Regular Expression or RegEx or RegExp

What is a Regular Expression or RegEx or RegExp ?

A regular expression is a special text string for describing a search pattern. Regular expressions are used in search engines, search and replace dialogs of word processors and text editors. Many programming languages provide regex capabilities, built-in, or via libraries.

The phrase regular expressions is often used to mean the specific, standard textual syntax for representing patterns that matching text need to conform to. Each character in a regular expression (that is, each character in the string describing its pattern) is understood to be a metacharacter (with its special meaning), or a regular character (with its literal meaning).

	abc…	Letters
	123…	Digits
	\d	Any Digit
	\D	Any Non-digit character
	.	Any Character
	\.	Period
	[abc]	Only a, b, or c
	[^abc]	Not a, b, nor c
	[a-z]	Characters a to z
	[0-9]	Numbers 0 to 9
	\w	Any Alphanumeric character
	\W	Any Non-alphanumeric character
	{m}	m Repetitions
	{m,n}	m to n Repetitions
	*	Zero or more repetitions
	+	One or more repetitions
	?	Optional character
	\s	Any Whitespace
	\S	Any Non-whitespace character
	^…$	Starts and ends
	(…)	Capture Group
	(a(bc))	Capture Sub-group
	(.*)	Capture all
	(abc|def)	Matches abc or def

The Dot ( . ) - Any character

    The dot ( . ) represents any character.

The ' .* ' matches zero or more of any character. 

Here the ' . ' matches only a single character.

Here the ' .. ' matches only a two characters.

Here the searching of 'th' characters.

Range
    Matches a character having a character code between the two specified characters inclusive.
Range of digits

Range of small alphabets

Range of capital alphabets

Range of digits, small alphabets, capital alphabets

Range with Repetitions {m}

Range with zero or more Repetitions ' * '

Negated set [^ ]

     Match any character that is not in the set.

Plus ( + )
     Matches 1 or more of the preceding token.

Alternation ( | ) and Optional ( ? )
     Acts like a boolean OR. Matches the expression before or after the |. 

Optional matches 0 or 1 of the preceding token, effectively making it optional.

Optional ( ? )

Quantifier {m,n}

     Matches the specified quantity of the previous token. {1,2} will match 1 to 2.

Anchors ( ^  $ )

   ' ^ '  matches the beginning of the string, or the beginning of a line.

   ' $ ' matches the end of the string, or the end of a line.

Regular expression for email id

To detect the below string:-
200 6248

Regular Expression is
[0-9]{3}\s[0-9]{4}

[0-9] -> range between 0-9

  {3}  -> 3 times digit

   \s    -> space character

To detect the below string:-
123.123.123.123

Regular Expression is
[0-9]{3}\.[0-9]{3}\.[0-9]{3}\.[0-9]{3}

[0-9] -> range between 0-9
  {3}  -> 3 times digit
   \.    -> Escape Dot (.) seperator

To detect the below string:-
[26/Apr/2000:00:23:48 -0400]

Regular Expression is
\[[0-9]{2}\W[A-Za-z]{3}\W[0-9]{4}\:[0-9]{2}\:[0-9]{2}\:[0-9]{2}\s\-[0-9]{4}\]

[0-9] -> range between 0-9
  {4}  ->4 times digit
   \[    -> Escape charactor  ' [ '
   \]    -> Escape charactor  ' ] '
   \:    -> Escape charactor  ' : '
   \W -> Escape special charactor