How to extract numbers from a string

Short answer

A Pattern describes what to look for, and results() returns a stream of all matches:

void main() {
    String text = "Order 15 contains 3 items for 120 euros";
 
    List<Integer> numbers = Pattern.compile("\\d+").matcher(text).results()
            .map(match -> Integer.parseInt(match.group()))
            .toList();
    IO.println(numbers);
}

Output:

[15, 3, 120]

With a find() loop

The classic way is to call find() until there are no more matches:

void main() {
    Matcher matcher = Pattern.compile("\\d+").matcher("a1b22c333");
    while (matcher.find()) {
        IO.println(matcher.group() + " at " + matcher.start());
    }
}

Output:

1 at 1
22 at 3
333 at 6

Decimals and negatives

Extend the pattern with an optional minus sign and a fractional part:

void main() {
    String text = "t = -3.5, then 12 and 0.75";
 
    var numbers = Pattern.compile("-?\\d+(\\.\\d+)?").matcher(text).results()
            .map(match -> Double.parseDouble(match.group()))
            .toList();
    IO.println(numbers);
}

Output:

[-3.5, 12.0, 0.75]

Just the digits

If you need all digits as one string, remove everything else:

void main() {
    IO.println("+1 (555) 010-99".replaceAll("\\D", ""));
}

Output:

155501099

Common mistake

Calling group() without a successful find(). The matcher has not found a match yet and throws:

void main() {
    Matcher matcher = Pattern.compile("\\d+").matcher("abc 42");
    IO.println(matcher.group());
}

Output:

Exception in thread "main" java.lang.IllegalStateException: No match found

Thanks for reading!

Short answer

With a find() loop

Decimals and negatives

Just the digits

Common mistake

Functions used in this recipe

Thanks for reading!