# JS Hoist Regexp

> Hoist RegExp creation outside render or memoize with useMemo(). Apply when using regular expressions in React components or frequently called functions. Use when this capability is needed.

- Skill: `tomevault-io/js-hoist-regexp` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add tomevault-io/js-hoist-regexp`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/js-hoist-regexp/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- Author: tomevault-io (https://skillmd.com/u/tomevault-io)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tomevault-io/js-hoist-regexp

---


## 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
```

---
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<!-- tomevault:4.0:skill_md:2026-04-11 -->