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What can be the regular expression or regex for a file path or path?

I am new to this. Please can anyone tell me what can be the regular expression for file path or any path :
Ex => "C:\\\\Users\\\\1700000\\\\Downloads\\\\BackendApp\\\\WebApplication\\\\WebAPI_APPL\\\\Data\\\\1\\\\FirstFol\\\\SecondFodler\\\\MainFolder\\\\File.xlsx"
or
"C:\\\\Users\\\\1700000\\\\Downloads\\\\BackendApp\\\\WebApplication\\\\WebAPI_APPL\\\\Data\\\\1\\\\FirstFol\\\\SecondFodler\\\\MainFolder"
File path or path can be small or big, it is not fixed. Path should start with "C(any drive):\\"
Please let me know what can I use also the expression should consider double backslash in it.
This expressions didn't work
#"^(?:[\w]\:|\\\\)(\\\\[a-z_\-\s0-9\.]+)+\.(txt|gif|pdf|doc|docx|xls|xlsx)$"
#"^(?:[a-zA-Z]\:|\\\\\\\\[\w\.]+\\\\[\w.$]+)\\\\(?:[\w]+\\\\)*\w([\w.])+$"

Here's one i've used in the past for unix based systems
(\/)((\w|-|_|[0-9])*\/(\w|-|_|[0-9])*)+[^.*\.]
For windows it will be essentially the same + detecting drive letter :
([A-Z]{1}:)(\/)((\w|-|_|[0-9])*\/(\w|-|_|[0-9])*)+[^.*\.]*
With backslashes :
([A-Z]{1}:)(\\\\)((\w|-|_|[0-9])*\\\\(\w|-|_|[0-9])*)+[^.*\.]*
When I used it I expected my strings to be paths only. It may capture text after a path so it's not perfect. But I hope this helps.

Try this one:
^(?:[\w]:|\\)([A-Za-z_\s0-9.-\\]+)\.(txt|gif|pdf|docx|doc|xlsx|xls)$
you could use https://regex101.com/ to try yours regex

Question Mark in Querystring with Server.MapPath returning an error

I am working on a web application. Here I am storing attachments/uploads on server physical directory. The parent folder of uploads may contain special characters like '?'
Example of URL
"~/ChapterFiles/Capgeminisdfsdf_BE CSE ?_CoverPic/CoverPic.jpg"
When I am doing, Server.MapPath() on this URL, I am getting an error "Illegal characters in path."
Can't remove question mark from folder name as it's part of requirement. Please suggest a solution, I need to fix it urgently.

You can use something like:
String absoluteDir = Server.MapPath("~");
String myRelativePath = "~/ChapterFiles/Capgeminisdfsdf_BE CSE ? _CoverPic/CoverPic.jpg".Replace("/","\\");
String absolutePath = Path.Combine(absoluteDir,myRelativePath);
It will work. I advice you to write some unit tests for this function.

Use HttpServerUtility.UrlEncode and UrlDecode to encode/decode the string.

Question marks are not allowed in folder names in Windows. Your requirement in it's current form is impossible to implement and there is no "fix". You need to rethink how to map URL's to folder and file names.

You need to use # sign before the string. Like below
#"~/ChapterFiles/Capgeminisdfsdf_BE CSE ?_CoverPic/CoverPic.jpg"
Reference link

How do I find the parent directory in C#?

I use this code for finding the debug directory
public string str_directory = Environment.CurrentDirectory.ToString();
"C:\\Users\\Masoud\\Documents\\Visual Studio 2008\\Projects\\MyProj\\MyProj\\bin\\Debug"
How can I find the parent folder as shown below?
"C:\\Users\\Masoud\\Documents\\Visual Studio 2008\\Projects\\MyProj\\MyProj"

You can use System.IO.Directory.GetParent() to retrieve the parent directory of a given directory.

string parent = System.IO.Directory.GetParent(str_directory).FullName;
See BOL

If you append ..\.. to your existing path, the operating system will correctly browse the grand-parent folder.
That should do the job:
System.IO.Path.Combine("C:\\Users\\Masoud\\Documents\\Visual Studio 2008\\Projects\\MyProj\\MyProj\\bin\\Debug", #"..\..");
If you browse that path, you will browse the grand-parent directory.

Edit: The normalization covered in this answer only happens when the path is used to access the file system, but not on the string itself. By contrast, this answer achieves the result and normalization purely using path strings, without using the file system at all.
I've found variants of System.IO.Path.Combine(myPath, "..") to be the easiest and most reliable. Even more so if what northben says is true, that GetParent requires an extra call if there is a trailing slash. That, to me, is unreliable.
Path.Combine makes sure you never go wrong with slashes.
.. behaves exactly like it does everywhere else in Windows. You can add any number of \.. to a path in cmd or explorer and it will behave exactly as I describe below.
Some basic .. behavior:
If there is a file name, .. will chop that off:
Path.Combine(#"D:\Grandparent\Parent\Child.txt", "..") => D:\Grandparent\Parent\
If the path is a directory, .. will move up a level:
Path.Combine(#"D:\Grandparent\Parent\", "..") => D:\Grandparent\
..\.. follows the same rules, twice in a row:
Path.Combine(#"D:\Grandparent\Parent\Child.txt", #"..\..") => D:\Grandparent\
Path.Combine(#"D:\Grandparent\Parent\", #"..\..") => D:\
And this has the exact same effect:
Path.Combine(#"D:\Grandparent\Parent\Child.txt", "..", "..") => D:\Grandparent\
Path.Combine(#"D:\Grandparent\Parent\", "..", "..") => D:\

To get a 'grandparent' directory, call Directory.GetParent() twice:
var gparent = Directory.GetParent(Directory.GetParent(str_directory).ToString());

Directory.GetParent is probably a better answer, but for completeness there's a different method that takes string and returns string: Path.GetDirectoryName.
string parent = System.IO.Path.GetDirectoryName(str_directory);

Like this:
System.IO.DirectoryInfo myDirectory = new DirectoryInfo(Environment.CurrentDirectory);
string parentDirectory = myDirectory.Parent.FullName;
Good luck!

No one has provided a solution that would work cross-form. I know it wasn't specifically asked but I am working in a linux environment where most of the solutions (as at the time I post this) would provide an error.
Hardcoding path separators (as well as other things) will give an error in anything but Windows systems.
In my original solution I used:
char filesep = Path.DirectorySeparatorChar;
string datapath = $"..{filesep}..{filesep}";
However after seeing some of the answers here I adjusted it to be:
string datapath = Directory.GetParent(Directory.GetParent(Directory.GetCurrentDirectory()).FullName).FullName;

You might want to look into the DirectoryInfo.Parent property.

IO.Path.GetFullPath(#"..\..")
If you clear the "bin\Debug\" in the Project properties -> Build -> Output path, then you can just use AppDomain.CurrentDomain.BaseDirectory

Since nothing else I have found helps to solve this in a truly normalized way, here is another answer.
Note that some answers to similar questions try to use the Uri type, but that struggles with trailing slashes vs. no trailing slashes too.
My other answer on this page works for operations that put the file system to work, but if we want to have the resolved path right now (such as for comparison reasons), without going through the file system, C:/Temp/.. and C:/ would be considered different. Without going through the file system, navigating in that manner does not provide us with a normalized, properly comparable path.
What can we do?
We will build on the following discovery:
Path.GetDirectoryName(path + "/") ?? "" will reliably give us a directory path without a trailing slash.
Adding a slash (as string, not as char) will treat a null path the same as it treats "".
GetDirectoryName will refrain from discarding the last path component thanks to the added slash.
GetDirectoryName will normalize slashes and navigational dots.
This includes the removal of any trailing slashes.
This includes collapsing .. by navigating up.
GetDirectoryName will return null for an empty path, which we coalesce to "".
How do we use this?
First, normalize the input path:
dirPath = Path.GetDirectoryName(dirPath + "/") ?? ""; // To handle nulls, we append "/" rather than '/'
Then, we can get the parent directory, and we can repeat this operation any number of times to navigate further up:
// This is reliable if path results from this or the previous operation
path = Path.GetDirectoryName(path);
Note that we have never touched the file system. No part of the path needs to exist, as it would if we had used DirectoryInfo.

To avoid issues with trailing \, call it this way:
string ParentFolder = Directory.GetParent( folder.Trim('\\')).FullName;

To get your solution
try this
string directory = System.IO.Directory.GetParent(System.IO.Directory.GetParent(Environment.CurrentDirectory).ToString()).ToString();

This is the most common way -- it really depends on what you are doing exactly:
(To explain, the example below will remove the last 10 characters which is what you asked for, however if there are some business rules that are driving your need to find a specific location you should use those to retrieve the directory location, not find the location of something else and modify it.)
// remove last 10 characters from a string
str_directory = str_directory.Substring(0,str_directory.Length-10);

You shouldn't try to do that. Environment.CurrentDirectory gives you the path of the executable directory. This is consistent regardless of where the .exe file is. You shouldn't try to access a file that is assumed to be in a backwards relative location
I would suggest you move whatever resource you want to access into a local location. Of a system directory (such as AppData)

In my case I am using .NET 6. When I use:
public string str_directory = Environment.CurrentDirectory.ToString();
returns C:\Projects\MyTestProject\bin\Debug\net6.0
In order to reach C:\Projects\MyTestProject I am using the following:
Directory.GetParent(Directory.GetCurrentDirectory()).Parent.Parent
You can chain Parent to Directory.GetParent(Environment.CurrentDirectory) in order to reach the directory you want.
Final version:
public string str_directory = Directory.GetParent(Environment.CurrentDirectory).Parent.Parent.ToString();

How to get filename from a file "template.docx.amp" in c#?

I have got a strange template whose extension is ".docx.amp" . Now i want to get fileName. i.e "template". Path.GetFileNameWithoutExtension() fails as it returns template.docx. Is there any other built in mechanism or i will have to go the ugle string comparison/split way. Please suggest and give some workaround

Nope, you will have to use some kind of string split, eg:
var nameWithoutExtension = filename.Split('.')[0];

How to get a filename from a path?

I need some Regular expression experts for an extra hand. :)
I have different paths,different folders,different amount of folders.
My question:How do I get the last thing - the filename?
For example in the path:
C:\a\b\c\d\e\fgh.ddj
How do I get "fgh.ddj" with regular expressions?

You don't need regex's, you can do it just like this, its a system.io helper function:
myfilename = Path.GetFileName(mypath);

You can also use FileInfo. When using FileInfo, it actually doesn't matter if the file is present or not.
var fileInfo = new FileInfo("C:\a\b\c\d\e\fgh.ddj");
var fileName = fileInfo.Name;
//this returns "fgh.ddj"
If the file is present, of course there's lots of info about file size, last accessed, etc.

If you have perl installed, then you can try something like this...
#!/usr/bin/perl
use strict;
my $fullname = 'C:\a\b\c\d\e\fgh.ddj';
my $file = (split /\\/, $fullname)[-1];
print $file;