1 files changed, 45 insertions, 0 deletions
diff --git a/Timeline/Models/Validation/UsernameValidator.cs b/Timeline/Models/Validation/UsernameValidator.cs
new file mode 100644
index 00000000..e4891400
--- /dev/null
+++ b/Timeline/Models/Validation/UsernameValidator.cs
@@ -0,0 +1,45 @@
+using System.Linq;
+using System.Text.RegularExpressions;
+
+namespace Timeline.Models.Validation
+{
+    public class UsernameValidator : Validator<string>
+    {
+        public const int MaxLength = 26;
+        public const string RegexPattern = @"^[a-zA-Z0-9_][a-zA-Z0-9-_]*$";
+
+        private readonly Regex _regex = new Regex(RegexPattern);
+
+        protected override bool DoValidate(string value, out string message)
+        {
+            if (value.Length == 0)
+            {
+                message = "An empty string is not permitted.";
+                return false;
+            }
+
+            if (value.Length > 26)
+            {
+                message = $"Too long, more than 26 characters is not premitted, found {value.Length}.";
+                return false;
+            }
+
+            foreach ((char c, int i) in value.Select((c, i) => (c, i)))
+                if (char.IsWhiteSpace(c))
+                {
+                    message = $"A whitespace is found at {i} . Whitespace is not permited.";
+                    return false;
+                }
+
+            var match = _regex.Match(value);
+            if (!match.Success)
+            {
+                message = "Regex match failed.";
+                return false;
+            }
+
+            message = ValidationConstants.SuccessMessage;
+            return true;
+        }
+    }
+}