sandbox-agent/.agents/skills/vercel-react-best-practices/rules/js-hoist-regexp.md

title	impact	impactDescription	tags
Hoist RegExp Creation	LOW-MEDIUM	avoids recreation	javascript, regexp, optimization, memoization

Hoist RegExp Creation

Don't create RegExp inside render. Hoist to module scope or memoize with useMemo().

Incorrect (new RegExp every render):

function Highlighter({ text, query }: Props) {
  const regex = new RegExp(`(${query})`, 'gi')
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
}

Correct (memoize or hoist):

const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/

function Highlighter({ text, query }: Props) {
  const regex = useMemo(
    () => new RegExp(`(${escapeRegex(query)})`, 'gi'),
    [query]
  )
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
}

Warning (global regex has mutable state):

Global regex (/g) has mutable lastIndex state:

const regex = /foo/g
regex.test('foo')  // true, lastIndex = 3
regex.test('foo')  // false, lastIndex = 0