Calls the method "IsMatch" on the Regex class (from the "System.Text.RegularExpressions" namespace), with the arguments of the current keycharacter (provided by an event you did not copy/paste) and a RegEx. Then it inverses the outcome.
Not very helpful, is it? It compares a key with a RegEx and reports whether they match. You'd have to look up the "\\d+" part on this page[^] from MSDN, I don't know the regex-code from my head.