JavaScript Regular Expressions
A few everyday patterns cover most of what regular expressions are used for.
In real work the same handful of patterns come up again and again.
It is also worth knowing when not to reach for one.
Example
javascript
const messy = " too many spaces ";
console.log(messy.trim().replace(/\s+/g, " "));The output has single spaces throughout.
Collapsing whitespace.
Pulling numbers out of text.
Turning a title into a URL slug.
Syntax
javascript
text.replace(/\s+/g, " ");\s+ matches any run of whitespace.
\s+ handles spaces, tabs and newlines together.
Example
javascript
const messy = "a b\t\tc";
console.log(messy.replace(/\s+/g, " "));Plain replaceAll(" ", " ") could not do this.
Pull every number out of a sentence.
Example
javascript
const text = "3 apples and 12 pears";
const numbers = text.match(/\d+/g).map(Number);
console.log(numbers.join(","));
console.log(numbers.reduce(function (a, b) { return a + b; }, 0));Converting with Number makes them usable.
Lowercase, strip anything unusual, then join with dashes.
Example
javascript
function slug(title) {
return title
.toLowerCase()
.replace(/[^a-z0-9]+/g, "-")
.replace(/^-|-$/g, "");
}
console.log(slug("Hello, World! 2026"));The output is hello-world-2026.
Text from a visitor may contain regex characters.
Escape it before building a pattern from it.
Example
javascript
function escape(text) {
return text.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
}
const pattern = new RegExp(escape("a.b"));
console.log(pattern.test("a.b"));
console.log(pattern.test("axb"));Without escaping, the dot would have matched any character.
For exact text, the string methods are clearer and faster.
For HTML or JSON, use a real parser.
Example
javascript
const text = "report.pdf";
console.log(text.endsWith(".pdf"));
console.log(/\.pdf$/.test(text));Both work, but the first says what it means at a glance.
Complete Example
html
<!DOCTYPE html>
<html>
<head>
<title>JavaScript Regex in Practice</title>
</head>
<body>
<h1>Regex in Practice</h1>
<p id="out"></p>
<script>
function slug(title) {
return title.toLowerCase().replace(/[^a-z0-9]+/g, "-").replace(/^-|-$/g, "");
}
const messy = " too many spaces ";
const text = "3 apples and 12 pears";
document.getElementById("out").innerHTML =
"Tidied: [" + messy.trim().replace(/\s+/g, " ") + "]" +
"<br>Numbers: " + text.match(/\d+/g).join(", ") +
"<br>Slug: " + slug("Hello, World! 2026");
</script>
</body>
</html>Run the above example in the Try It Editor.
Try your own title:
Put punctuation and capitals in and see the slug that comes out.
\s+ collapses any run of whitespace.\d+ with g extracts every number.A small set of patterns covers most day-to-day needs.
Escaping user input matters as soon as a pattern is built dynamically.
Knowing when not to use a regular expression is as useful as knowing how.