err := assembler.Run(contents)
+
+	s.Require().NoError(err)
+	s.Equal(contents, output)
+}