MM-55267 Add ability for server-side Markdown code to understand emojis (#25332)

* MM-55267 Add ability for server-side Markdown code to understand emojis

* Remove unused regex

server/public/shared/markdown/emoji.go

@@ -0,0 +1,42 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package markdown
+
+import (
+	"regexp"
+)
+
+// Based off the mobile app's emoji parsing from https://github.com/mattermost/commonmark.js
+
+var (
+	emojiRegex = regexp.MustCompile(`^:([a-z0-9_\-+]+):\B`)
+)
+
+// parseEmoji attempts to parse a named emoji (eg. :taco:) starting at the current parser position. If an emoji is
+// found, it adds that to p.inlines and returns true. Otherwise, it returns false.
+func (p *inlineParser) parseEmoji() bool {
+	// Only allow emojis after non-word characters
+	if p.position > 1 {
+		prevChar := p.raw[p.position-1]
+
+		if isWordByte(prevChar) {
+			return false
+		}
+	}
+
+	remaining := p.raw[p.position:]
+
+	loc := emojiRegex.FindStringIndex(remaining)
+	if loc == nil {
+		return false
+	}
+
+	// Note that there may not be a system or custom emoji that exists with this name
+	p.inlines = append(p.inlines, &Emoji{
+		Name: remaining[loc[0]+1 : loc[1]-1],
+	})
+	p.position += loc[1] - loc[0]
+
+	return true
+}