Simplify regex for user/community name validation (#5164)

* Add lowercase only check for community names

* Lint

* Remove redundant check

* Lint

* Add newline escape tests

* Eliminate confounding factor in community name test

* Use same check for user names and community names

* Use min/max length check

---------

Co-authored-by: Dessalines <dessalines@users.noreply.github.com>
Co-authored-by: Felix Ableitner <me@nutomic.com>

crates/utils/src/utils/validation.rs

@@ -85,26 +85,19 @@ fn has_newline(name: &str) -> bool {
 }
 
 pub fn is_valid_actor_name(name: &str, actor_name_max_length: usize) -> LemmyResult<()> {
-  static VALID_ACTOR_NAME_REGEX_EN: LazyLock<Regex> =
-    LazyLock::new(|| Regex::new(r"^[a-zA-Z0-9_]{3,}$").expect("compile regex"));
-  static VALID_ACTOR_NAME_REGEX_AR: LazyLock<Regex> =
-    LazyLock::new(|| Regex::new(r"^[\p{Arabic}0-9_]{3,}$").expect("compile regex"));
-  static VALID_ACTOR_NAME_REGEX_RU: LazyLock<Regex> =
-    LazyLock::new(|| Regex::new(r"^[\p{Cyrillic}0-9_]{3,}$").expect("compile regex"));
-
-  let check = name.chars().count() <= actor_name_max_length && !has_newline(name);
-
   // Only allow characters from a single alphabet per username. This avoids problems with lookalike
   // characters like `o` which looks identical in Latin and Cyrillic, and can be used to imitate
   // other users. Checks for additional alphabets can be added in the same way.
-  let lang_check = VALID_ACTOR_NAME_REGEX_EN.is_match(name)
-    || VALID_ACTOR_NAME_REGEX_AR.is_match(name)
-    || VALID_ACTOR_NAME_REGEX_RU.is_match(name);
+  static VALID_ACTOR_NAME_REGEX: LazyLock<Regex> = LazyLock::new(|| {
+    Regex::new(r"^(?:[a-zA-Z0-9_]+|[0-9_\p{Arabic}]+|[0-9_\p{Cyrillic}]+)$").expect("compile regex")
+  });
 
-  if !check || !lang_check {
-    Err(LemmyErrorType::InvalidName.into())
-  } else {
+  min_length_check(name, 3, LemmyErrorType::InvalidName)?;
+  max_length_check(name, actor_name_max_length, LemmyErrorType::InvalidName)?;
+  if VALID_ACTOR_NAME_REGEX.is_match(name) {
     Ok(())
+  } else {
+    Err(LemmyErrorType::InvalidName.into())
   }
 }
 
@@ -441,6 +434,15 @@ mod tests {
     assert!(is_valid_actor_name("a", actor_name_max_length).is_err());
     // empty
     assert!(is_valid_actor_name("", actor_name_max_length).is_err());
+    // newline
+    assert!(is_valid_actor_name(
+      r"Line1
+
+Line3",
+      actor_name_max_length
+    )
+    .is_err());
+    assert!(is_valid_actor_name("Line1\nLine3", actor_name_max_length).is_err());
   }
 
   #[test]