Intro to Regular Expressions

Daniel C Reyes
2 min readApr 18, 2021


RegEx for Beginners in JavaScript

What is a regular expression?

RegEx is a sequence of characters that define a search pattern in a form or text. It is used in popular languages like Javascript, Go, Python, Java, C# which supports regex fully. In JavaScript, regular expressions are objects to find patterns of char combinations in strings. Some typical use cases of regular expressions are validating a string with the pattern, searching within a string, replace substrings in a string, extract some meta-information from a string.

Popular Regex and String Methods


The test() method executes a search for a match between a regular expression and a specified string. The return is true or false.

const string = "This is an example of test"
const regex = /exam/;
console.log(regex.test(string)) =>> true


There is a method of String. It finds matches in a string and returns an array of the matches.

const string = "This is an example of the match method";
var regex =/ match/
console.log(str.match(regex)); ==> [ "match" ]

To find all matches, we use the g (global) flag(see below)

const string = "This is a test to test match method";
const regex =/test/g
console.log(str.match(regex)); ==>[ "test", "test" ]

In case of no matches, null is returned (and NOT an empty array. Important to remember while applying array methods).

var string = "This is a test" ;
console.log(str.match(/hello/)); ==> null


Flags are optional parameters that can be added to a regular expression to affect its matching behavior. There are two main flags that modify the search in different ways

  • i: Ignores casing (/e/i will match both ‘e’ and ‘E’)
  • g: Global search returning all matches for a given expression inside a string — without it, only the first match is returned

These flags can be used separately or together in any order as seen down below.

conts string = "Hello, hello";
==> Array(2)[ "He","he"]

Groups and ranges

Several characters or character classes inside square brackets […] means we want to “search for any of these characters"

For example [bd] will match for ‘b’ or ‘d’

console.log(/[bd]/.test("bed")); =>>true
console.log(/[bd]/.test("medical")); ==>true
console.log(/[bd]/.test("zoo")); ==>false

Character classes

Character classes are shorthands for certain character sets.

\d = Any digit character (from 0 to 9)

\D = Non-digit: any character except \d

\w = Any alphanumeric character from the basic Latin alphabet (including. digit), including the underscore

\W = Non-wordly character: anything but \w.

\s = a single white space character, including space, tab, form feed, line feed, and other Unicode spaces

\S = Non-space: any character except \s, for instance a letter