Question

I’m not understanding why my regex pattern doesn’t seem to work. Here is an example:

String token = "23030G40KT";

Pattern p = Pattern
                .compile("(\\d{3}|VRB)|(\\d{2,3})|(G\\d{2,3})?|(KT|MPS|KMH)");
Matcher m = p.matcher(token);

while(m.find()){
    System.out.println(m.group());
}

That prints out:

230
30
G40

(With two following blank lines that aren’t showing here)

I’d like to print:

230
30
G40
KT

with no blank lines. What do I need to change?

Answer 1

The reason your original regex doesn’t work is described very well in other answers, such as @Reimus’s. However, I want to help you simplify it further. Your regex looks complicated but is actually very simple if you break it down.

Let’s talk about what your original regex does:

\\d{3} – Three decimals

| – Or

VRB – “VRB”

| – Or

\\d{2,3} – 2 or 3 decimals

| – Or

G\\d{2,3} – “G” followed by 2 or 3 decimals

| – Or

(KT|MPS|KMH) – “KT” or “MPS” or “KMH”

So basically you just have a bunch of things or’d together. Some of them are redundant (such as “3 decimals” and “2 or 3 decimals”). Combine them together and you get fewer cases with no grouping needed.

You can achieve the same results with this simpler regex:

Pattern.compile("G?\\d{2,3}|KT|MPS|KMH|VRB");