diff --git a/index.ts b/index.ts
index 6fbe9e2..c1f8047 100644
--- a/index.ts
+++ b/index.ts
@@ -11,7 +11,9 @@ export * as nip06 from './nip06'
 export * as nip10 from './nip10'
 export * as nip13 from './nip13'
 export * as nip19 from './nip19'
+export * as nip21 from './nip21'
 export * as nip26 from './nip26'
+export * as nip27 from './nip27'
 export * as nip39 from './nip39'
 export * as nip42 from './nip42'
 export * as nip57 from './nip57'
diff --git a/nip21.test.js b/nip21.test.js
new file mode 100644
index 0000000..acc9562
--- /dev/null
+++ b/nip21.test.js
@@ -0,0 +1,42 @@
+/* eslint-env jest */
+const {nip21} = require('./lib/nostr.cjs')
+
+test('test', () => {
+  expect(
+    nip21.test(
+      'nostr:npub108pv4cg5ag52nq082kd5leu9ffrn2gdg6g4xdwatn73y36uzplmq9uyev6'
+    )
+  ).toBe(true)
+  expect(
+    nip21.test(
+      'nostr:note1gmtnz6q2m55epmlpe3semjdcq987av3jvx4emmjsa8g3s9x7tg4sclreky'
+    )
+  ).toBe(true)
+  expect(
+    nip21.test(
+      ' nostr:npub108pv4cg5ag52nq082kd5leu9ffrn2gdg6g4xdwatn73y36uzplmq9uyev6'
+    )
+  ).toBe(false)
+  expect(nip21.test('nostr:')).toBe(false)
+  expect(
+    nip21.test(
+      'nostr:npub108pv4cg5ag52nQq082kd5leu9ffrn2gdg6g4xdwatn73y36uzplmq9uyev6'
+    )
+  ).toBe(false)
+  expect(nip21.test('gggggg')).toBe(false)
+})
+
+test('parse', () => {
+  const result = nip21.parse(
+    'nostr:note1gmtnz6q2m55epmlpe3semjdcq987av3jvx4emmjsa8g3s9x7tg4sclreky'
+  )
+
+  expect(result).toEqual({
+    uri: 'nostr:note1gmtnz6q2m55epmlpe3semjdcq987av3jvx4emmjsa8g3s9x7tg4sclreky',
+    value: 'note1gmtnz6q2m55epmlpe3semjdcq987av3jvx4emmjsa8g3s9x7tg4sclreky',
+    decoded: {
+      type: 'note',
+      data: '46d731680add2990efe1cc619dc9b8014feeb23261ab9dee50e9d11814de5a2b'
+    }
+  })
+})
diff --git a/nip21.ts b/nip21.ts
new file mode 100644
index 0000000..a5608a2
--- /dev/null
+++ b/nip21.ts
@@ -0,0 +1,41 @@
+import * as nip19 from './nip19'
+import * as nip21 from './nip21'
+
+/**
+ * Bech32 regex.
+ * @see https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki#bech32
+ */
+export const BECH32_REGEX =
+  /[\x21-\x7E]{1,83}1[023456789acdefghjklmnpqrstuvwxyz]{6,}/
+
+/** Nostr URI regex, eg `nostr:npub1...` */
+export const NOSTR_URI_REGEX = new RegExp(`nostr:(${BECH32_REGEX.source})`)
+
+/** Test whether the value is a Nostr URI. */
+export function test(value: unknown): value is `nostr:${string}` {
+  return (
+    typeof value === 'string' &&
+    new RegExp(`^${NOSTR_URI_REGEX.source}$`).test(value)
+  )
+}
+
+/** Parsed Nostr URI data. */
+export interface NostrURI {
+  /** Full URI including the `nostr:` protocol. */
+  uri: `nostr:${string}`
+  /** The bech32-encoded data (eg `npub1...`). */
+  value: string
+  /** Decoded bech32 string, according to NIP-19. */
+  decoded: nip19.DecodeResult
+}
+
+/** Parse and decode a Nostr URI. */
+export function parse(uri: string): NostrURI {
+  const match = uri.match(new RegExp(`^${nip21.NOSTR_URI_REGEX.source}$`))
+  if (!match) throw new Error(`Invalid Nostr URI: ${uri}`)
+  return {
+    uri: match[0] as `nostr:${string}`,
+    value: match[1],
+    decoded: nip19.decode(match[1])
+  }
+}
diff --git a/nip27.test.js b/nip27.test.js
new file mode 100644
index 0000000..24c63ac
--- /dev/null
+++ b/nip27.test.js
@@ -0,0 +1,49 @@
+/* eslint-env jest */
+const {nip27} = require('./lib/nostr.cjs')
+
+test('matchAll', () => {
+  const result = nip27.matchAll(
+    'Hello nostr:npub108pv4cg5ag52nq082kd5leu9ffrn2gdg6g4xdwatn73y36uzplmq9uyev6!\n\nnostr:note1gmtnz6q2m55epmlpe3semjdcq987av3jvx4emmjsa8g3s9x7tg4sclreky'
+  )
+
+  expect([...result]).toEqual([
+    {
+      uri: 'nostr:npub108pv4cg5ag52nq082kd5leu9ffrn2gdg6g4xdwatn73y36uzplmq9uyev6',
+      value: 'npub108pv4cg5ag52nq082kd5leu9ffrn2gdg6g4xdwatn73y36uzplmq9uyev6',
+      decoded: {
+        type: 'npub',
+        data: '79c2cae114ea28a981e7559b4fe7854a473521a8d22a66bbab9fa248eb820ff6'
+      },
+      start: 6,
+      end: 75
+    },
+    {
+      uri: 'nostr:note1gmtnz6q2m55epmlpe3semjdcq987av3jvx4emmjsa8g3s9x7tg4sclreky',
+      value: 'note1gmtnz6q2m55epmlpe3semjdcq987av3jvx4emmjsa8g3s9x7tg4sclreky',
+      decoded: {
+        type: 'note',
+        data: '46d731680add2990efe1cc619dc9b8014feeb23261ab9dee50e9d11814de5a2b'
+      },
+      start: 78,
+      end: 147
+    }
+  ])
+})
+
+test('replaceAll', () => {
+  const content =
+    'Hello nostr:npub108pv4cg5ag52nq082kd5leu9ffrn2gdg6g4xdwatn73y36uzplmq9uyev6!\n\nnostr:note1gmtnz6q2m55epmlpe3semjdcq987av3jvx4emmjsa8g3s9x7tg4sclreky'
+
+  const result = nip27.replaceAll(content, ({decoded, value}) => {
+    switch (decoded.type) {
+      case 'npub':
+        return '@alex'
+      case 'note':
+        return '!1234'
+      default:
+        return value
+    }
+  })
+
+  expect(result).toEqual('Hello @alex!\n\n!1234')
+})
diff --git a/nip27.ts b/nip27.ts
new file mode 100644
index 0000000..7fa5be8
--- /dev/null
+++ b/nip27.ts
@@ -0,0 +1,63 @@
+import * as nip19 from './nip19'
+import * as nip21 from './nip21'
+
+/** Regex to find NIP-21 URIs inside event content. */
+export const regex = () =>
+  new RegExp(`\\b${nip21.NOSTR_URI_REGEX.source}\\b`, 'g')
+
+/** Match result for a Nostr URI in event content. */
+export interface NostrURIMatch extends nip21.NostrURI {
+  /** Index where the URI begins in the event content. */
+  start: number
+  /** Index where the URI ends in the event content. */
+  end: number
+}
+
+/** Find and decode all NIP-21 URIs. */
+export function * matchAll(content: string): Iterable<NostrURIMatch> {
+  const matches = content.matchAll(regex())
+
+  for (const match of matches) {
+    const [uri, value] = match
+
+    yield {
+      uri: uri as `nostr:${string}`,
+      value,
+      decoded: nip19.decode(value),
+      start: match.index!,
+      end: match.index! + uri.length
+    }
+  }
+}
+
+/**
+ * Replace all occurrences of Nostr URIs in the text.
+ *
+ * WARNING: using this on an HTML string is potentially unsafe!
+ *
+ * @example
+ * ```ts
+ * nip27.replaceAll(event.content, ({ decoded, value }) => {
+ *   switch(decoded.type) {
+ *     case 'npub':
+ *       return renderMention(decoded)
+ *     case 'note':
+ *       return renderNote(decoded)
+ *     default:
+ *       return value
+ *   }
+ * })
+ * ```
+ */
+export function replaceAll(
+  content: string,
+  replacer: (match: nip21.NostrURI) => string
+): string {
+  return content.replaceAll(regex(), (uri, value) => {
+    return replacer({
+      uri: uri as `nostr:${string}`,
+      value,
+      decoded: nip19.decode(value)
+    })
+  })
+}