Validating file types of Doc / Docx / Pdf / Jpeg / Bmp / Gif / Png by regular expression

^.+\.(?:(?:[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

 Similarly you can validate any file type as the above,

For JPG / BMP / Gif :- 

(.*\.([Jj][Pp][Gg])|([Bb][Mm][Pp])|([Gg][Ii][Ff])|([Pp][Nn][Gg])$)

Validating For Email:-

^([a-zA-Z0-9_\-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.)|

  (([a-zA-Z0-9\-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$

Validating USA Phone No.

((\(\d{3}\) ?)|(\d{3}[-.]))?\d{3}[-.]\d{4}( ext\d{0,}) 

it will accept as: 123.456.7890 

or

123-456-7890 or 123.456-7890(1 hyphen and 1 dot)
You can test it here: http://www.regextester.com/ 

 You can embed case insensitity into the regular expression like so: 

 \.(?i:)(?:jpg|gif|doc|pdf)$