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A regular expression is a sequence of characters that forms a search pattern.

When you search for data in a text, you can use this search pattern to describe what you are searching for.

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.

 
/pattern/modifiers;
/w3Schools/i
w3Schools = pattern 
i = modifier    // makes it case- sensitive search 
 

<script>
function myFunction() {
    var str = "Visit W3Schools!"; 
    var n = str.search("s");              // returns 2
    var n = str.search(/w3Schools/i);       // this will return 6
    document.getElementById("demo").innerHTML = n;
}
</script>


<script>
function myFunction() {
    var str = document.getElementById("demo").innerHTML; 
    var txt = str.replace(/microsoft/i,"W3Schools");  // replaces microsot by"W3Scools"
    document.getElementById("demo").innerHTML = txt;
}
</script>

test() and exec() methods for regular expressions.