How to use regex to get file extension?

Question

I'm trying extract a substring from a string that contains a filename or directory. The substring being extracted should be the file extension. I've done some searching online and discovered I can use regular expressions to accomplish this. I've found that ^\.[\w]+$ is a pattern that will work to find file extensions. The problem is that I'm not fully familiar with regular expressions and its functions.

Basically If i have a string like C:\Desktop\myFile.txt I want the regular expression to then find and create a new string containing only .txt

For you: regular-expressions.info (A great resource that has many language-independent and language-specific aids.) — BlackVegetable
– BlackVegetable, Commented Apr 3, 2015 at 16:35
OP just wants to extract extension which is not what has been asked in dupe question. — anubhava
– anubhava, Commented Jan 29, 2021 at 5:45

anubhava · Accepted Answer · 2015-04-03 16:37:13Z

22

Regex to capture file extension is:

(\\.[^.]+)$

Note that dot needs to be escaped to match a literal dot. However [^.] is a character class with negation that doesn't require any escaping since dot is treated literally inside [ and ].

\\.        # match a literal dot
[^.]+      # match 1 or more of any character but dot
(\\.[^.]+) # capture above test in group #1
$          # anchor to match end of input

answered Apr 3, 2015 at 16:37

anubhava

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3 Comments

Inigo Over a year ago

I don't think a space can be part of a file extension? Perhaps [\w] or [^\s] instead of [^.]?

anubhava Over a year ago

@Inigo: A unix file system allows any character to be used as extension. I can create a file with the name a.b c as well

Inigo Over a year ago

Sure, but I really doubt that would count as a file extension, just a file with a dot in its name. My 2¢.

Touchstone · Accepted Answer · 2015-04-03 18:36:16Z

6

You could use String class split() function. Here you could pass a regular expression. In this case, it would be "\.". This will split the string in two parts the second part will give you the file extension.

public class Sample{
  public static void main(String arg[]){
    String filename= "c:\\abc.txt";

    String fileArray[]=filename.split("\\.");

    System.out.println(fileArray[fileArray.length-1]); //Will print the file extension
  }
}

edited Apr 3, 2015 at 18:36

answered Apr 3, 2015 at 16:38

Touchstone

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Comments

Frank Andres · Accepted Answer · 2015-04-03 17:01:58Z

2

If you don't want to use RegEx you can go with something like this:

String fileString = "..." //this is your String representing the File
int lastDot = fileString.lastIndexOf('.');
String extension = fileString.subString(lastDot+1);

answered Apr 3, 2015 at 17:01

Frank Andres

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