Fix parsing of single hex-digits.

crates/aiken-lang/src/parser/lexer.rs

@@ -112,7 +112,7 @@ pub fn lexer() -> impl Parser<char, Vec<(Token, Span)>, Error = ParseError> {
         .ignore_then(
             one_of("0123456789abcdefABCDEF")
                 .repeated()
-                .at_least(2)
+                .at_least(1)
                 .collect::<String>(),
         )
         .validate(|value: String, span, emit| {

crates/aiken-lang/src/tests/format.rs

@@ -11,6 +11,18 @@ fn format_comment_at_end_of_file() {
     );
 }
 
+#[test]
+fn format_single_hex_digit() {
+    assert_format!(
+        r#"
+        const a = 0xa
+        const b = 0x0f
+        const c = 0x0000000f
+        const d = 0x123
+    "#
+    );
+}
+
 #[test]
 fn format_simple_module() {
     assert_format!(

crates/aiken-lang/src/tests/snapshots/format_single_hex_digit.snap

@@ -0,0 +1,11 @@
+---
+source: crates/aiken-lang/src/tests/format.rs
+description: "Code:\n\nconst a = 0xa\nconst b = 0x0f\nconst c = 0x0000000f\nconst d = 0x123\n"
+---
+const a = 0xa
+
+const b = 0xf
+
+const c = 0xf
+
+const d = 0x123