The confusing RegEx

Regular Expression

Teacher: Today we will learn regex and how to use it in Java.

Jonny & Alice screaming with fear, they said in chorus, sir it is the most confusing thing which makes our life miserable as a programmer.

Teacher: Smiled!!! And replied yes I also used to think in this way when I am student :), But it is not hard as you think, Just need some key points to remember, Yes you have to remember key points like you remember History and Geography.

I try to point out those keys which will break your fear.

Before that tell me why you are saying regex confusing and also share the confusion to me.

Alice: Sir, The common problem is, it is hard to read and write, What we mean by that is , If we have a string to validate with a complex pattern, say email validation if you look the regex for that, it is a one liner with multiple backslashes, third brackets, first bracket etc. so we often perplex how to understand what it says. In simple word just seeing a regex solution, we don’t understand what it tries to say.

Teacher: So you mean readability right, you prefer to write more code to avoid RegEx, so code increases readability but lets me tell you regex is a Holy Grail if you unleash its power you can write concise and readable code.

Jonny: Sir another problem is there is no fix solution for a problem let takes the example of email validation if you search it in google you can see a ton of different solutions to validate email. So it is hard to take the right one?

Teacher: This is because you are not understood the crux of regex. Any other problem??

Students: Pin drop silence there.

Teacher slowly takes a step towards board and start his lesson.

What is Regex?

The teacher said Regular Expression is a technique  for search a pattern in a String, This search pattern can be very simple to very complex, a word to a sentence, or an expression made by different meta-characters or symbol used in the regex.

To understand regex correctly we need to know metacharacters/symbols and it’s meaning, This is the only thing you need to remember.

We found regex hard because we are not able to understand the usage of symbols.

Let take a look what are the different symbols used in the regex.

We can classify regex symbols in 3 brackets.

1. Meta-Characters.
2. Ranges & reserved symbols.
3. Quantifiers.

Meta-Characters : In regex, there are some reserved metacharacters which have

pre-defined meanings to express some common patterns like the digit, whitespace etc in a compact way.

Meta Character	Expression	Alternate Expr.	Definition
To Express digit	\d	[0-9] or [^\D]	By this we represent a digit character
To Express anything but not digit	\D	[^0-9] or [^\d]	By this we represent a non-digit character
To Express a word	\w	[a-zA-Z_0-9] or [^\W]	By this we represent a word character
To Express anything but not a  word	\W	[^a-zA-Z_0-9] or [^\w]	By this we represent a non-word character
To Express a whitespace	\s	[\t\n\x0b\r\f] or [^\S]	By this we represent any whitespace like \r,\t,\n etc
To Express anything but not a whitespace	\S	[^\t\n\x0b\r\f] or [^\s]	By this we represent any non whitespace
To Express a boundary	\b	[a-zA-Z0-9_]	By this we represent a boundary

Ranges & reserved symbols :  In regex when we try to match pattern, some information has to mention like how many times a pattern will be matched or you want to match the beginning of the string or end of the string or more complex pattern like maximum how many times a pattern can be a String or minimum etc. we defined them using ranges and reserved symbols.

Symbol	Description	Example	Example Definition
.	Any character	.ha.	Start with any character followed by ha then any character -- sham match:  gyan: not match
^	Check beginning of the line	^sha	If line starts with sha matched else false sham : match :Aha “ not match
$	Check end of the line	tra$	If line ends with tra matched else false Mitra: match :Chakra “ not match
[xyz]	Match either x or y or z	a[xyz]	ax : Matched aa : not matched
[xyz][abc]	Match x,y or z followed by a or b or c	s[hwo][abc]	sha : Matched sou : Not matched
XA	Exactly X followed by A	sm	sm: Matched Sa : Not Matched
X|A	X or A	s[X|A]	sX: Matched sZ: Not Matched
[^abc]	Remember : When ^ uses in side third braces act as Negate.	s[^abc]m	shm:Matched sam:Not Matched
[a-c1-10]	Match between a to c and digit 1 to 10 remember	s[x-z1-10]	sy:Matched sb : Not Matched
()	Used for Grouping	(s[^yz])(a|b)([a-c1-10]	sab1: Matched shac: Matched syab: Not Matched sabb: Matched

Quantifiers: Quantifiers say how many times a pattern can be found in a String.

Quantifiers	Description	Example	Example Definition
*	Pattern can occurs zero to many times	s(\s)*m	sm:Matched s    m : Matched s:m:Not Matched
+	Pattern can occurs one to many times	s(\s)+m	s    m : Matched sm:Not Matched
?	Pattern can occur no or one time	s(h)?a	sha:Matched sa:Matched shha:Not Matched
{X}	Pattern must occurs exactly X times	s(\d)(3)	s123:Matched s1234:Not Matched s1:Not Matched
{X,Y}	Pattern must occurs at least X and at maximum Y	s(\d)(2,4)	s12:Matched s1:Not Matched s12345:Not Matched

Email Validation :

Teacher : So Jonny earlier you said that Email validation is  confusing, Now can you guys tell us what below email validation says,

^[A-Za-z0-9]+(\\.[A-Za-z0-9-]+)*
     @[A-Za-z0-9-]+(\\.[A-Za-z0-9]+)*(\\.[A-Za-z]{2,})$;

Jonny: Yes sir, first part says ^[A-Za-z0-9-\\+]+, email must start with any characters and there must be one occurrence,^ denotes the start of the line and + says one or more, so email can start with any characters with any length.

Sir: Very good, Alice you tell me the second part.

Alice : (\\.[A-Za-z0-9-]+)*, this says that after first part it followed by a dot then again any length of characters but at least one and this part is optional as * is in the last.

Sir: Impressive.

Jonny: @[A-Za-z0-9-]+  Then it strictly matches @ and then at least one character. As + is there.

Alice : (\\.[A-Za-z0-9]+)* again it follows by the dot and at least one character and it is optional again.

Jonny : (\\.[A-Za-z]{2,})$ then email ends($) with a dot and any character in a-z or A-z and length between two to any.

Sir: Great, Now Alice, tell me a Valid Email according to this regex.

Alice: shamik.mitra@gmail.co.in or shamik@gmail.com

Sir: good, Jonny tell me an invalid one

Jonny: shamik.mitra@co.i or .mitra@gmail.co.uk

Sir: Well it seems you are learning regex very quickly. So before finish today's lesson I give you one tip, stick above tables in your desk so every day you can go through the regex symbols then easily you will remember the Regex.