Why is Regular Expression Not working in .NET - asp.net

Question

Why would a regular expression not work in an ASP.net application when it is working in online regex?
I tried to implement the same code in ASP.net but it is not working properly. Can anyone figure out these problem?
Unrestricted File Upload as
The application should use a whitelist of allowed file types. This list determines the types of files that can be uploaded and rejects all files that do not match approved types.
The application should use server-side input validation to ensure evasion techniques have not been used to bypass the whitelist filter. These evasion techniques could include appending a second file type to the file name (e.g. image.jpg.php) or using trailing space or dots in the file name.
Only alphanumeric characters and Hyphen should be allowed in the filename.
<asp:RegularExpressionValidator ID="Rg_FrontPage" ValidationExpression="^(\[a-z\]|\[A-Z\]|\[0-9\]|\[-\]|\[(0-9)+\]+)+\.(jpg|JPG|JPEG|jpeg|png|PNG)$"
ControlToValidate="FUp_Front" runat="server" Font-Size="Small" ForeColor="Red"
ErrorMessage="Max size 100KB. Upload only JPEG/JPG/PNG format" ValidationGroup="Submit"
Display="Dynamic" />

Answer 1

An alternative: You could perform the check in the Click event where you could validate the file name (i.e. FUp_Front.FileName) or better the content type (i.e. FUp_Front.PostedFile.ContentType) of the uploaded file. See the FileUpload control tutorial and the Uploading Files (C#) documentation for further information.
To fix the regex:
You don't need to escape the brackets in your regex, e.g. (a little simplified version):
<asp:RegularExpressionValidator ID="Rg_FrontPage" ValidationExpression="^[a-zA-Z0-9.-]+(jpg|JPG|JPEG|jpeg|png|PNG)$" ViewStateIgnoresCase="true"
ControlToValidate="FUp_Front" runat="server" Font-Size="Small" ForeColor="Red"
ErrorMessage="Max size 100KB. Upload only JPEG/JPG/PNG format" ValidationGroup="Submit"
Display="Dynamic" />
See the docs for further information.

Answer 2

i am using javacript method in client side which is solve my problems onclientclick i do not know why regular expression is fails
<asp:ImageButton ID="btnGenerateOtp" runat="server" ImageUrl="images_new/generate_new.jpg" OnClientClick="return complainformvalidate();"ValidationGroup="Submit
" OnClick="btnGenerateOtp_Click" Visible="false" />
function complainformvalidate() {
var flg = true;
if (document.getElementById('<%=FUp_Front.ClientID %>') != null) {
var fu1 = document.getElementById('<%=FUp_Front.ClientID %>').value;
if (fu1 != '') {
if (fu1.split(".").length - 1 > 1) {
document.getElementById('lblFUp_Front').innerHTML = 'Please enter valid file';
document.getElementById('lblFUp_Front').style.color = "red";
document.getElementById('<%=CV_FUp_Front.ClientID %>').style.display = 'none';
flg = false;
}
if (fu1.split(" ").length - 1 > 0) {
//alert('Inavlid file');
document.getElementById('lblFUp_Front').innerHTML = 'Please enter valid file';
document.getElementById('lblFUp_Front').style.color = "red";
document.getElementById('<%=CV_FUp_Front.ClientID %>').style.display = 'none';
flg = false;
}
var filenameWithExtension = fu1.replace(/^.*[\\\/]/, '');
var num = filenameWithExtension;
var pattern = /(^([a-z]|[A-Z]|[0-9]|[-]|[(0-9)+]+)+\.(jpg|JPG|JPEG|jpeg|png|PNG)$)/;
if (num.search(pattern) == -1) {
document.getElementById('lblFUp_Front').innerHTML = 'Please enter valid file';
document.getElementById('lblFUp_Front').style.color = "red";
document.getElementById('<%=CV_FUp_Front.ClientID %>').style.display = 'none';
flg = false;
}
}
}
<asp:ImageButton ID="btnGenerateOtp" runat="server" ImageUrl="images_new/generate_new.jpg" OnClientClick="return complainformvalidate();"
ValidationGroup="Submit" />
}
on server side i have use custom validator which is solve my problmes
<asp:CustomValidator ID="CV_FUp_Front" runat="server" ErrorMessage="Max size 100KB. Upload only JPEG/JPG/PNG format"
ClientValidationFunction="ValidateFile" OnServerValidate="ValidateFile_FUp_Front" Display="Dynamic" ValidationGroup="Submit"
Font-Size="8pt" CssClass="error errorMsg" />
protected void ValidateFile_FUp_Front(object source, ServerValidateEventArgs args)
{
int sizeinbytes_, fileSize_;
string fileName = "";
FileInfo fi;
string ext;
//added by santosh on 25Feb2020 prevent file is accept double extension
var regex = #"(^([a-z]|[A-Z]|[0-9]|[-]|[(0-9)+]+)+\.(jpg|JPG|JPEG|jpeg|png|PNG)$)";
var match = Regex.Match(FUp_Front.FileName, regex, RegexOptions.IgnoreCase);
if (!match.Success)
{
args.IsValid = false;
CV_FUp_Front.ErrorMessage = "Please enter valid file";
Rg_FrontPage.Enabled = false;
Rg_BackPage.Enabled = false;
}
int FUp_Front_count = FUp_Front.FileName.Split('.').Length - 1;
if (FUp_Front_count > 1)
{
args.IsValid = false;
CV_FUp_Front.ErrorMessage = "Please enter valid file";
}
//end by santosh
if (Session["FUp_Front"] == null && !FUp_Front.HasFile)
{
args.IsValid = false;
CV_FUp_Front.ErrorMessage = "Max size 100KB. Upload only JPEG/JPG/PNG format";
Rg_FrontPage.Enabled = false;
Rg_BackPage.Enabled = false;
}
else if (FUp_Front.HasFile)
{
fileName = FUp_Front.PostedFile.FileName;
fi = new FileInfo(FUp_Front.PostedFile.FileName);
ext = fi.Extension.ToLower();
if (ext != ".jpg" && ext != ".jpeg" && ext != ".png")
{
args.IsValid = false;
CV_FUp_Front.ErrorMessage = "Max size 100KB. Upload only JPEG/JPG/PNG format";
Rg_FrontPage.Enabled = false;
Rg_BackPage.Enabled = false;
}
sizeinbytes_ = FUp_Front.PostedFile.ContentLength;
fileSize_ = sizeinbytes_ / 1024;
if (fileSize_ > 100)
{
args.IsValid = false;
CV_FUp_Front.ErrorMessage = "Max size 100KB. Upload only JPEG/JPG/PNG format";
Rg_FrontPage.Enabled = false;
Rg_BackPage.Enabled = false;
}
}
else
{
args.IsValid = true;
}
}