JavaScript Regular Expressions

Below is the example of the JavaScript Regular Expressions. 

• Example:
  

JAVASCRIPT

<script>
function GFGFun() {
    var str = "Visit geeksforGeeks";
    var n = str.search(/GeeksforGeeks/i);
    document.write(n);
}
GFGFun();
</script>

• Output: 
  

6

A regular expression is a sequence of characters that forms a search pattern. The search pattern can be used for text search and text to replace operations. A regular expression can be a single character or a more complicated pattern. Regular expressions can be used to perform all types of text search and text replace operations.
Syntax:

/pattern/modifiers;

Example:

var patt = /GeeksforGeeks/i;

Explanation:
/GeeksforGeeks/i is a regular expression.
GeeksforGeeks is a pattern (to be used in a search).
i is a modifier (modifies the search to be Case-Insensitive).
Regular Expression Modifiers :
Modifiers can be used to perform multiline searches: 
Examples: 

Expressions	Description
[abc]	Find any of the character inside the brackets
[0-9]	Find any of the digits between the brackets 0 to 9
(x | y)	Find any of the alternatives between x or y separated with |

Regular Expression Patterns :
Metacharacters are characters with a special meaning: 
Examples: 

Metacharacter	Description
\d	Used to find a digit
\s	Used to find a whitespace character
\b	Used to find a match at beginning or at the end of a word
\uxxxx	Used to find the Unicode character specified by the hexadecimal number xxxxx

Quantifiers define quantities: 
Examples: 

Quantifier	Description
n+	Used to match any string that contains at least one n
n*	Used to match any string that contains zero or more occurrences of n
n?	Used to matches any string that contains zero or one occurrences of n

Using String Methods:
In JavaScript, regular expressions are often used with the two string methods: search() and replace().
The search() method uses an expression to search for a match, and returns the position of the match.
The replace() method returns a modified string where the pattern is replaced.
Using String search() With a Regular Expression :
Use a regular expression to do a case-insensitive search for “GeeksforGeeks” in a string:
Example:

JAVASCRIPT

<script>
function myFunction() {

    // input string
    var str = "Visit geeksforGeeks!";

    // searching string with modifier i
    var n = str.search(/GeeksforGeeks/i);

    document.write(n + '<br>');

    // searching string without modifier i
    var n = str.search(/GeeksforGeeks/);

    document.write(n);
}
myFunction();
</script>

Output: 

6
-1

Use String replace() With a Regular Expression :
Use a case insensitive regular expression to replace gfG with GeeksforGeeks in a string:
Example:

JAVASCRIPT

<script>
function myFunction() {

    // input string
    var str = "Please visit gfG!";

    // replacing with modifier i
    var txt = str.replace(/gfg/i, "geeksforgeeks");

    document.write(txt);
}
myFunction();
</script>

Output: 

Please visit geeksforgeeks!