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Regexes in programs

Theory

Illegal identifiers

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Suppose, you want to create a new programming language that supports variables.

There is a set of rules for identifiers (names) of variables:

  • It can include lower and upper letters, digits and the underscore character _;
  • It can only start with a letter or an underscore;
  • If an identifier starts with an underscore, the second character should be either a letter or a digit, but not an underscore;

Note that a single _ is not a valid identifier.

Using the provided template, write a program that reads n identifiers and then outputs all invalid ones in the same order as they appear in the input data. We hope that you will use regexes to solve the problem.

Sample Input 1: 

6
ident
i
__
1a
b33
_n1

Sample Output 1: 

__
1a
Write a program in Java 17
import java.util.Scanner;

public class Main {

public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);

int numberOfVariables = Integer.parseInt(scanner.nextLine());

String regex = "write-your-regex-here";

for (int i = 0; i < numberOfVariables; i++) {
String identifier = scanner.nextLine();
if (!identifier.matches(regex)) {
System.out.println(identifier);
}
}
}
}
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