JavaScript Regular Expressions

Creating Patterns

Create a regex with forward slashes for literal patterns.

const pattern = /hello/;
pattern.test("hello world"); // true

Use the RegExp constructor for dynamic patterns.

const word = "hello";
const pattern = new RegExp(word);

Character classes match any single character from a set.

/[aeiou]/ matches any vowel
/[0-9]/ matches any digit
/[a-z]/ matches any lowercase letter

Use . to match any character except newline.

/h.llo/ matches "hello", "hallo", "h9llo"

Anchors match positions, not characters.

/^hello/ matches "hello" at start
/world$/ matches "world" at end

Quantifiers

Use * for zero or more matches of the previous element.
```
/a*b/ matches "b", "ab", "aab", "aaab"
```
Use + for one or more matches.
```
/a+b/ matches "ab", "aab" but not "b"
```
Use ? for zero or one match.
```
/a?b/ matches "b", "ab" but not "aab"
```

Use {n} for exactly n matches, {n,m} for a range.

/a{3}/ matches "aaa"
/a{2,4}/ matches "aa", "aaa", or "aaaa"

Use {n,} for n or more matches.
```
/a{2,}/ matches "aa", "aaa", "aaaa"...
```

Flags and Methods

Use the i flag for case-insensitive matching.
```
/hello/i.test("HELLO"); // true
```
Use the g flag to find all matches, not just the first.
```
"hello hello".match(/hello/g); // ["hello", "hello"]
```
Use the m flag for multiline mode where ^ and $ match line breaks.
```
/^line/m matches "line" at start of any line
```
The test() method returns true or false.
```
/[0-9]/.test("abc123"); // true
```

The exec() method returns match details or null.

/(\w+)@(\w+)/.exec("user@example.com");
// ["user@example.com", "user", "example"]

Matching and Replacing

Use match() to find all matching strings.

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

Use matchAll() for detailed match information with groups.

const matches = "a1b2c3".matchAll(/(\w)(\d)/g);
for (const m of matches) console.log(m[1], m[2]);

Use replace() to change matched strings.

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

Use replaceAll() to replace every match globally.

"hello hello".replaceAll(/hello/, "hi"); // "hi hi"

Use a function in replace() to compute replacements.

"hello world".replace(/\w+/, (m) => m.toUpperCase());
// "HELLO world"

Common Patterns

Email validation pattern.

/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test("user@example.com");

Phone number pattern.

/^\d{3}-\d{3}-\d{4}$/.test("123-456-7890");

URL validation pattern.

/^https?:\/\/.+/.test("https://example.com");

Extract numbers from a string.

"Price: $19.99".match(/\d+\.?\d*/g); // ["19.99"]

Remove whitespace from a string.

"  hello  world  ".replace(/\s+/g, " ").trim();

Tip: Use online regex testers like regex101.com to build and debug patterns before using them in code.

Warning: The g flag affects the lastIndex property of regex objects, so reusing them can cause bugs — create new regex for each use or reset lastIndex to 0.