Regex Lookahead and Lookbehind: Match Without Consuming

What are Zero-Width Assertions?

Most regex patterns consume characters — matching abc advances the cursor three positions. Zero-width assertions match a position in the string without consuming characters. They let you say "match X only when surrounded by Y" without including Y in the result.

Positive Lookahead: (?=...)

Matches a position followed by a specific pattern, without including that pattern in the match.

"16px 2rem 8px".match(/d+(?=px)/g)
// → ["16", "8"]  — "px" is checked but not captured

Negative Lookahead: (?!...)

// Password must have uppercase, lowercase, and a digit
/^(?=.*[a-z])(?=.*[A-Z])(?=.*d).{8,}$/.test("MyPass1word") // → true

// Match "foo" only when NOT followed by "bar"
"foobar foobaz".match(/foo(?!bar)/g) // → ["foo"] (only in "foobaz")

Positive Lookbehind: (?<=...)

"Price: $42.50, $100".match(/(?<=$)d+.?d*/g)
// → ["42.50", "100"]  — "$" is checked but not captured

Negative Lookbehind: (?<!...)

"wildcat housecat".match(/(?

// Remove leading zeros
"007 042 100".replace(/(?<=^|s)0+(?=d)/g, "")
// → "7 42 100"

// Split on commas not inside parentheses
"a,b,(c,d),e".split(/,(?![^(]*))/)
// → ["a", "b", "(c,d)", "e"]