Question

I’m trying to match the username with a regex. Please don’t suggest a split.

USERNAME=geo

Here’s my code:

    String input = "USERNAME=geo";
    Pattern pat = Pattern.compile("USERNAME=(\\w+)");
    Matcher mat = pat.matcher(input);
    if(mat.find()) {
        System.out.println(mat.group());
    }

why doesn’t it find geo in the group? I noticed that if I use the .group(1), it finds the username. However the group method contains USERNAME=geo. Why?

Answer 1

Because group() is equivalent to group(0), and group 0 denotes the entire pattern.

From the documentation:

public String group(int group)

Group zero denotes the entire pattern, so the expression m.group(0) is equivalent to m.group()

As you’ve found out, with your pattern, group(1) gives you what you want.

If you insist on using group(), you’d have to modify the pattern to something like "(?<=USERNAME=)\\w+".