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

public class DockerArgRegex {
    public static void main(String[] args) {
        System.out.println(extract("$foo"));
        System.out.println(extract("${foo}"));
    }

    private static String extract(String input) {
        Pattern argPattern = Pattern.compile("\\$(?:\\{([^{}]+)\\}|(\\w+))");
        Matcher matcher = argPattern.matcher(input);
        if (matcher.find()) {
            return (matcher.group(1));
        }
        return null;
    }
}