How does this group() catch the text?

Question

I've come across this Hackerrank problem and the regex should match string between the HTML tags. The regex and the string is

String str="<h1>Hello World!</h1>";
String regex="<(.+)>([^<]+)</\\1>";

Also what if the 'str' has more than one HTML tags like String str="<h1><h1>Hello World!</h1></h1>" and how ([^<]+) catches this 'str'.

My question is how ([^<]+) matches the 'str' and not ([a-zA-Z]+).

Here if the full source code :

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

/* Solution assumes we can't have the symbol "<" as text between tags */
public class Solution{
    public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        int testCases = Integer.parseInt(scan.nextLine());

        while (testCases-- > 0) {
            String line = scan.nextLine();

            boolean matchFound = false;
            Pattern r = Pattern.compile(regex);
            Matcher m = r.matcher(line);

            while (m.find()) {
                System.out.println(m.group(2));
                matchFound = true;
            }
            if ( ! matchFound) {
                System.out.println("None");
            }
        }
    }
}

Don't mind if I'm stupid to ask this question and thank you in advance!

Answer 1

This regex guarantees that your string only contains one tag, assuming well formed HTML input.

The initial <(.+)> captures the name of your tag. The capture group will also get any attributes it can. Since + is a greedy quantifier, it will capture multiple tags if it can.

The trailing </\\1> matches against whatever the first group captured. That's why, if your HTML is well formed, the expression won't capture multiple tags or tags with attributes:

• Opening tag <h1>, closing tag </h1> ✓
• Opening tag <h1 attr="value">, closing tag </h1>, but expecting </h1 attr="value">
• Opening tag <h1><h2>, closing tag </h2></h1>, but expecting </h1><h2>

That's why the tag can be matche with .+ rather safely, while the contents must be matched with [^<]+. You want to make sure you don't grab any stay tags in the content, but any other character at all is allowed. [^<]+ (pronounced. "not <, at least once) allows things like !, while [A-za-z] certainly would not.