(*) regex is available in <regex> header file

(*) It's used for pattern match

(*) [a-z] - means any one char from a to z e.g. a , b, r 

(*) [ ] - parenthesis are used for a group.

(**) \\  - to use any special character in regular expression like " or [ etc

(*) [a-z]+ - Any one or more characters from a to z. e.g. aa, axghdk, dgholgh
			 + means "Match the expression one or more times in addition to matching it once."
			 
(*) abc+ - means abc plus any number of times c. e.g abccccc, abc, but not aabc

(*) [abc]+ or (abc)+ - any number of times abc e.g. abcabcabc

(*) [a-z]+ - In the range a to z, repeat any number of times. a, aa, tehte, hfod 
			 
(*) [a-z]* - Same as above but includes empty string also e.g. , aa, dklgkj

(*) [a-zA-Z0-9] -  Matches single lowercase letter, uppercase letter, or digit

(*) abc8 is also one of the valid regular expression which will exactly match with same

(*) [\\[0-9] - A char either [ or number. notice \\(double escape char used before [
			without \\, [ has special meaning.

(*) c[a-z]*t - first letter c, last letter t. In middle any number of times a to z.
				e.g. caaabbt, cttt, ct,
				
(*) c[a-su-z]t - Same as above in middle there shouldn't be t.

(*) One example asked in tally. Match with pattern "abc=def; xyz=lmn"
	1. start and end there should be doulbe quotes
	2. key and value pair should be separated by ;(semicolon), except the last pair.
	3. between two key-pair you can have space or without space. "abc=def; xyz=lmn" or "abc=def;xyz=lmn"
	4. kay an value should be spearated by = without space
	
	Let's start:
	
	Step 1: we want start and end double quotes
		regex rx("\"[a-zA-Z0-9]+\""); 
		
		// in middle there could be anything but start and 
		end should be "(doulbe quotes). e.g "sfs" , "389hkjds"
		
	Step 2: key value should be separated by equal
		regex rx("\"[a-zA-Z0-9]+\\=[a-zA-Z0-9]+\""); 
		
		//Repeat the same patter in left and right and = in middle
		
	Step 3: It should be ending with ;
		"\"[a-zA-Z0-9]+\\=[a-zA-Z0-9]+\\;\"" 
		
		// this is correct but doesn't respect if
		the last pair shouldn't have ;(secolon).
	
	Step 4: It should repeat this for any number of pairs
		"\"([a-zA-Z0-9]+\\=[a-zA-Z0-9]+\\;)+\""  
		
		//Put whole thing in bracket and add +
		
	Step 5: It should be ending with ; but not for last pair.	
			"\"([a-zA-Z0-9]+\\=[a-zA-Z0-9]+\\;)+([a-zA-Z0-9]+\\=[a-zA-Z0-9]+\\)\""
			
			//THis is interesting. copied the whole thing without ; at end. seond pair
			//is not repeating
			
	Step 6: The first part can be there or not; eg. "abc=def". it's valid to have
			without semicolon.
			
			"\"[[a-zA-Z0-9]+\\=[a-zA-Z0-9]+\\;]*([a-zA-Z0-9]+\\=[a-zA-Z0-9]+\\)\""
			
			//Changed the first part to []*. Means it can be null also
			
	Step 7: There can be space between two pairs after ;. "abc=def; lmn=pqr"
			"\"[[a-zA-Z0-9]+\\=[a-zA-Z0-9]+\\;[ ]*]*([a-zA-Z0-9]+\\=[a-zA-Z0-9]+\\)\""
			
			//Added [ ]* in first part at end. 0 or more space.
			
			This is the final solution.
			a) there are two parts, first part can be null also
			b) Only in beginning and end it has "
			c) First part should be ending with ;(semicolon)
			d) There can be space after secolon
			e) Each part can have one or more combination of character separated by
				=.
			f) The second part is not repeating.