JavaScript Regular Expressions
A regular expression (regex) is a pattern that describes text. You use it to search, check or replace text.
Writing a Pattern
A regex is written between two slashes, with optional flags after it:
Example
const pattern = /karachi/i; // i = ignore upper/lower case
console.log(pattern.test("I live in Karachi")); // true
console.log(pattern.test("I live in Lahore")); // falsetest() returns true if the pattern is found anywhere in the text.
Flags
| Flag | Meaning |
|---|---|
i | Ignore case: A and a match each other |
g | Global: find all matches, not only the first |
m | Multiline: ^ and $ work on each line |
u | Unicode: handles emoji and non-English letters correctly |
Building Blocks
These are the pieces you will use most:
| Pattern | Matches |
|---|---|
abc | The exact text "abc" |
. | Any single character (except a new line) |
\d | A digit, 0 to 9 |
\w | A letter, digit or underscore |
\s | A space, tab or new line |
[aeiou] | Any one of these characters |
[a-z] | Any character in the range |
[^0-9] | Any character not in the brackets |
^ | The start of the text |
$ | The end of the text |
A pipe between two patterns means "or": /cat|dog/ matches either "cat" or "dog".
How Many Times?
Quantifiers say how many times the thing before them may repeat:
| Quantifier | Means |
|---|---|
* | 0 or more |
+ | 1 or more |
? | 0 or 1 (optional) |
{4} | Exactly 4 |
{2,5} | Between 2 and 5 |
{2,} | 2 or more |
So \d{4} means "four digits", and colou?r matches both "color" and "colour".
Checking a Pakistani Mobile Number
A mobile number like 0300-1234567 is: 03, two digits, an optional dash, then seven digits. ^ and $ make sure the whole text matches, not just part of it.
Example
<!DOCTYPE html>
<html>
<body>
<input id="phone" placeholder="0300-1234567">
<button id="check">Check</button>
<p id="msg"></p>
<script>
const mobile = /^03\d{2}-?\d{7}$/;
document.getElementById("check").addEventListener("click", () => {
const value = document.getElementById("phone").value.trim();
document.getElementById("msg").textContent = mobile.test(value)
? "Valid number: " + value
: "Please enter a number like 0300-1234567";
});
</script>
</body>
</html>Try 03001234567, 0300-1234567, 0300-123 and 1234.
Finding Matches
match() with the g flag returns an array of every match:
Example
const sms = "Your order of Rs 1200 is ready. Delivery charge Rs 150. Total Rs 1350.";
const amounts = sms.match(/\d+/g);
console.log(amounts);[ '1200', '150', '1350' ]
Without g, match() returns only the first match, with extra details. If nothing matches, it returns null.
Replacing Text
replace() and replaceAll() accept a regex:
Example
const text = "Call me at 0321 5550000";
console.log(text.replace(/\s+/g, " ")); // many spaces become one
const card = "4111-2222-3333-4444";
console.log(card.replace(/\d{4}(?=-)/g, "****"));Call me at 0321 5550000 ****-****-****-4444
(?=-) is a lookahead: it means "followed by a dash", without including the dash in the match. So only the first three groups are hidden.
Capture Groups
Round brackets ( ) capture part of the match, so you can read it on its own. You can also name a group with (?<name>...):
Example
<!DOCTYPE html>
<html>
<body>
<p id="demo"></p>
<script>
const datePattern = /(?<day>\d{2})\/(?<month>\d{2})\/(?<year>\d{4})/;
const result = "Admission date: 05/11/2026".match(datePattern);
const { day, month, year } = result.groups;
document.getElementById("demo").innerHTML =
"Full match: " + result[0] + "<br>" +
"Day: " + day + ", Month: " + month + ", Year: " + year + "<br>" +
"ISO format: " + year + "-" + month + "-" + day;
</script>
</body>
</html>Full match: 05/11/2026 Day: 05, Month: 11, Year: 2026 ISO format: 2026-11-05
In replace(), you can use $1, $2 (or $<name>) to put a group back into the new text:
Example
console.log("Khan, Ayesha".replace(/(\w+), (\w+)/, "$2 $1")); // Ayesha KhanEscaping Special Characters
Characters like . * + ? ( ) [ ] { } / \ ^ $ | have special meanings. To match them as normal text, put a backslash before them: \. matches a real dot, and \$ matches a real dollar sign.
Regex can get hard to read fast. Keep patterns short, write a comment explaining each one, and test them with many sample inputs, both good and bad.
Exercise
Exercise
Which regex matches text that is exactly five digits, like "75300"?