Update identifier Unicode character validation to match Python spec

crates/ruff_python_stdlib/Cargo.toml

@@ -13,3 +13,4 @@ license = { workspace = true }
 [lib]
 
 [dependencies]
+unic-ucd-ident = "0.9.0"

crates/ruff_python_stdlib/src/identifiers.rs

@@ -1,16 +1,18 @@
+use unic_ucd_ident::{is_xid_continue, is_xid_start};
+
 use crate::keyword::is_keyword;
 
 /// Returns `true` if a string is a valid Python identifier (e.g., variable
 /// name).
 pub fn is_identifier(name: &str) -> bool {
     // Is the first character a letter or underscore?
     let mut chars = name.chars();
-    if !chars.next().is_some_and(|c| c.is_alphabetic() || c == '_') {
+    if !chars.next().is_some_and(|c| c == '_' || is_xid_start(c)) {
         return false;
     }
 
     // Are the rest of the characters letters, digits, or underscores?
-    if !chars.all(|c| c.is_alphanumeric() || c == '_') {
+    if !chars.all(|c| c == '_' || is_xid_continue(c)) {
         return false;
     }
 
@@ -76,7 +78,24 @@ pub fn is_migration_name(name: &str) -> bool {
 
 #[cfg(test)]
 mod tests {
-    use crate::identifiers::{is_migration_name, is_module_name};
+    use crate::identifiers::{is_identifier, is_migration_name, is_module_name};
+
+    #[test]
+    fn valid_identifiers() {
+        assert!(is_identifier("_abc"));
+        assert!(is_identifier("abc"));
+        assert!(is_identifier("_"));
+        assert!(is_identifier("a_b_c"));
+        assert!(is_identifier("abc123"));
+        assert!(is_identifier("abc_123"));
+        assert!(is_identifier("漢字"));
+        assert!(is_identifier("ひらがな"));
+        assert!(is_identifier("العربية"));
+        assert!(is_identifier("кириллица"));
+        assert!(is_identifier("πr"));
+        assert!(!is_identifier("percentile_co³t"));
+        assert!(!is_identifier("HelloWorld❤️"));
+    }
 
     #[test]
     fn module_name() {