I was using attributes from the System.ComponentModel.DataAnnotations   namespace for model validation.  This namespace includes a few very useful validation attributes such as

1. Required Attribute – Validates the field has a value
2. Range Attribute – Validates the field is within a given range
3. RegularExpression Attribute – Validates the field matches a given regular expression.

The regular expression attribute is very useful since you can describe exactly what format you want a string property to be in.  While using this though I ran into a problem.  The attribute doesn’t let you specify RegexOptions.  This was an issue for me since I wanted to use the regex to validate that the users input was between 5 and 200 characters long , so I had attribute that property as such:

[RegularExpression(@".{5,200}")]
public string Text{get;set;}

However this doesn’t work since by default the wildcard . does not match new lines (which I am allowing in the input).  The way to fix this is to specify the RegexOptions.SingleLine option to either the Regex constructor or Match function.  The problem is I have no way of doing that here, and there is no argument on the attribute constructor to specify those options.  I was considering overriding the attribute to create one that allows specifying the attribute but then I stumbled upon this:

Regular Expression Options

You are able to specify the regex option inside of the regular expression text! (which I thought was a huge discovery until my co-worker said he knew this all along but never let me know!). 

So I just changed the expression to look like this:

[RegularExpression(@"(?s).{5,200}")]
public string Text{get; set;}

The (?s) is the inline regex option definition so say I want this in SingleLine mode! And now the validation works the way I wanted!