# How to replace string using JavaScript RegExp?

In this tutorial, we will explore how we can replace a particular sub string in a string by using regular expressions in JavaScript. Sometimes we may want to replace a recurrent sub string in a string with something else. In such cases, regular expressions can be beneficial. Before using them to solve our current problem, let us see what regular expressions exactly are.

Regular expressions are basically a pattern of characters that can be used to search different occurrences of that pattern in a string. Regular expressions make it possible to search for a particular pattern in a stream of characters and replace it easily. For example, consider the regular expression /ab*c/ which denotes that we are looking for a pattern where we have one ‘a’, followed by 0 or more b characters followed by one ‘c’.

Now that we know what regular expressions are, let us look at how we can use them to replace a sub string in a string in JavaScript. As regular expressions are used to denote the different patterns of characters, they can be beneficial in finding these patterns in a string and also replacing them.

For this very reason, JavaScript has an in-built method for the string data type. One such method is the replace() method of JavaScript. Using this function we can replace the occurrences of a particular pattern in a string with whatever we want and we can use regular expressions to denote this pattern with which we want to replace. Let us look at how this function works.

### Syntax and Parameters

The syntax of the replace() function is −

s.replace(pattern, replacement);


Here s is the string in which we want to replace some pattern. Let us look at both the parameters in detail −

• pattern − This refers to the sub string of the string s which is to be converted into something else, the sub string that is to be replaced. This parameter can be a string or a regular expression that denotes the pattern to be found and replaced in string s. Only the first occurrence of the pattern in s will be replaced, but if a regular expression is provided with the global modifier ‘g’ in this parameter, all matches of that regular expression in the string s will be replaced. Naturally, this is a required parameter.

• replacement − This denotes the string which will take the place of the pattern in the string s. This is what every occurrence of the pattern will be replaced with. Note that this parameter can also be a function that returns the string which will be used as a replacement for each occurrence of pattern in s.

This function gives another string with the required replacements as output. For example,

"abcdea".replace(/a/i, "b");


The resulting string will change from "abcdea" to "bbcdea" after this operation. Note that the last a was not replaced as the regular expression provided as a pattern does not contain the global modifier ‘g’.

Let us take a look at how we can achieve this through the code for both with and without the global modifier ‘g’.

### Example 1: Without the global modifier ‘g’.

In the below example, we replace a string using the string replace() method without global modifier 'g'.

<!DOCTYPE html>
<html>
<body>
<script>
var st = "She sells sea shells on the seashore";
document.write("Original string : " + st);
var regExp = /Sea/i;

var modifiedSt = st.replace(regExp, "river");
document.write("<br>Modified string : " + modifiedSt);
</script>
</body>
</html>


In the above code, we used the regular expression with only one modifier: ‘i’ to achieve replacing sub strings of a string in JavaScript and as you can notice it only changed the first occurrence of "sea" to "river" and not the second one which is in the last word "seashore".

### Example 2: With the globab modifier ‘g’.

In the below example, we replace a string using the string replace() method with global modifier 'g'.

<!DOCTYPE html>
<html>
<body>
<script>
var st = "She sells sea shells on the seashore";
document.write("Original string : " + st);
var regExp = /Sea/ig;

var modifiedSt = st.replace(regExp, "river");
document.write("<br>Modified string : " + modifiedSt);
</script>
</body>
</html>


In the above code, we used the regular expression with two modifiers: ‘i’ and ‘g’ to achieve replacing sub strings of a string in JavaScript and as you can notice the last word was changed from "seashore" to "rivershore" which did not happen in the last example.

## Conclusion

In this tutorial, we saw how we can replace a string in JavaScript using regular expressions. This was achieved by using the string.replace() method in JavaScript which uses regular expressions to identify and replace patterns in a string.