Regular Expression problem (in asp.net MVC5)

Question

hoping someone can point out what I'm missing here with a regular expression.

Here is the data item from my model :-

[Display(Name = "Serial to Search")]
[MaxLength(12)]
[RegularExpression(@"ABC|WXYZ\w{8,9}")]
public string SerialNo { get; set; }

This should allow me to match a serial that begins with either ABC or WXYZ and has another 8 or 9 characters/numbers.

In my view I'm using jquery unobtrusive validation and an @Html.ValidationMessageFor control to display errors.

I have tested this on regex101.com using the following test string :-

ABCGC1000BC5

and it passes fine, but in my view I get a validation error, specifically that the string doesn't match the regex requirements. Can anyone see what I'm missing ? Thanks.

regex101.com screenshot

Answer 1

Your regex matches two type of strings: 1) ABC or 2) WXYZ followed with 8 or 9 word chars. Remember that RegularExpressionAttribute pattern must match the whole string. Even if regex101.com shows a match for ABC12 , it won't match in your environment.

You need to use a grouping,

(ABC|WXYZ)\w{8,9}
^        ^

A non-capturing group would fit even better since you are only validating a string and not using captures later:

(?:ABC|WXYZ)\w{8,9}
^^^        ^

Answer 2

RegularExpressionAttribute searches for an exact match: if the Regex isn't anchored to beginning or end of the string, then RegularExpressionAttribute will effectively do that anchoring for you. See the implementation on ReferenceSource .

That is the difference between regex101.com and your test where it fails. If you anchor the regex on regex101.com, as ^ABC|WXYZ\\w{8,9}$ , you will see that it fails.

The reason, as Wiktor Stribiżew pointed out in the comments, is that your regex looks for ABC OR WXYZ\\w{8,9} . Neither ^ABC$ nor ^WXYZ\\w{8,9}$ match your test string of ABCGC1000BC5 .

Edit: (Please accept Wiktor Stribiżew's answer as the solution. This answer just aims to explain the difference between running it on regex101.com and in ASP.NET).