Java Matcher class not matching a simple pattern

Question

My regex matcher is returning false, and I can't understand why. It works fine on regexr.com

Here's the code:

        String test = "Calm_fit calm://listpage?pageid=fitnesstab SomeText";
        Pattern pattern = Pattern.compile("/\bcalm?:\/\/\S+/gi");
        Matcher matcher = pattern.matcher(test);
        System.out.println("Here 1");

        if (matcher.find()) {
            System.out.println("Here 2 Matched: " + matcher.group(1));
        }

Here 2 Matched: is not getting printed. Control doesn't enter if statement. I'm trying to extract the url calm://listpage?pageid=fitnesstab from the text.

Am I missing something here ? Thanks in advance!

Use `Pattern.compile("calm?://\\S+")` and change `if ` to `while`. Java regex is set with string literals, not regex literals, so you can't use `/` delimiters with flags. You will also need to use `matcher.group()`, not `matcher.group(1)`, as the regex contains no capturing groups. — Wiktor Stribiżew, Sep 30 '21 at 08:15
Use `Pattern.compile("calm?://\\S+", Pattern.CASE_INSENSITIVE);` and `.group(0)`. — Joachim Sauer, Sep 30 '21 at 08:20
I'm just looking to find the first match. @RobbyCornelissen so should I use group(0) instead ? — Ayusch Jain, Sep 30 '21 at 08:26
If you need the first match, why did you add `g` at all? Just keep your `if`, no need of `while`. — Wiktor Stribiżew, Sep 30 '21 at 08:27
Not a big deal, it is `"\\bcalm?://\\S+"` in Java. regex101.com produces the code you can copy/paste into your code, no need to mess with the regex input field text. — Wiktor Stribiżew, Sep 30 '21 at 08:32

Robby Cornelissen · Answer 1 · 2021-09-30T09:11:54.293

To summarize:

Java regular expressions are defined as strings, not as regex literals. This means that
- You don't need to delimit your expression using forward slashes.
- Backslashes need to be escaped.
- Literal forward slashes don't need to be escaped.
Flags like Pattern.CASE_INSENSITIVE can be passed as an argument to the Pattern.compile() method.
Your expression has no group 1. Either use group 0 or add additional parentheses to define a group.

Putting it all together:

import java.util.regex.*;

public class MyClass {
    public static void main(String args[]) {
        String test = "Calm_fit calm://listpage?pageid=fitnesstab SomeText";
        Pattern pattern = Pattern.compile("\\bcalm?://\\S+",
                Pattern.CASE_INSENSITIVE);
        Matcher matcher = pattern.matcher(test);

        if (matcher.find()) {
            System.out.println(matcher.group(0));
        }
    }
}

This produces the following output:

calm://listpage?pageid=fitnesstab

I'm just looking to find the first match. So should I just ignore while loop ? I can put it in if statement. — Ayusch Jain, Sep 30 '21 at 08:28
Yeah, that should be fine, but don't understand why you put a global flag in your expression then. — Robby Cornelissen, Sep 30 '21 at 08:32

Java Matcher class not matching a simple pattern

1 Answers1