^(([a-zA-Z]:)|(\{2}w+)$?)(\(w[w].*))(.jpg|.JPG|.gif|.GIF|.doc|.DOC|.pdf|.PDF)$

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

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

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

^           = 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

.(?i:)(?:jpg|gif|doc|pdf)$

.(jpg|gif|doc|pdf)$

ValidationExpression="^.+.(([pP][dD][fF])|([jJ][pP][gG])|([pP][nN][gG])))$"