Regular Expression Patterns (RegExp Patterns)
In JavaScript, regular expression (RegExp) patterns define the structure and rules for matching strings. A pattern can include literal characters, metacharacters, character classes, quantifiers, assertions, groups, alternation, and flags to specify exactly what to match and how.
1. Literal Patterns
Literal patterns match exact characters in a string.
const str = "Hello World";
console.log(/Hello/.test(str)); // true
console.log(/World/.test(str)); // true
console.log(/world/.test(str)); // false (case-sensitive)
Case-insensitive matching: Use the i flag.
console.log(/world/i.test(str)); // true
2. Character Classes
Character classes match any one character from a set:
console.log(/[aeiou]/.test("hello")); // true (matches 'e')
console.log(/[0-9]/.test("123")); // true (matches '1')
console.log(/[^0-9]/.test("123")); // false (no non-digit)
- Combine ranges:
[a-zA-Z0-9]matches letters or digits. - Shorthand classes:
\\d→ any digit[0-9]\\w→ any word character[a-zA-Z0-9_]\\s→ any whitespace
Example:
console.log("a1b2".match(/[a-z]\\d/g)); // ["a1", "b2"]
3. Metacharacters
Metacharacters define special matching rules:
| Metacharacter | Meaning |
|---|---|
. | Any character except newline |
^ | Start of string/line |
$ | End of string/line |
* | 0 or more repetitions |
+ | 1 or more repetitions |
? | 0 or 1 occurrence / non-greedy |
() | Groups pattern and captures match |
[] | Character class |
{n,m} | Repetition between n and m |
| ` | ` |
Examples:
const str = "cat or dog";
console.log(/cat|dog/.test(str)); // true
console.log(/c.t/.test("cot")); // true
4. Quantifiers
Quantifiers specify how many times a part of the pattern repeats:
console.log(/a{2,4}/.test("aaa")); // true (2 to 4 'a')
console.log(/a*/.test("")); // true (0 or more 'a')
console.log(/a+/.test("a")); // true (1 or more 'a')
console.log(/a?/.test("")); // true (0 or 1 'a')
Greedy vs Lazy Matching:
const str2 = "<p>one</p><p>two</p>";
console.log(str2.match(/<p>.*<\\/p>/)); // greedy → matches both paragraphs
console.log(str2.match(/<p>.*?<\\/p>/g)); // lazy → matches each paragraph separately
5. Assertions
Assertions match positions without consuming characters:
| Assertion | Meaning |
|---|---|
\\b | Word boundary |
\\B | Non-word boundary |
^ | Start of string/line |
$ | End of string/line |
(?=...) | Positive lookahead |
(?!...) | Negative lookahead |
(?<=...) | Positive lookbehind |
(?<!...) | Negative lookbehind |
Examples:
console.log(/\\bcat\\b/.test("the cat sat")); // true
console.log(/(?<=\\$)\\d+/.test("$100")); // true
6. Groups
( … )→ Capturing group for backreferences or replacements(?: … )→ Non-capturing group
Examples:
console.log("abcabc".match(/(abc)\\1/)); // ["abcabc", "abc"]
console.log("abc def".match(/(?:abc) def/)); // ["abc def"]
7. Alternation (OR Operator)
Matches one of multiple patterns:
console.log("cat".match(/cat|dog/)); // ["cat"]
8. Flags
Flags modify how patterns behave:
| Flag | Meaning |
|---|---|
g | Global search (all matches) |
i | Case-insensitive |
m | Multiline (^/$ match lines) |
s | Dot matches newline |
u | Unicode mode |
y | Sticky match (from current position) |
Example:
console.log("Cat cat".match(/cat/gi)); // ["Cat", "cat"]
Practical Examples
Email Validation:
const emailPattern = /^[\\w.-]+@[\\w.-]+\\.\\w+$/;
console.log(emailPattern.test("[email protected]")); // true
Phone Number (XXX-XXX-XXXX):
const phonePattern = /^\\d{3}-\\d{3}-\\d{4}$/;
console.log(phonePattern.test("123-456-7890")); // true
Date Validation (DD/MM/YYYY):
const datePattern = /^\\d{2}\\/\\d{2}\\/\\d{4}$/;
console.log(datePattern.test("13/10/2025")); // true
Extract URLs:
const urlPattern = /https?:\\/\\/[^\\s]+/g;
console.log("Visit <http://example.com>".match(urlPattern)); // ["<http://example.com>"]
Match Repeated Words:
const repeatPattern = /\b(\w+)\b\s+\1\b/;
console.log("hello hello world".match(repeatPattern)); // ["hello hello", "hello"]