Fix Telegram HTML links broken by italic regex matching inside href URLs (#2164)

reItalic (_text_) ran after reLink converted [text](url) to <a href>,
injecting <i> tags into URLs containing underscores (e.g. Google Flights
URL-safe base64 in the tfs param). Telegram silently dropped such malformed
<a> tags, causing only 1 of 3 links to appear in messages.

Fix: extract markdown links into placeholders before any formatting runs,
restore them as <a href> last — same pattern used for code blocks.

pkg/channels/telegram/parser_markdown_to_html.go

@@ -16,14 +16,15 @@ func markdownToTelegramHTML(text string) string {
 	inlineCodes := extractInlineCodes(text)
 	text = inlineCodes.text
 
+	links := extractLinks(text)
+	text = links.text
+
 	text = reHeading.ReplaceAllString(text, "$1")
 
 	text = reBlockquote.ReplaceAllString(text, "$1")
 
 	text = escapeHTML(text)
 
-	text = reLink.ReplaceAllString(text, `<a href="$2">$1</a>`)
-
 	text = reBoldStar.ReplaceAllString(text, "<b>$1</b>")
 
 	text = reBoldUnder.ReplaceAllString(text, "<b>$1</b>")
@@ -40,6 +41,12 @@ func markdownToTelegramHTML(text string) string {
 
 	text = reListItem.ReplaceAllString(text, "• ")
 
+	for i, lnk := range links.links {
+		label := escapeHTML(lnk[0])
+		url := lnk[1]
+		text = strings.ReplaceAll(text, fmt.Sprintf("\x00LK%d\x00", i), fmt.Sprintf(`<a href="%s">%s</a>`, url, label))
+	}
+
 	for i, code := range inlineCodes.codes {
 		escaped := escapeHTML(code)
 		text = strings.ReplaceAll(text, fmt.Sprintf("\x00IC%d\x00", i), fmt.Sprintf("<code>%s</code>", escaped))
@@ -57,6 +64,29 @@ func markdownToTelegramHTML(text string) string {
 	return text
 }
 
+type linkMatch struct {
+	text  string
+	links [][2]string // [label, url]
+}
+
+func extractLinks(text string) linkMatch {
+	matches := reLink.FindAllStringSubmatch(text, -1)
+
+	extracted := make([][2]string, 0, len(matches))
+	for _, match := range matches {
+		extracted = append(extracted, [2]string{match[1], match[2]})
+	}
+
+	i := 0
+	text = reLink.ReplaceAllStringFunc(text, func(m string) string {
+		placeholder := fmt.Sprintf("\x00LK%d\x00", i)
+		i++
+		return placeholder
+	})
+
+	return linkMatch{text: text, links: extracted}
+}
+
 type codeBlockMatch struct {
 	text  string
 	codes []string

pkg/channels/telegram/parser_markdown_to_html_test.go

@@ -0,0 +1,66 @@
+package telegram
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_markdownToTelegramHTML(t *testing.T) {
+	cases := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name:     "plain text",
+			input:    "hello world",
+			expected: "hello world",
+		},
+		{
+			name:     "bold",
+			input:    "**bold text**",
+			expected: "<b>bold text</b>",
+		},
+		{
+			name:     "italic",
+			input:    "_italic text_",
+			expected: "<i>italic text</i>",
+		},
+		{
+			name:     "link without underscores in URL",
+			input:    "[click here](https://example.com/path)",
+			expected: `<a href="https://example.com/path">click here</a>`,
+		},
+		{
+			name: "link with underscores in URL is not corrupted by italic regex",
+			// Google Flights URLs use URL-safe base64 with underscores in the tfs param.
+			// Previously reItalic ran after reLink, matching _text_ inside href and injecting
+			// <i> tags into the URL, which broke the link in Telegram.
+			input:    "[3 → 10 сентября — от $202](https://www.google.com/travel/flights/search?tfs=CBwQAho_EgoyURL_safe_base64)",
+			expected: `<a href="https://www.google.com/travel/flights/search?tfs=CBwQAho_EgoyURL_safe_base64">3 → 10 сентября — от $202</a>`,
+		},
+		{
+			name:     "multiple links all survive",
+			input:    "[first](https://a.com/path_one) and [second](https://b.com/path_two_x)",
+			expected: `<a href="https://a.com/path_one">first</a> and <a href="https://b.com/path_two_x">second</a>`,
+		},
+		{
+			name:     "link label with HTML special chars is escaped",
+			input:    "[a & b](https://example.com)",
+			expected: `<a href="https://example.com">a &amp; b</a>`,
+		},
+		{
+			name:     "HTML special chars in plain text are escaped",
+			input:    "a & b < c > d",
+			expected: "a &amp; b &lt; c &gt; d",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			actual := markdownToTelegramHTML(tc.input)
+			require.Equal(t, tc.expected, actual)
+		})
+	}
+}