EmailAddressAttribute.cs

namespace System.ComponentModel.DataAnnotations {
    using System;
    using System.ComponentModel.DataAnnotations.Resources;
    using System.Text.RegularExpressions;

    [AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter, AllowMultiple = false)]
    public sealed class EmailAddressAttribute : DataTypeAttribute {

        // This attribute provides server-side email validation equivalent to jquery validate,
        // and therefore shares the same regular expression.  See unit tests for examples.
        private static Regex _regex = CreateRegEx();

        public EmailAddressAttribute()
            : base(DataType.EmailAddress) {
            
            // DevDiv 468241: set DefaultErrorMessage not ErrorMessage, allowing user to set
            // ErrorMessageResourceType and ErrorMessageResourceName to use localized messages.
            DefaultErrorMessage = DataAnnotationsResources.EmailAddressAttribute_Invalid;
        }

        public override bool IsValid(object value) {
            if (value == null) {
                return true;
            }

            string valueAsString = value as string;

            // Use RegEx implementation if it has been created, otherwise use a non RegEx version.
            if (_regex != null) { 
                return valueAsString != null && _regex.Match(valueAsString).Length > 0;
            }
            else {
                int atCount = 0;
                
                foreach (char c in valueAsString) {
                    if (c == '@') {
                        atCount++;
                    }
                }

                return (valueAsString != null
                && atCount == 1
                && valueAsString[0] != '@'
                && valueAsString[valueAsString.Length - 1] != '@');
            }
        }

        private static Regex CreateRegEx() {
            // We only need to create the RegEx if this switch is enabled.
            if (AppSettings.DisableRegEx) {
                return null;
            }

            const string pattern = @"^((([a-z]|\d|[!#\$%&'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])+(\.([a-z]|\d|[!#\$%&'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])+)*)|((\x22)((((\x20|\x09)*(\x0d\x0a))?(\x20|\x09)+)?(([\x01-\x08\x0b\x0c\x0e-\x1f\x7f]|\x21|[\x23-\x5b]|[\x5d-\x7e]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(\\([\x01-\x09\x0b\x0c\x0d-\x7f]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF]))))*(((\x20|\x09)*(\x0d\x0a))?(\x20|\x09)+)?(\x22)))@((([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])*([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])))\.)+(([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])*([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])))\.?$";
            const RegexOptions options = RegexOptions.Compiled | RegexOptions.IgnoreCase | RegexOptions.ExplicitCapture;

            // Set explicit regex match timeout, sufficient enough for email parsing
            // Unless the global REGEX_DEFAULT_MATCH_TIMEOUT is already set
            TimeSpan matchTimeout = TimeSpan.FromSeconds(2);

            try {
                if (AppDomain.CurrentDomain.GetData("REGEX_DEFAULT_MATCH_TIMEOUT") == null) {
                    return new Regex(pattern, options, matchTimeout);
                }
            }
            catch {
                // Fallback on error
            }

            // Legacy fallback (without explicit match timeout)
            return new Regex(pattern, options);
        }
    }
}