Validating file types by regular expression

Your regex seems a bit too complex in my opinion. Also, remember that the dot is a special character meaning "any character". The following regex should work (note the escaped dots):

^.*\.(jpg|JPG|gif|GIF|doc|DOC|pdf|PDF)$

You can use a tool like Expresso to test your regular expressions.

^.+\.(?:(?:[dD][oO][cC][xX]?)|(?:[pP][dD][fF]))$

Will accept .doc, .docx, .pdf files having a filename of at least one character:

^           = beginning of string.+          = at least one character (any character)\.          = dot ('.')(?:pattern) = match the pattern without storing the match)[dD]        = any character in the set ('d' or 'D')[xX]?       = any character in the set or none               ('x' may be missing so 'doc' or 'docx' are both accepted)|           = either the previous or the next pattern$           = end of matched string

Warning! Without enclosing the whole chain of extensions in (?:), an extension like .docpdf would pass.

You can test regular expressions at http://www.regextester.com/

Are you just looking to verify that the file is of a given extension? You can simplify what you are trying to do with something like this:

(.*?)\.(jpg|gif|doc|pdf)$

Then, when you call IsMatch() make sure to pass RegexOptions.IgnoreCase as your second parameter. There is no reason to have to list out the variations for casing.

Edit: As Dario mentions, this is not going to work for the RegularExpressionValidator, as it does not support casing options.