From 83e8139da2acd7fc7c24794c379f4a369ea5fb4d Mon Sep 17 00:00:00 2001
From: Junjun Zhang <topcheer@me.com>
Date: Fri, 25 Sep 2026 12:33:32 +0800
Subject: [PATCH] fix(im): lowercase matrix mention needle for mixed-case
 localparts (#2749 follow-up)

The post-match boundary rewrite dropped the old (?i) case-insensitivity:
the body is lowered by callers but the needle ('@' + localPart) was not,
so mixed-case localparts (@Bot-:server on self-hosted homeservers) never
matched - the #2749 miss resurfacing as a case variant. Lower the needle
in both mentionsLocalPart and stripLocalPartMention.

Tests: mixed-case non-word ending, uppercase body, lowercase-body-of-
uppercase-localpart, and stripMention round trip.
---
 internal/im/matrix_adapter.go    |  8 ++++++--
 internal/im/zz_issue2749_test.go | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/internal/im/matrix_adapter.go b/internal/im/matrix_adapter.go
index 6feb55bab..9515d4a17 100644
--- a/internal/im/matrix_adapter.go
+++ b/internal/im/matrix_adapter.go
@@ -832,7 +832,10 @@ const matrixLocalPartContChars = "abcdefghijklmnopqrstuvwxyz0123456789._=-+/"
 // mentionsLocalPart reports whether text contains "@localpart" NOT followed
 // by a legal localpart continuation character (RE2-safe boundary).
 func mentionsLocalPart(lower, localPart string) bool {
-	needle := "@" + localPart
+	// #2749 case follow-up: the old (?i) regex was case-insensitive, and
+	// self-hosted homeservers may issue mixed-case localparts (@Bot:...).
+	// The body is already lowered by callers; lower the needle too.
+	needle := "@" + strings.ToLower(localPart)
 	for start := 0; ; {
 		i := strings.Index(lower[start:], needle)
 		if i < 0 {
@@ -851,7 +854,8 @@ func mentionsLocalPart(lower, localPart string) bool {
 // prefixes of a longer handle (same boundary rule as mentionsLocalPart).
 func stripLocalPartMention(text, localPart string) string {
 	lower := strings.ToLower(text)
-	needle := "@" + localPart
+	// #2749 case follow-up: lower the needle to match the (?i) behavior.
+	needle := "@" + strings.ToLower(localPart)
 	var b strings.Builder
 	for i := 0; i < len(text); {
 		if strings.HasPrefix(lower[i:], needle) {
diff --git a/internal/im/zz_issue2749_test.go b/internal/im/zz_issue2749_test.go
index 5247e1d8e..52a1e228d 100644
--- a/internal/im/zz_issue2749_test.go
+++ b/internal/im/zz_issue2749_test.go
@@ -58,3 +58,22 @@ func TestIssue2749_WordEndingLocalpartUnchanged(t *testing.T) {
 		t.Errorf("prefix confusion regression")
 	}
 }
+
+func TestIssue2749_MixedCaseLocalpart(t *testing.T) {
+	// #2749 case follow-up: self-hosted homeservers may issue mixed-case
+	// localparts; the old (?i) regex matched them. Lower both sides.
+	a := &matrixAdapter{userID: "@Bot-:matrix.org"}
+	if !a.hasMention("@Bot- check the build", nil) {
+		t.Errorf("mixed-case non-word-ending mention must match")
+	}
+	if !a.hasMention("hey @BOT- run tests", nil) {
+		t.Errorf("uppercase body mention of mixed-case localpart must match")
+	}
+	b := &matrixAdapter{userID: "@Al:matrix.org"}
+	if !b.hasMention("@al do it", nil) {
+		t.Errorf("lowercase body mention of uppercase localpart must match")
+	}
+	if got := a.stripMention("@Bot- check the build"); got != "check the build" {
+		t.Errorf("stripMention mixed-case = %q, want %q", got, "check the build")
+	}
+}