diff --git a/lib/reader/src/lexer.rs b/lib/reader/src/lexer.rs
index 1c422a8027..ca315d3b58 100644
--- a/lib/reader/src/lexer.rs
+++ b/lib/reader/src/lexer.rs
@@ -38,6 +38,7 @@ pub enum Token<'a> {
     JumpTable(u32), // jt2
     FuncRef(u32), // fn2
     SigRef(u32), // sig2
+    UserRef(u32), // u345
     Name(&'a str), // %9arbitrary_alphanum, %x3, %0, %function ...
     HexSequence(&'a str), // #89AF
     Identifier(&'a str), // Unrecognized identifier (opcode, enumerator, ...)
@@ -320,6 +321,7 @@ impl<'a> Lexer<'a> {
             "jt" => Some(Token::JumpTable(number)),
             "fn" => Some(Token::FuncRef(number)),
             "sig" => Some(Token::SigRef(number)),
+            "u" => Some(Token::UserRef(number)),
             _ => None,
         }
     }
@@ -604,4 +606,17 @@ mod tests {
         assert_eq!(lex.next(), token(Token::Name("ebb11"), 1));
         assert_eq!(lex.next(), token(Token::Name("_"), 1));
     }
+
+    #[test]
+    fn lex_userrefs() {
+        let mut lex = Lexer::new("u0 u1 u234567890 u9:8765");
+
+        assert_eq!(lex.next(), token(Token::UserRef(0), 1));
+        assert_eq!(lex.next(), token(Token::UserRef(1), 1));
+        assert_eq!(lex.next(), token(Token::UserRef(234567890), 1));
+        assert_eq!(lex.next(), token(Token::UserRef(9), 1));
+        assert_eq!(lex.next(), token(Token::Colon, 1));
+        assert_eq!(lex.next(), token(Token::Integer("8765"), 1));
+        assert_eq!(lex.next(), None);
+    }
 }
diff --git a/lib/reader/src/parser.rs b/lib/reader/src/parser.rs
index 55cf342ad3..03516fa91d 100644
--- a/lib/reader/src/parser.rs
+++ b/lib/reader/src/parser.rs
@@ -890,24 +890,26 @@ impl<'a> Parser<'a> {
         match self.token() {
             Some(Token::Name(s)) => {
                 self.consume();
-                Ok(ExternalName::new(s))
+                Ok(ExternalName::testcase(s))
             }
-            Some(Token::HexSequence(s)) => {
-                if s.len() % 2 != 0 {
-                    return err!(
-                        self.loc,
-                        "expected binary external name to have length multiple of two"
-                    );
-                }
-                let mut bin_name = Vec::with_capacity(s.len() / 2);
-                let mut i = 0;
-                while i + 2 <= s.len() {
-                    let byte = u8::from_str_radix(&s[i..i + 2], 16).unwrap();
-                    bin_name.push(byte);
-                    i += 2;
-                }
+            Some(Token::UserRef(namespace)) => {
                 self.consume();
-                Ok(ExternalName::new(bin_name))
+                match self.token() {
+                    Some(Token::Colon) => {
+                        self.consume();
+                        match self.token() {
+                            Some(Token::Integer(index_str)) => {
+                                let index: u32 = u32::from_str_radix(index_str, 10).map_err(|_| {
+                                    self.error("the integer given overflows the u32 type")
+                                })?;
+                                self.consume();
+                                Ok(ExternalName::user(namespace, index))
+                            }
+                            _ => err!(self.loc, "expected integer"),
+                        }
+                    }
+                    _ => err!(self.loc, "expected colon"),
+                }
             }
             _ => err!(self.loc, "expected external name"),
         }