From b0fa52c3afd4ed33899473c0f6c69aad7a202b5b Mon Sep 17 00:00:00 2001
From: Kevin Newton <kddnewton@gmail.com>
Date: Tue, 23 Jun 2026 14:39:06 -0400
Subject: [PATCH] Fix up regex delimited by \r\n

---
 src/prism.c               | 10 +++++++++-
 test/prism/regexp_test.rb | 19 +++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/src/prism.c b/src/prism.c
index 0960e39747..e5e7a0005d 100644
--- a/src/prism.c
+++ b/src/prism.c
@@ -2249,7 +2249,15 @@ pm_regular_expression_flags_create(pm_parser_t *parser, const pm_token_t *closin
     if (closing->type == PM_TOKEN_REGEXP_END) {
         pm_buffer_t unknown_flags = { 0 };
 
-        for (const uint8_t *flag = closing->start + 1; flag < closing->end; flag++) {
+        // The closing delimiter is normally a single byte, so the options
+        // follow it. A `\r\n` newline delimiter is two bytes, however, so we
+        // skip past it to avoid misreading the trailing `\n` as an option.
+        const uint8_t *flag = closing->start + 1;
+        if ((closing->end - closing->start) >= 2 && closing->start[0] == '\r' && closing->start[1] == '\n') {
+            flag++;
+        }
+
+        for (; flag < closing->end; flag++) {
             switch (*flag) {
                 case 'i': flags |= PM_REGULAR_EXPRESSION_FLAGS_IGNORE_CASE; break;
                 case 'm': flags |= PM_REGULAR_EXPRESSION_FLAGS_MULTI_LINE; break;
diff --git a/test/prism/regexp_test.rb b/test/prism/regexp_test.rb
index cde0c23f97..41c51a111c 100644
--- a/test/prism/regexp_test.rb
+++ b/test/prism/regexp_test.rb
@@ -237,8 +237,27 @@ def test_last_encoding_option_wins
       assert_equal Regexp::NOENCODING, option
     end
 
+    # A `%r` regexp can use a newline (LF or CRLF) as its delimiter. The closing
+    # delimiter terminates the regexp and must not be misread as an option -- in
+    # particular the trailing `\n` of a `\r\n` delimiter.
+    def test_newline_delimiter
+      assert_newline_regexp("%r\nfoo\n", "foo", 0)
+      assert_newline_regexp("%r\r\nfoo\r\n", "foo", 0)
+      assert_newline_regexp("%r\r\nfoo\r\ni", "foo", Regexp::IGNORECASE)
+      assert_newline_regexp("%r\r\nfoo\r\nmix", "foo", Regexp::IGNORECASE | Regexp::MULTILINE | Regexp::EXTENDED)
+      assert_newline_regexp("%r\r\n\r\n", "", 0)
+    end
+
     private
 
+    def assert_newline_regexp(source, content, options)
+      node = Prism.parse_statement(source)
+
+      assert_kind_of RegularExpressionNode, node
+      assert_equal content, node.content
+      assert_equal options, node.options
+    end
+
     def assert_valid_regexp(source)
       assert Prism.parse_success?("/#{source}/ =~ \"\"")
     end