var input = "hello222";
var ok = false;

var dob_regex = /^([0-9]){2}(\/){1}([0-9]){2}(\/)([0-9]){4}$/;   // DD/MM/YYYY
var email_regex = /^[a-zA-Z0-9._-]+@([a-zA-Z0-9.-]+\.)+[a-zA-Z0-9.-]{2,4}$/;  // email address
var username_regex = /^[\w.-]+$/;  // allowed characters: any word . -, ( \w ) represents any word character (letters, digits, and the underscore _ ), equivalent to [a-zA-Z0-9_]
var num_regex = /^\d+$/; // numeric digits only
var search_regex = "/hello/"; 
var password_regex = /^[A-Za-z\d]{6,8}$/;  // any upper/lowercase characters and digits, between 6 to 8 characters in total
var phone_regex = /^\(\d{3]\) \d{3}-\d{4}$/;  // (xxx) xxx-xxxx  
var question_regex = /\?$/; // ends with a question mark

1. Check that there are at least 3 numeric characters in input

if ( input.match(/d/g) == null ) {
	 ok = false;
} else if ( input.match(/d/g).length < 3 )  {
	 ok = false;	
} else {
	 ok = true;	
}

2. Check that the input has a minimum of 8 characters

if ( input.length >= 8 ) {
	 ok = true;	
}

3. Check that the input is in a correct date format (DD/MM/YYYY)

if ( dob_regex.test(input) ) {
	ok = true;	
}

4. Check that the input is in a correct email format (username@example.com)

if ( email_regex.test(input) ) {
	ok = true;	
}

5. Check that the input matches the specified username format

if ( username_regex.test(input) ) {
	ok = true;	
}

6. Check that the input contains only numeric characters

if ( input.match(num_regex) ) {
	ok = true;	
}

7. Search for a specific word in the input

if ( input.search(search_regex) != -1 ) {
	ok = true;	
}

8. Check that the input matches the specified password format

if ( input.match(password_regex) ) {
	ok = true;	
}

9. Check that the input matches the specified phone number format

if ( input.match(phone_regex) ) {
	ok = true;	
}

10. Check that the input is a question (ends with a question mark)

if ( question_regex.test(input) ) {
	ok = true;	
}

To test the Regular Expressions (RegEx) mentioned in this blog, you can use this tool.