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Keshav Mohta

All About Regex

/ 3 min read

Regular Expression or Regex

Regular expressions are useful to parse a file and validate or replace with our need, few of examples are regex are

  • email address validation
  • number validation
  • password validation and strength check

in this article we will explore more on regex

Syntax Variation

there are 2 variation of writing regex pattern in #JavaScript

using back slash / /

  • when using back slashes , we do not need to wrap the expression in quotes

using string pattern

using ” then no need to wrap pattern in / / and then to create a regexp using new RegExp('pattern')


const rex = /[a-zA-Z0-9]/;
const stringPattern = "[a-zA-Z0-9]";
const stringRegex = new RegExp(stringPattern);
// here rex and stringRegex are equivalent

also there are subtle difference between these 2 variation and

also when we need to use character class such as \d ( digit only ) \b (word boundary) then in string pattern we need to use extra \ and we can get regex pattern from string pattern using regex.source method

const rex = /\d*\bcolor\b/;
const stringPattern = "\\d*\\bcolor\\b";
const stringRegex = new RegExp(stringPattern);
console.log(stringRegex.source); // return \d*\bcolor\b


flags are very distinctive usage while using regex, the very common flags we use mostly are

  • /g —> do global search
  • /i —> do case insensitive search
  • /m —> do multiline search

and tis is how do we write flag in both regex syntax

in backslash pattern, write after ending /, for eg. /[a-z0-9]/gi in string pattern, we set as second argument, for eg. new RegExp('[a-z0-9]', 'gi')

apart from these common flags there are few other useful flags are which we talk here

/d flag

do not confuse this with \d character class

this flag is useful when we use capture groups and it provide the capture group and matched group index array

also note this works only when we have /g flag , means both comes together

/y flag

this is conditional search in regex. In Regex we can not search from a specific range like we want to search after particular match

for eg. I want to capture all property of a css declaration block , so first search for opening bracket { and after that we search for property and value and so on but this is not possible as regex always start from start of page

here sticky flag /y comes handy, we can set index of regex pattern and then match it

/u flag

this is unicode match flag as if we have smiley, or some unicode pattern in our string.

also when we use \p character class then it must have \u flag

\p is very useful character class , so we talk about character class

character class

very common character class are

  • \d to capture digit
  • \w to capture all word
  • \s to capture white space
  • \p pre defined class, /u flag is mandatory when we use this

RegEx return

normally we use while loop for regex.match to get all matched pattern, but es2023 introduce new method .matchAll() which is easier to work on

with while loop

const stringPattern = "[a-zA-Z0-9]*";
const stringRegex = new RegExp(stringPattern, "g");
const str = "there are 33 states and 7 union territory in india.";
const matches = str.match(stringRegex);