Can anyone tell me why this C# email validation regular expression (regex) hangs?
I got a good email vaidation regex from: Email regular expression
public static void Main(string[] args)
{
string value = @"cvcvcvcvvcvvcvcvcvcvcvvcvcvcvcvcvvccvcvcvc";
var regex = new Regex(
@"^([0-9a-zA-Z]([-.w]*[0-9a-zA-Z])*@([0-9a-zA-Z][-w]*[0-9a-zA-Z].)+[a-zA-Z]{2,9})$",
RegexOptions.Compiled);
var x = regex.Match(value); // Hangs here !?!
return;
}
It works in most cases, but the code above hangs, burning 100% CPU... I've tested in a W8 metro App. and on a standard .Net 4.5 app.
Can anyone tell me why this happens, and if there is a good email validation REGEX that doesn't hang, or if there is a way to fix this one?
Many thanks, Jon
The explanation why it hangs: Catastrophic backtracking.
Let's simplify the crucial part of the regex:
(w*[0-9a-zA-Z])*@
You have
w* that can match the same characters as the following part [0-9a-zA-Z] , so the two combined translate, in essence, to w+ (w+)* This means that, given s = "cvcvcvcvvcvvcvcvcvcvcvvcvcvcvcvcvvccvcvcvc" , this part of the regex needs to check all possible permutations of s (which number at 2**(len(s)-1) ) before deciding on a non-match when the following @ is not found.
Since you cannot validate an e-mail address with any regex (there are far too many corner cases in the spec), it's usually best to
^.*@.*$ ) Just for completeness, you can avoid the backtracking issues with the help of atomic groups:
var regex = new Regex(
@"^([0-9a-zA-Z](?>[-.w]*[0-9a-zA-Z])*@(?>[0-9a-zA-Z][-w]*[0-9a-zA-Z].)+[a-zA-Z]{2,9})$",
RegexOptions.Compiled);
Never ever use regex to validate an email..
You can use MailAddress class to validate it
try
{
address = new MailAddress(address).Address;
//address is valid
}
catch(FormatException)
{
//address is invalid
}
guess it's because of [-.w] in regex, try to use this:
^[a-zA-Z0-9_-]+(?:.[a-zA-Z0-9_-]+)*@(?:([[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.)|(([a-zA-Z0-9-]+.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(]?)$
Also, in .net 4.5 EmailAttribute should be available, not sure though
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