JavaScript Regular Expression

A regular expression (regex) is a pattern that describes a set of strings. JavaScript uses regex to search, match, replace, and validate text — like emails, phone numbers, and passwords.

A regular expression (regex or RegExp) is a tiny language for describing text patterns. JavaScript supports regex as both a literal /pattern/flags and via the RegExp constructor — useful for search, replace and validation.

• Regex literal: /abc/. Constructor: new RegExp("abc").
• Flags: g global, i ignore case, m multi-line.
• Test: regex.test(str) returns true / false.
• Match: str.match(regex) returns array or null.

<script>
let str = "12345";

let pattern = /^\d+$/;

document.write(pattern.test(str));
</script>

<script>
var regexDigit = /^\d{10}$/;

document.write(
    regexDigit.test("9995484545")
);
</script>

<script>
let email = "hello@pbainst.in";

let re =
/^[\w.-]+@[\w-]+\.[a-z]{2,}$/i;

document.write(re.test(email));
</script>

<script>
let phone = "9876543210";

let re = /^[6-9]\d{9}$/;

document.write(re.test(phone));
</script>

<script>
var dateRegex =
/^([0-3][0-9]-)([0-1][0-2]-)(\d{2})?\d{2}$/;

document.write(
    dateRegex.test("01-02-10")
);
</script>

<script>
let str = "Order 23 books and 45 pens";

let nums = str.match(/\d+/g);

document.write(nums);
</script>

<script>
let s = "JavaScript";

document.write(
    s.replace(/[aeiou]/gi, "*")
);
</script>

<script>
let pwd = "Abc@1234";

/*
At least:
1 lowercase,
1 uppercase,
1 digit,
1 special character,
minimum 8 characters
*/

let re =
/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$/;

document.write(re.test(pwd));
</script>

• Use the i flag to ignore case.
• Use the g flag to find all matches, not only the first.
• Always escape special characters with \: \., \?.
• Test regex on regex101.com before using in code.

• Validate a 10-digit Indian phone number using regex.
• Check whether a string contains only letters and digits (alphanumeric).
• Count the number of vowels in a string using regex.
• Remove all extra spaces from a sentence using replace().
• Validate a PAN card format like ABCDE1234F.