diff --git a/nip27.ts b/nip27.ts
index c63804b..f8a6d3b 100644
--- a/nip27.ts
+++ b/nip27.ts
@@ -13,7 +13,7 @@ export interface NostrURIMatch extends NostrURI {
 }
 
 /** Find and decode all NIP-21 URIs. */
-export function* matchAll(content: string): Iterable<NostrURIMatch> {
+export function * matchAll(content: string): Iterable<NostrURIMatch> {
   const matches = content.matchAll(regex())
 
   for (const match of matches) {
diff --git a/nip30.test.ts b/nip30.test.ts
new file mode 100644
index 0000000..cc7809f
--- /dev/null
+++ b/nip30.test.ts
@@ -0,0 +1,32 @@
+import {matchAll, replaceAll} from './nip30.ts'
+
+test('matchAll', () => {
+  const result = matchAll('Hello :blobcat: :disputed: ::joy:joy:')
+
+  expect([...result]).toEqual([
+    {
+      name: 'blobcat',
+      shortcode: ':blobcat:',
+      start: 6,
+      end: 15
+    },
+    {
+      name: 'disputed',
+      shortcode: ':disputed:',
+      start: 16,
+      end: 26
+    }
+  ])
+})
+
+test('replaceAll', () => {
+  const content = 'Hello :blobcat: :disputed: ::joy:joy:'
+
+  const result = replaceAll(content, ({name}) => {
+    return `<img src="https://ditto.pub/emoji/${name}.png" />`
+  })
+
+  expect(result).toEqual(
+    'Hello <img src="https://ditto.pub/emoji/blobcat.png" /> <img src="https://ditto.pub/emoji/disputed.png" /> ::joy:joy:',
+  )
+})
diff --git a/nip30.ts b/nip30.ts
new file mode 100644
index 0000000..8465c85
--- /dev/null
+++ b/nip30.ts
@@ -0,0 +1,54 @@
+/** Regex for a single emoji shortcode. */
+export const EMOJI_SHORTCODE_REGEX = /:(\w+):/
+
+/** Regex to find emoji shortcodes in content. */
+export const regex = () => new RegExp(`\\B${EMOJI_SHORTCODE_REGEX.source}\\B`, 'g')
+
+/** Represents a Nostr custom emoji. */
+export interface CustomEmoji {
+  /** The matched emoji name with colons. */
+  shortcode: `:${string}:`
+  /** The matched emoji name without colons. */
+  name: string
+}
+
+/** Match result for a custom emoji in text content. */
+export interface CustomEmojiMatch extends CustomEmoji {
+  /** Index where the emoji begins in the text content. */
+  start: number
+  /** Index where the emoji ends in the text content. */
+  end: number
+}
+
+/** Find all custom emoji shortcodes. */
+export function * matchAll(content: string): Iterable<CustomEmojiMatch> {
+  const matches = content.matchAll(regex())
+
+  for (const match of matches) {
+    try {
+      const [shortcode, name] = match
+
+      yield {
+        shortcode: shortcode as `:${string}:`,
+        name,
+        start: match.index!,
+        end: match.index! + shortcode.length
+      }
+    } catch (_e) {
+      // do nothing
+    }
+  }
+}
+
+/** Replace all emoji shortcodes in the content. */
+export function replaceAll(
+  content: string,
+  replacer: (match: CustomEmoji) => string
+): string {
+  return content.replaceAll(regex(), (shortcode, name) => {
+    return replacer({
+      shortcode: shortcode as `:${string}:`,
+      name,
+    })
+  })
+}