Regular Expressions: Whitespace Exercise

Problem:

Validate those texts "1.  abc" , "2.      abc" and "3.              abc" and skip this text "4.abc" in this regular expression problem.

Output:

Solution:

Let's solve this step-by-step; as you know to solve this regular expression; you have to take it character-by-character:

Step 1: The first character is integer (or number)! This means that you need to tell the regular expression that the first character is a digit (number). To do that you have to add: \d - which designates a digit.

Step 2: You add the dot "." and you escape it: \.

Step 3: This is the trickiest part. First the white space could be normal space or a tab! To designate a tab you need to add \t and a normal space you add a _; since you might have both. You add \s which designates both tabs and normal spaces! Now \s designates one whitespace only; so you have to tell you the regular expression you might have more whitespaces which you indicate \s* but wait a second! The star means you might have at least zero whitespace and it goes into skip text; you have to tell the regular expression that you can't have zero whitespaces; one and more only! This is the where the Kleene "+" comes into place. To indicate one and more whitespace you add \s+.

Step 4: Really as easy as counting abc now ;) 

So, at the end: \d\.\s+abc