Refactoring code for modular approach

models/api_models.py

@@ -2,6 +2,7 @@
 from pydantic import BaseModel, Field, field_validator, EmailStr, ConfigDict
 from typing import Optional, List
 from datetime import datetime
+import re
 
 
 # User Authentication Models
@@ -13,8 +14,8 @@ class UserRegister(BaseModel):
     @field_validator('username')
     @classmethod
     def validate_username(cls, v):
-        if not v.isalnum():
-            raise ValueError('Username must contain only alphanumeric characters')
+        if not re.match(r'^[a-zA-Z0-9_]+$', v):
+            raise ValueError('Username must contain only letters, numbers, or underscores')
         return v.lower().strip()
 
     @field_validator('password')