JavaScript regular expressions

What Is a Regular Expression?

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.




                    var patt = /mdbootstrap/i;

Example explained:

/mdbootstrap/i  is a regular expression.

mdbootstrap  is a pattern (to be used in a search).

i  is a modifier (modifies the search to be case-insensitive).

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 String

The search() method searches a string for a specified value and returns the position of the match:

                      var str = "Visit mdbootstrap!";
                      var n ="mdbootstrap");  // Return 6

Using String search() With a Regular Expression

Use a regular expression to do a case-insensitive search for "MDBootstrap" in a string:

                    var str = "Visit our website mdbootstrap";
                    var n =;   // Return 18

Using String replace() With a String

The replace() method replaces a specified value with another value in a string:

                      <button onclick="myFunction()">Try it</button>
                      <p id="example-1">Please visit Microsoft!</p>
                        function myFunction() {
                            var str = document.getElementById("example-1").innerHTML;   // Get the string "Please visit Microsoft!"
                            var txt = str.replace("Microsoft", "MDBootstrap");          // Replace "Microsoft" with "MDBootstrap"
                            document.getElementById("example-1").innerHTML = txt;       // Put replaced string into a paragraph with ID = "example-1"

Live preview

Please visit Microsoft!

Use String replace() With a Regular Expression

Use a case insensitive regular expression to replace Microsoft with MDBootstrap in a string:

                    <button onclick="myFunction2()">Try it</button>
          <p id="example-2">Please visit Microsoft!</p>
            function myFunction2() {
                var str = document.getElementById("example-2").innerHTML;  // Get the string "Please visit Microsoft!"
                var txt = str.replace(/microsoft/i, "MDBootstrap");         // Replace "microsoft" (event if it's lowercase) with "MDBootstrap"
                document.getElementById("example-2").innerHTML = txt;      // Put replaced string into a paragraph with ID = "example-1"

Note: Regular expression arguments (instead of string arguments) can be used in the methods above. Regular expressions can make your search much more powerful (case insensitive for example).

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About the author

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Michal Szymanski
Co-Founder at MDBootstrap & BrandFlow. Entrepreneur, web developer, UI/UX designer, marketing analyst. Dancer and nerd in one.