A regular expression (regex, for short) is a sequence of characters that defines a search pattern. Instead of searching for one exact string, a regex describes a whole family of strings — “any digit repeated three times,” “anything that looks like an email address,” “a word that starts with a capital letter.”
Regex isn’t a programming language on its own — it’s a small, standalone syntax that almost every programming language, text editor, and command-line tool understands: JavaScript, Python, Java, grep, VS Code’s search bar, SQL, and dozens more.
Why regex exists
Before regex, matching text patterns meant writing custom string-parsing code by hand — loops, character-by-character comparisons, endless edge cases. Regex compresses all of that into a compact notation that a regex engine can execute directly.
Compare these two ways of checking “does this string look like a US-style phone number?”:
// Without regex — verbose and easy to get wrong
function looksLikePhone(s) {
if (s.length !== 12) return false;
for (let i = 0; i < s.length; i++) {
const c = s[i];
if (i === 3 || i === 7) {
if (c !== "-") return false;
} else if (c < "0" || c > "9") {
return false;
}
}
return true;
}// With regex — one line, and easier to read once you know the syntax
const looksLikePhone = (s) => /^\d{3}-\d{3}-\d{4}$/.test(s);Where you’ll actually use regex
- Form validation — checking that an email, phone number, or password meets a format before submitting
- Search and replace — renaming variables across a codebase, cleaning up messy data
- Parsing logs — extracting timestamps, error codes, or IP addresses from log files
- Data extraction — pulling structured data (dates, prices, URLs) out of unstructured text
- Input sanitization — stripping unwanted characters or whitespace
Tip
Regex is best for shallow, pattern-based matching — things with a flat, repeatable shape. It struggles with deeply nested or context-sensitive structures, which is why “don’t parse HTML with regex” is a common piece of advice you’ll hear later in this course.
The building blocks
Every regex is built from a small set of ingredients that you’ll learn one at a time in this course:
| Concept | Example | Meaning |
|---|---|---|
| Literal characters | cat |
Matches the exact text “cat” |
| Character classes | [aeiou] |
Matches any one vowel |
| Quantifiers | a{2,4} |
Matches 2 to 4 a’s in a row |
| Anchors | ^Hello |
Matches “Hello” only at the start of a string |
| Groups | (ab)+ |
Matches “ab” repeated one or more times |
Don’t worry if none of that fully makes sense yet — each of these gets its own lesson, with interactive demos you can experiment with directly in the browser.
Try it yourself
Open the demo above and change the test text. Try inputs like "call 555.123.4567" or "555-1234" — notice how the pattern only matches the exact \d{3}-\d{3}-\d{4} shape, nothing looser.
What should happen
"call 555.123.4567" won’t match at all, because the pattern requires hyphens (-), not dots, between the digit groups. "555-1234" won’t match either, because it’s missing the first three digits. The pattern ^\d{3}-\d{3}-\d{4}$ is strict on purpose — you’ll learn how to loosen or tighten patterns like this throughout the course.
What’s next
In the next lesson, you’ll learn about regex engines — the actual programs that take a pattern and a piece of text and figure out whether (and where) they match.