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JavaScript RegExp(Regular Expression)

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  • Difficulty Level : Medium
  • Last Updated : 06 Jan, 2023
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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 replacement operations.




var patt = /GeeksforGeeks/i;

Explanation : 

/GeeksforGeeks/i is a regular expression.
GeeksforGeeks is the pattern (to be used in a search).
i is a modifier (modifies the search to be Case-Insensitive).

Regular Expression Modifiers can be used to perform multiline searches which can also be set to case-insensitive matching: 

gFind the character globally
iFind a character with case-insensitive matching
mFind multiline matching

Regular Expression Brackets can be Find characters in a specified range

[abc]Find any of the characters inside the brackets
[^abc]Find any character, not inside the brackets
[0-9]Find any of the digits between the brackets 0 to 9
[^0-9]Find any digit not in between the brackets
(x | y)Find any of the alternatives between x or y separated with |

Regular Expression Metacharacters are characters with a special meaning:

\.Search single characters, except line terminator or newline.
\wFind the word character i.e. characters from a to z, A to Z, 0 to 9
\dFind a digit
\DSearch non-digit characters i.e all the characters except digits
\sFind a whitespace character
\SFind the non-whitespace characters.
\bFind a match at the beginning or at the end of a word
\BFind a match that is not present at the beginning or end of a word.
\0Find the NULL character.
\nFind the newline character.
\fFind the form feed character
\rFind the carriage return character
\tFind the tab character
\vFind the vertical tab character
\uxxxxFind the Unicode character specified by the hexadecimal number xxxxx

Regular Expression Quantifiers are used to define quantitiesoccurrence

n+Match any string that contains at least one n
n*Match any string that contains zero or more occurrences of n
n?Match any string that contains zero or one occurrence of n
m{X}Find the match of any string that contains a sequence of m, X times
m{X, Y}Find the match of any string that contains a sequence of m, X to Y times
m{X,}Find the match of any string that contains a sequence of m, at least X times
m$Find the match of any string which contains m at the end of it
^mFind the match of any string which contains m at the beginning of it
?!mFind the match of any string which is not followed by a specific string m.

Regular Expression Object Properties:

constructorReturn the function that created the RegExp object’s prototype
globalSpecify whether the “g” modifier is set or not
ignorecaseSpecify whether the “i” modifier is set or not
lastindexSpecify the index at which to start the next match
multilineSpecify whether the “m” modifier is set or not
sourceReturn the text of RegExp pattern

Regular Expression Object Methods:

compile()Used to compile the regular expression while executing of script
exec()Used to test for the match in a string.
test()Used to test for a match in a string
toString()Return the string value of the regular expression

Below is an example of the JavaScript Regular Expressions. 



function GFGFun() {
    var str = "Visit geeksforGeeks";
    var 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:



function myFunction() {
    // input string
    var str = "Visit geeksforGeeks!";
    // searching string with modifier i
    var n =;
    // searching string without modifier i
    var n =;



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



function myFunction() {
    // input string
    var str = "Please visit gfG!";
    // replacing with modifier i
    var txt = str.replace(/gfg/i, "geeksforgeeks");


Please visit geeksforgeeks! 

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