regex works in online regex tester but not in .NET

Question

the following demo works just fine online but not when I attempt to run it in c#/.NET

 var regex = new RegularExpressionAttribute(@\"@(?!.*?\\.\\.)[^@]+$\");
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Answer 1

The RegularExpressionAttribute requires a full match of the string.

In your case, you are only matching starting at the @ character, so you start somewhere in between of your string. This is just not good enough for the RegularExpressionAttribute. You can think of it adding an implicit ^ at the start to your expression (and an implicit $ at the end too, so you can drop that as well).

So to fix this, you need to make sure that it has something to match at the start to, e.g. a .*?:

new RegularExpressionAttribute(@".*?@(?!.*?\.\.)[^@]+");

And then it works. You don’t need any other regular expression flags.

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Unfortunately, this behavior of RegularExpressionAttribute is mentioned nowhere in the documentation. You can verify it in the reference source however:

// We are looking for an exact match, not just a search hit. This matches what
// the RegularExpressionValidator control does
return (m.Success && m.Index == 0 && m.Length == stringValue.Length);

So it’s technically not an added ^ and $ but after finding a match, the start and end index are required to match the string’s start and end index (which is essentially the same thing).

And if you think about it, this behavior makes a lot of sense for validation. After all, you want to make sure that a string matches a certain full-specified format, and not just check if some substring is valid.