Understanding Regular Expression

Now the most complicated or the biggest issue that we all face is that how to define the regex pattern.

Lets say for example below is the regex for evaluation a email address.

^[a-zA-Z0-9]{1,10}@[a-zA-Z]{1,10}.(com|org)$

Now as this is target to newbie or the beginner they might be scratching there head that what the hell is this. And this shouldn't be making any sense to them at the moment. But let me tell you that this is the regular expression or regex which will can be used to validate a email address.

Note: Let me clear a thing regular expression can only be used to check the syntax of certain pattern. They don't guarantee the existence of any such things.

For eg: [email protected] is a valid expression but regex will not tell you whether this email id in actual exists or not.
But where as xyz@@@xyz.com is invalid email id as it don't match the expression.

So, lets dissect the expression. So, that it may make sense to you all.

All regular expression can be defined/created no matter how complex they are they can be defined using 3 symbols.
Which are famously be called the BCD

B stands for Brackets
C stands for Carrot
D stands for Dollar

Now lets further deep down into it.

As we know now that B stands for Bracket

But Brackets in regular expression are being of 3 types. They are as follows

Square Brackets [], Curly Brackets {}, Round Brackets ()

Now square brackets [] are used for specifying the characters which needed to be matched.
Where as curly brackets {} are used for specifying how many characters are allowed. Lastly the round brackets () which are used for grouping or certain words together.

So, that was all about Brackets or the B in regular expression. Now let's move ahead

C stands for Carrot ^ this marks the start of any regular expression.

And lastly D stands for Dollar $ which marks the end of any regular expression.

So, to sum up we can see the below pictorial representation of a regular expression using the BCD of regular expression

So, I hope now the above expression which I showed you earlier for validating any email address must be making some sense to you. If not still let's understand it by dividing it into the BCD of regular expression for better understanding.

^[a-zA-Z0-9]{1,10}@[a-zA-Z]{1,10}.(com|org)$

This is our regular expression.

We can see that and now we all know that Carrot "^" and Dollar "$". Are used for marking the start and the end of any regular expression which can also be seen in above regex expression.

And after carrot we have Square brackets [] which denotes the characters which are allowed. So, in this case we have lower case a-z and upper case A-Z and lastly numeric values 0-9. Now this means that any other characters apart from this will result in invalid entry. Meaning if we try to pass !@#$%^&*() any of these characters it will be treated as invalid characters as only numeric and alphabets are allowed in the Square brackets. After that we have Curly brackets {} which denotes the number of characters allowed. Now we see 2 digits separated by comma 1 and 10. This means that the minimum of 1 character is allowed and maximum of 10 characters. And in the end we have Dollar $ for denoting the end of the expression and before that we have Round brackets () which have 2 domain name separated by pipe sign | this means either of two meaning. Either com or org are allowed.

So, lets sum it up.

If we evaluate this email address : [email protected] or [email protected] or [email protected].

These all are valid

But if we try @[email protected] or [email protected]

These are invalid

Because our first bracket the square bracket only allows [a-zA-Z0-9] characters and our second square bracket only allow [a-zA-Z] characters. And only com or org domain are allowed. No other domain apart from it.

So, I hope this must be helpful. Kindly do comment if anything is not making any sense. And do post any constructive comment or in what area I need to focus in more. As this is my first article.

Regards