Question

I have files in the format *C:\Temp\myfile_124.txt*

I need a regular expression which will give me just the number “124” that is whatever is there after the underscore and before the extension.

I tried a number of ways, latest is

(.+[0-9]{18,})(_[0-9]+)?\\.txt$

I am not getting the desired output. Can someone tell me what is wrong?

       Matcher matcher = FILE_NAME_PATTERN.matcher(filename);
        if (matcher.matches() && matcher.groupCount() == 2) {
            try {
                String index = matcher.group(2);
                if (index != null) {
                    return Integer.parseInt(index.substring(1));
                }
            }
            catch (NumberFormatException e) {

            }   

Answer 1

The first part [0-9]{18,} states you have atleast 18 digits which you don’t have.

Usually with regex its a good idea to make the expression as simple as possible. I suggest trying

_([0-9]+)?\\.txt$

Note: you have to call find() to make it perform the lookup, otherwise it says “No match found”

---

This example

String s = "C:\\Temp\\myfile_124.txt";
Pattern p = Pattern.compile("_(\\d+)\\.txt$");
Matcher matcher = p.matcher(s);
if (matcher.find())
    for (int i = 0; i <= matcher.groupCount(); i++)
        System.out.println(i + ": " + matcher.group(i));

prints

0: _124.txt
1: 124