RegExp

Added in ves6 · Updated March 16, 2026 · Built-in Objects

javascript regexp es6 string

RegExp is a built-in object that represents regular expressions for pattern matching and text manipulation. Regular expressions are powerful tools for searching, replacing, and extracting text based on patterns.

Creating RegExp Objects

Create a RegExp using the constructor or a literal notation:

// Literal notation (preferred)
const regex1 = /pattern/;
const regex2 = /pattern/flags;

// Constructor
const regex3 = new RegExp("pattern");
const regex4 = new RegExp("pattern", "flags");

Flags

Flags modify how the regular expression behaves:

Flag	Description
`g`	Global — find all matches
`i`	Case-insensitive matching
`m`	Multiline — `^` and `$` match line boundaries
`s`	Dotall — `.` matches newlines
`u`	Unicode — enable full Unicode support
`y`	Sticky — match only from lastIndex

const caseInsensitive = /hello/i;
const globalMatch = /o/g;
const multiline = /^line/m;

Basic Patterns

Character Classes

/\d/       // Matches any digit [0-9]
/\D/       // Matches any non-digit
/\w/       // Matches word character [a-zA-Z0-9_]
/\W/       // Matches non-word character
/\s/       // Matches whitespace
/\S/       // Matches non-whitespace
/[abc]/    // Matches any character in brackets
/[^abc]/   // Matches any character NOT in brackets
/[a-z]/    // Matches any character in range

Anchors

/^hello/   // Matches at start
/world$/  // Matches at end
/\bword/  // Matches at word boundary
/\Bword/  // Matches at non-word boundary

Quantifiers

/a*/      // Zero or more
/a+/      // One or more
/a?/      // Zero or one
/a{3}/    // Exactly 3
/a{2,}/   // 2 or more
/a{2,5}/  // Between 2 and 5

Special Characters

/\./      // Escaped dot (matches literal .)
/\n/      // Newline
/\t/      // Tab
/\r/      // Carriage return
/\\/      // Backslash

Methods

test()

Tests if a pattern exists in a string. Returns true or false:

const regex = /hello/;
regex.test("hello world");  // true
regex.test("goodbye");     // false

exec()

Executes a search and returns an array with match details or null:

const regex = /(\d+)-(\d+)-(\d+)/;
const result = regex.exec("2024-03-16");

result[0];      // "2024-03-16" (full match)
result[1];      // "2024" (first capture group)
result[2];      // "03" (second capture group)
result[3];      // "16" (third capture group)
result.index;   // 0 (position of match)

String Methods

match()

Returns matches found in a string:

"hello world".match(/o/g);    
// ["o", "o"] (with g flag)

"hello".match(/o/);          
// ["o", index: 4, input: "hello"] (without g flag)

matchAll()

Returns an iterator of all matches with capture groups:

const regex = /(\d+)-(\d+)/g;
const matches = "12-34 56-78".matchAll(regex);

for (const match of matches) {
  console.log(match[0], match[1], match[2]);
}
// "12-34" "12" "34"
// "56-78" "56" "78"

replace()

Replaces matched text:

"hello world".replace(/world/, "there");
// "hello there"

"cat cat cat".replace(/cat/g, "dog");
// "dog dog dog"

"John Smith".replace(/(\w+) (\w+)/, "$2, $1");
// "Smith, John"

split()

Splits string by pattern:

"a,b,c".split(/,/);
// ["a", "b", "c"]

"one1two2three".split(/\d/);
// ["one", "two", "three"]

search()

Returns index of first match or -1:

"hello world".search(/world/);  // 6
"hello".search(/xyz/);          // -1

Practical Examples

Email Validation

function isValidEmail(email) {
  const regex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
  return regex.test(email);
}

isValidEmail("user@example.com");  // true
isValidEmail("invalid");            // false

Extracting Numbers

const text = "Price: $19.99";
const match = text.match(/\d+\.\d+/);

if (match) {
  console.log(parseFloat(match[0]));  // 19.99
}

Password Strength

function isStrongPassword(password) {
  return /.{8,}/.test(password) &&    // At least 8 chars
         /[A-Z]/.test(password) &&     // Uppercase
         /[a-z]/.test(password) &&     // Lowercase
         /[0-9]/.test(password);       // Number
}