diff --git a/README.adoc b/README.adoc
index 0a170fa..5673ad1 100644
--- a/README.adoc
+++ b/README.adoc
@@ -13190,6 +13190,8 @@ ERROR: package host-nodejs installs executables without proper RPATH
 
 Examples:
 
+* String
+** link:rootfs_overlay/lkmc/nodejs/alphanumeric.js[]: https://stackoverflow.com/questions/4444477/how-to-tell-if-a-string-contains-a-certain-character-in-javascript/58359106#58359106
 * `process`
 ** link:rootfs_overlay/lkmc/nodejs/command_line_arguments.js[]
 * `fs`
diff --git a/rootfs_overlay/lkmc/nodejs/alphanumeric.js b/rootfs_overlay/lkmc/nodejs/alphanumeric.js
new file mode 100755
index 0000000..a628e85
--- /dev/null
+++ b/rootfs_overlay/lkmc/nodejs/alphanumeric.js
@@ -0,0 +1,50 @@
+#!/usr/bin/env node
+
+// https://cirosantilli.com/linux-kernel-module-cheat#node-js
+
+const assert = require('assert');
+
+const char_is_alphanumeric = function(c) {
+  let code = c.codePointAt(0);
+  return (
+    // 0-9
+    (code > 47 && code < 58) ||
+    // A-Z
+    (code > 64 && code < 91) ||
+    // a-z
+    (code > 96 && code < 123)
+  )
+}
+
+const is_alphanumeric = function (str) {
+  for (let c of str) {
+    if (!char_is_alphanumeric(c)) {
+      return false;
+    }
+  }
+  return true;
+};
+
+// Arbitrarily defined as alphanumeric or '-' or '_'.
+const is_almost_alphanumeric = function (str) {
+  for (let c of str) {
+    if (
+      !char_is_alphanumeric(c) &&
+      !is_almost_alphanumeric.almost_chars.has(c)
+    ) {
+      return false;
+    }
+  }
+  return true;
+};
+is_almost_alphanumeric.almost_chars = new Set(['-', '_']);
+
+assert( is_alphanumeric('aB0'));
+assert(!is_alphanumeric('aB0_-'));
+assert(!is_alphanumeric('aB0_-*'));
+assert(!is_alphanumeric('你好'));
+
+assert( is_almost_alphanumeric('aB0'));
+assert( is_almost_alphanumeric('aB0_-'));
+assert(!is_almost_alphanumeric('aB0_-*'));
+assert(!is_almost_alphanumeric('你好'));