Lookarounds in JavaScript

(?=x)   Positive lookahead
(?!x)   Negative lookahead
(?<=x)  Positive lookbehind
(?<!x)  Negative lookbehind

// Lookahead examples
/\d+(?=px)/.exec("100px")        // ["100"] (number before px)
/\d+(?!px)/.exec("100em")        // ["100"] (number not before px)

// Password validation with lookahead
/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$/
// Requires: lowercase, uppercase, digit, 8+ chars

// Lookbehind examples
/(?<=\$)\d+/.exec("$100")        // ["100"] (number after $)
/(?<!\$)\d+/.exec("€100")        // ["100"] (number not after $)

// Practical examples
"test@email.com".match(/\w+(?=@)/)      // ["test"]
"$1,234.56".match(/(?<=\$)[\d,]+/)      // ["1,234"]
"not.a.file.txt".match(/\w+(?=\.txt$)/) // ["file"]