Matching characters reversely using regex

i need a regex that matches a string from specified position to first character reversely. strings are some file names.

• i m using Delphi 2010
• my example string is New Document.extension
• if specified position is 4, it should match: New Docu

You can get from "New Document.extension" to "New docu" following those steps:

• First strip the extension. You end up with "New Document"
• Remove the last 4 characters. You get "New Docu".

For the "This Is My Longest Document.ext1.ext2" example:

• Strip the extension, you end up with: "This Is My Longest Document.ext1"
• Strip the last 4 characters. You get: "Thi开发者_如何学编程s Is My Longest Document."

---

So you want the entire string up to the fourth-to-last position before the final dot? No problem:

Delphi .NET:

ResultString := Regex.Match(SubjectString, '^.*(?=.{4}\.[^.]*$)').Value;

Explanation:

^       # Start of string
.*      # Match any number of characters
(?=     # Assert that it's possible to match, starting at the current position:
 .{4}   # four characters
 \.     # a dot (the last dot in the string!) because...
 [^.]*  # from here one only non-dots are allowed until...
 $      # the end of the string.
)       # End of lookahead.

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Since I can't post the regex because I came up with the exact same Regex as Tim, I'm going to post a piece of procedural code that does the exact same thing.

function FileNameWithoutExtension(const FileName:string; const StripExtraNumChars: Integer): string;
var i: Integer;
begin
  i := LastDelimiter('.', FileName); // The extension starts at the last dot
  if i = 0 then i := Length(FileName) + 1; // Make up the extension position if the file has no extension
  Dec(i, StripExtraNumChars + 1); // Strip the requested number of chars; Plus one for the dot itself
  Result := Copy(FileName, 1, i); // This is the result!
end;

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You accepted the answer giving a regex for

The entire string up to the fourth-to-last position before the final dot.

If that's what you want then you do it best without a regex:

procedure RemoveExtensionAndFinalNcharacters(var s: string; N: Integer);
begin
  s := ChangeFileExt(s, '');//remove extension
  s := Copy(s, 1, Length(s)-N);//remove final N characters
end;

This more efficient than a regex and, much more importantly, it is much clearer and more intelligible.

Regexes are not the only fruit.

---

Edit based on comments

I'm not sure how Delphi does regex, but this works in most systems.

^.*(?=.{4}\.\w+$)

^                #the start of the string
.*               #Any characters.
(?=              #A lookahead meaning followed by...
.{4}             #Any 4 chars.
\.               #A literal .
\w+              #an actual extension.
$                #the end of the string
)                #closing the lookahead

You could also use \w{3}$ instead of \w+ at the end if you wanted to make sure that the extension was three charaters long.