Pyupgrade: Printf string formatting (#1803)

src/python/identifiers.rs

@@ -1,5 +1,15 @@
-use once_cell::sync::Lazy;
-use regex::Regex;
+/// Returns `true` if a string is a valid Python identifier (e.g., variable
+/// name).
+pub fn is_identifier(s: &str) -> bool {
+    // Is the first character a letter or underscore?
+    if !s
+        .chars()
+        .next()
+        .map_or(false, |c| c.is_alphabetic() || c == '_')
+    {
+        return false;
+    }
 
-pub static IDENTIFIER_REGEX: Lazy<Regex> =
-    Lazy::new(|| Regex::new(r"^[A-Za-z_][A-Za-z0-9_]*$").unwrap());
+    // Are the rest of the characters letters, digits, or underscores?
+    s.chars().skip(1).all(|c| c.is_alphanumeric() || c == '_')
+}