Regular Expression Metacharacters
Metacharacters are special symbols in JavaScript regular expressions that control pattern matching behavior. They allow you to match patterns beyond literal characters.
1. Common Metacharacters
. (Dot)
- Matches any single character except newline (
\\n). - With the
sflag, it also matches newlines.
console.log("cat".match(/c.t/)); // ["cat"]
console.log("c\\nt".match(/c.t/s)); // ["c\\nt"]
^ (Caret)
- Matches the start of a string.
- In multiline (
m) mode, matches the start of each line.
console.log(/^hello/.test("hello world")); // true
console.log(/^world/m.test("hello\\nworld")); // true
$ (Dollar)
- Matches the end of a string.
- In multiline mode, matches the end of each line.
console.log(/world$/.test("hello world")); // true
console.log(/hello$/m.test("hello\\nworld")); // true
(Asterisk)
Matches 0 or more occurrences of the preceding character or group.
console.log("ct".match(/ca*t/)); // ["ct"]
console.log("caat".match(/ca*t/)); // ["caat"]
+ (Plus)
Matches 1 or more occurrences of the preceding character or group.
console.log("ct".match(/ca+t/)); // null
console.log("caat".match(/ca+t/)); // ["caat"]
? (Question Mark)
- Matches 0 or 1 occurrence of the preceding character or group.
- Also used for non-greedy matching.
console.log("cat".match(/ca?t/)); // ["cat"]
console.log("ct".match(/ca?t/)); // ["ct"]
| (Pipe)
Acts as an OR operator, matching either the pattern before or after it.
console.log("cat".match(/cat|dog/)); // ["cat"]
console.log("dog".match(/cat|dog/)); // ["dog"]
() (Parentheses)
Groups patterns together and can capture matched substrings for later use.
console.log("abc".match(/(ab)c/)); // ["abc", "ab"]
console.log("abc abc".replace(/(ab)c/g, "$1d")); // "abd abd"
[] (Square Brackets)
Defines a character class, matching any single character inside the brackets.
console.log("cat".match(/[a-c]/g)); // ["c", "a"]
{} (Curly Braces)
Specifies number of occurrences of the preceding character or group:
{n}→ exactly n times{n,}→ at least n times{n,m}→ between n and m times
console.log("caaat".match(/ca{2,4}t/)); // ["caaat"]
\ (Backslash)
- Escapes a metacharacter to match it literally.
- Also introduces special sequences like
\\d,\\w, etc.
console.log("a.b".match(/a\\.b/)); // ["a.b"]
console.log("123".match(/\\d+/)); // ["123"]
\b (Word Boundary)
Matches the position between a word character (\\w) and a non-word character (\\W).
console.log("hello world".match(/\\bworld\\b/)); // ["world"]
\B (Non-Word Boundary)
Matches a position that is NOT a word boundary.
console.log("educate".match(/\\Bcat\\B/)); // ["cat"]
4. Special Sequences (Shorthand)
| Sequence | Meaning | Example |
|---|---|---|
\\d | Any digit [0-9] | /\\d/.test("5") → true |
\\D | Any non-digit | /\\D/.test("a") → true |
\\w | Any word character [a-zA-Z0-9_] | /\\w/.test("G") → true |
\\W | Any non-word character | /\\W/.test("@") → true |
\\s | Any whitespace (space, tab, newline) | /\\s/.test(" ") → true |
\\S | Any non-whitespace | /\\S/.test("a") → true |
. | Any character except newline | /./.test("x") → true |
Practical Examples
// Dot matches any character
console.log(/a.c/.test("abc")); // true
// Start and end boundaries
console.log(/^Hello/.test("Hello World")); // true
console.log(/World$/.test("Hello World")); // true
// Optional character
console.log(/colou?r/.test("color")); // true
console.log(/colou?r/.test("colour")); // true
// Quantifiers
console.log(/a{2,4}/.test("aa")); // true
console.log(/a{2,4}/.test("aaaaa")); // false
// Lookahead
console.log(/\d(?=px)/.test("10px")); // true
// Word boundary
console.log(/\bcat\b/.test("The cat sat")); // true
// OR operator
console.log(/cat|dog/.test("dog")); // true
// Escaping
console.log(/a\.b/.test("a.b")); // true