ES6 RegEx

A RegEx is the short form of the Regular Expression, which is the sequence of characters that define a pattern. The characters in a RegEx can be alphabets, numbers or special characters. Usually, RegEx is used to find a pattern in a string to replace, delete or modify them. In ES6, a RegEx can be defined by 2 different notations.

Literal notation: The pattern is enclosed between slashes. It is executed in compile time. So, constant patterns can be given in literal notation.
```
 var regex = /pattern/
```
Constructor function: The pattern is given inside single quotes. It is executed in runtime. So, flexible patterns can be given in constructor function.
```
var regex = new RegExp( pattern, optional arguments)
```

There are different ways of mentioning patterns which are listed below:

Use of Brackets: It is used to mention a particular range of characters in a pattern.

Patterns with brackets	Description
[pattern]	Any one character from the pattern.
[^pattern]	Any one character not from the pattern.
[0-9]	Any one decimal number between 0-9.
[a-z]	Any one character from the lower case alphabets.
[A-Z]	Any one character from the upper case alphabets.

Use of Quantifiers: It is used to specify the frequency and position of characters in a pattern.

Quantifiers	Description
pattern+	Matches strings with atleast one or more pattern.
pattern*	Matches strings with zero or more patterns.
pattern{n}	Matches strings with ‘n’ number of patterns.
pattern{n1, n2}	Matches strings with patterns in range from n1 to n2 (Both inclusive).
pattern{n, }	Matches strings with minimum ‘n’ number of patterns.
pattern$	Matches strings with pattern as the end sequence.
^pattern	Matches strings with pattern as the beginning sequence.

Use of literal characters: It is used to specify the escape characters.

Literal Characters	Description
\0	It denotes a NULL character.
\t	It denotes a tab space.
\n	It denotes a newline.
\r	It denotes a carriage return.

Use of metacharacters: It is used to specify only the type of characters.

Meta Characters	Description
\s	It denotes a blank or whitespace.
\S	It denotes a non-blank or no space character.
\d	It denotes a decimal digit character.
\D	It denotes a non-digit character.
\w	It denotes a word character (Any character sequence).
\W	It denotes a non-word character.

Example:

<script>
    var text1 = "Welcome to GeeksforGeeks ";
  
    var text2 = "The numbers are - 0, 1, 2, "
              + "3, 4, 5, 6, 7, 8, 9. The "
              + "special characters are - @,"
              + " #, $, %, ^, &, *";
  
    // RegExp.match() and RegExp.replace()
    // have been used in the following
    // examples
  
    // Bracket notations
    // 1
    var regexp = new RegExp('[geeks]');
    console.log(text1.match(regexp));
      
    // 2
    var regexp = new RegExp('[^geeks]');
    console.log(text1.match(regexp));
      
    // 3
    var regexp = new RegExp('[0-9]');
    console.log(text2.match(regexp));
  
    // Quantifiers
    // 4
    var regexp = new RegExp('[geeks]*');
    console.log(text1.match(regexp));
      
    // 5
    var regexp = new RegExp('s{2}');
    console.log(text1.match(regexp));
  
    // Literal characters and meta characters
    // 6
    var regexp1 = /(\w+)\s(\w+)\s(\w+)/
    let newtext = text1.replace(regexp1, '$3 $2 $1');
    console.log(newtext);
  
    // 7
    var regexp1 = /(\d+)/;
    let newtext1 = text2.replace(regexp1, 'number');
    console.log(newtext1);
  
    // 8
    var regexp2 = /(\s+)/;
    console.log(text2.replace(regexp2, '\t'));
</script>