Find ASCII "arrows" in text

Question

I'm trying to find all the occurrences of "Arrows" in text, so in

"<----=====><==->>"

the arrows are:

"<----", "=====>", "<==", "->", ">"

This works:

 String[] patterns = {"<=*", "<-*", "=*>", "-*>"};
    for (String p : patterns) {
      Matcher A = Pattern.compile(p).matcher(s);
       while (A.find()) {
        System.out.println(A.group());
      }         
    }

but this doesn't:

      String p = "<=*|<-*|=*>|-*>";
      Matcher A = Pattern.compile(p).matcher(s);
       while (A.find()) {
        System.out.println(A.group());
      }         

No idea why. It often reports "<" instead of "<====" or similar.

What is wrong?

Answer 1

Solution

The following program compiles to one possible solution to the question:

import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class A {
  public static void main( String args[] ) {
    String p = "<=+|<-+|=+>|-+>|<|>";
    Matcher m = Pattern.compile(p).matcher(args[0]);
    while (m.find()) {
      System.out.println(m.group());
    }
  }
}

Run #1:

$ java A "<----=====><<---<==->>==>"
<----
=====>
<
<---
<==
->
>
==>

Run #2:

$ java A "<----=====><=><---<==->>==>"
<----
=====>
<=
>
<---
<==
->
>
==>

Explanation

An asterisk will match zero or more of the preceding characters. A plus (+) will match one or more of the preceding characters. Thus <-* matches < whereas <-+ matches <- and any extended version (such as <--------).

Answer 2

When you match "<=*|<-*|=*>|-*>" against the string "<---", it matches the first part of the pattern, "<=*", because * includes zero or more. Java matching is greedy, but it isn't smart enough to know that there is another possible longer match, it just found the first item that matches.