diff --git a/src/html/usage.rs b/src/html/usage.rs
index 3351c3cd..688949de 100644
--- a/src/html/usage.rs
+++ b/src/html/usage.rs
@@ -8,7 +8,7 @@ use regex::Regex;
 use serde::Serialize;
 
 lazy_static! {
-  static ref IDENTIFIER_RE: Regex = Regex::new(r"[^a-zA-Z$_]").unwrap();
+  static ref IDENTIFIER_RE: Regex = Regex::new(r"[^a-zA-Z1-9$_]").unwrap();
 }
 
 fn render_css_for_usage(name: &str) -> String {