diff --git a/src/gitcomp/gitcomp_core.py b/src/gitcomp/gitcomp_core.py
index 6ed8bec..960c3e3 100644
--- a/src/gitcomp/gitcomp_core.py
+++ b/src/gitcomp/gitcomp_core.py
@@ -1,7 +1,9 @@
 import re
+import sys
+
 from .user import User
 from .repository import Repository
-from typing import List, Dict
+from typing import List, Dict, Pattern
 from .net_mod import NetMod
 
 
@@ -10,8 +12,14 @@ class GitComp:
     repos: List[str]
     user_data: Dict[str, User] = None
     repo_data: Dict[str, Repository] = None
-    __username_regex = r'^[a-zA-Z0-9]+'
-    __repo_regex = r'^[A-Za-z0-9]+/[A-Za-z0-9]+'
+    # GitHub usernames can be alpha numerical characters with at-most 1 - except in the beginning and the end
+    # https://regex101.com/r/sYIEPE/2
+    __username_regex: Pattern = re.compile(r'^([A-Za-z0-9])([A-Za-z0-9]*)(-)?([A-Za-z0-9]*)([A-Za-z0-9]$)')
+    # Repository names are currently limited to 100 characters, some special names like . are reserved
+    # https://regex101.com/r/ibfDFk/2
+    __repo_regex: Pattern = re.compile(
+        r'^([A-Za-z0-9])([A-Za-z0-9]*)(-)?([A-Za-z0-9]*)([A-Za-z0-9])(/)((?=[^.])([^!#@%^&+=()]*)$|((\.+)(['
+        r'^!#@%^&+=()]+)$))')
 
     def __init__(self, users: List[str] = None, repos: List[str] = None):
         self.users = users
@@ -25,27 +33,34 @@ def __init__(self, users: List[str] = None, repos: List[str] = None):
             self.__fetch_repo_data()
 
     def __fetch_user_data(self):
-        self.__validate_user_names()
+        try:
+            self.__validate_user_names()
+        except ValueError as e:
+            sys.exit(e)
         response = NetMod().fetch_users_data(self.users)
         for user in response:
             self.user_data[user] = User(response[user])
 
     def __validate_user_names(self):
         for user in self.users:
-            if not re.match(GitComp.__username_regex, user):
+            if not GitComp.__username_regex.fullmatch(user):
                 raise ValueError(f"""
                 Improper username {user} 
                 """)
 
     def __fetch_repo_data(self):
-        self.__validate_repo_string()
+        try:
+            self.__validate_repo_string()
+        except ValueError as e:
+            sys.exit(e)
+
         response = NetMod().fetch_repos_data(self.repos)
         for repo in response:
             self.repo_data[repo] = Repository(response[repo])
 
     def __validate_repo_string(self):
         for repo in self.repos:
-            if not re.match(GitComp.__repo_regex, repo):
+            if not GitComp.__repo_regex.fullmatch(repo):
                 raise ValueError("""
                 Improper repository format.
                 Provide the repository name as: <user-name>/<repository-name>