Js Hoist Regexp

---
name: js-hoist-regexp
description: Hoist RegExp Creation. Use when you need help with js hoist regexp.
license: CC-BY-NC-SA-4.0
metadata:
  risk: unknown
  source: community
  kind: mode
  category: rules
---

## Hoist RegExp Creation

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

**Incorrect (new RegExp every render):**

```tsx
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):**

```tsx
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:

```typescript
const regex = /foo/g;
regex.test("foo"); // true, lastIndex = 3
regex.test("foo"); // false, lastIndex = 0
```