diff --git a/README.md b/README.md
index 09570ad..2c51382 100644
--- a/README.md
+++ b/README.md
@@ -39,7 +39,7 @@ $ make repl
   - arithmetic expressions
   - built-in functions
   - first-class and higher-order functions • closures
-- Unicode identifiers (UAX #31, XID). No Emojis yet.
+- Unicode identifiers (UAX #31, XID) plus monetary extensions (`[¥$_]`). No Emojis yet.
 - Full double-quoted string syntax from Rust-lang.
 - Ridiculous naming for the Aba-aba. Comparison with Monkey:
 
diff --git a/src/lexer/mod.rs b/src/lexer/mod.rs
index 3adcb43..20d72c9 100644
--- a/src/lexer/mod.rs
+++ b/src/lexer/mod.rs
@@ -48,27 +48,25 @@ pub fn is_whitespace(c: char) -> bool {
 }
 
 /// True if `c` is valid as a first character of an identifier.
-/// See [Rust language reference](https://doc.rust-lang.org/reference/identifiers.html) for
-/// a formal definition of valid identifier name.
+/// Compared to Rust, we additionally allow $ and ¥.
 fn is_id_start(c: char) -> bool {
-    // This is XID_Start OR '_' (which formally is not a XID_Start).
-    // We also add fast-path for ascii idents
     ('a'..='z').contains(&c)
         || ('A'..='Z').contains(&c)
         || c == '_'
+        || c == '$'
+        || c == '¥'
         || (c > '\x7f' && unicode_xid::UnicodeXID::is_xid_start(c))
 }
 
 /// True if `c` is valid as a non-first character of an identifier.
-/// See [Rust language reference](https://doc.rust-lang.org/reference/identifiers.html) for
-/// a formal definition of valid identifier name.
+/// Compared to Rust, we additionally allow $ and ¥.
 fn is_id_continue(c: char) -> bool {
-    // This is exactly XID_Continue.
-    // We also add fast-path for ascii idents
     ('a'..='z').contains(&c)
         || ('A'..='Z').contains(&c)
         || ('0'..='9').contains(&c)
         || c == '_'
+        || c == '$'
+        || c == '¥'
         || (c > '\x7f' && unicode_xid::UnicodeXID::is_xid_continue(c))
 }
 
diff --git a/web/src/js/editor/monkey-mode.js b/web/src/js/editor/monkey-mode.js
index d70cff1..afb78fc 100644
--- a/web/src/js/editor/monkey-mode.js
+++ b/web/src/js/editor/monkey-mode.js
@@ -16,7 +16,7 @@ CodeMirror.defineSimpleMode('monkey', {
     { regex: /[-+\/*=<>!]|对齐|联动|差异|倾斜/, token: 'operator' },
     { regex: /[\{\[\(]/, indent: true },
     { regex: /[\}\]\)]/, dedent: true },
-    { regex: /\p{XID_Start}\p{XID_Continue}*|[a-z$][\w$]*/u, token: 'variable' },
+    { regex: /[\p{XID_Start}$¥_][\p{XID_Continue}$¥]*/u, token: 'variable' },
   ],
   comment: [],
   meta: {},