Find where Regex.Match failed

Question

I want to know which Regex Groupcollection failed to match.

eg:

My pattern detects VIEW start page or VIEW end via:

match = Regex.Match(string, @"(^VIEW)\s*((?(1)(?:(start)\s*((?(1)page$))|(end$))))");

I want to know if I give the input string as "VIEW go" how to find that the second group failed.

Answer 1

You don't have to check the latter group to verify if your regex matches something or not.

The Match Object returned by the method allow you to check this using:

if ( match.Success )
    Console.Write("Success!");
else
    Console.Write("Expected start page or end after VIEW");

For you purpose you can also simplify your regex as this:

match = Regex.Match(string, @"^VIEW\s+(?:start\s+page|end)$");

See the online demo .

UPDATE

If you want to refine the error message you can try something like this:

match = Regex.Match(string, @"^VIEW\s+(?:(start\s+page|end)|(.*))$");

if (match.Success) {
    if ( match.Groups[1].Success)
        Console.Write("Success!");    
    if ( match.Groups[2].Success)
        Console.Write("Expected start page or end after VIEW");
} else {
    Console.Write("usage: VIEW [start page|end]");
}

A running code demo on ideone