バリデーションと小さな注意点

文字列全体の検証

アンカーを実際に使う

// Validate 4–6 digits ONLY (no spaces, no letters)
function isPin(text) {
  const re = /^\\d{4,6}$/;
  return re.test(text);
}

console.log("1234 ->", isPin("1234"));
console.log("12 34 ->", isPin("12 34"));
console.log("a123 ->", isPin("a123"));

過剰一致を避ける

// Greedy (.*) vs lazy (.*?)
const text = "say \"hi\" and \"bye\"";

// Greedy: eats too much
const g = text.match(/".*"/);
console.log("greedy:", g && g[0]);

// Lazy: shortest between quotes
const l = text.match(/".*?"/g);
console.log("lazy:", l);

単語境界 \\b

// Use \\b to match a whole word (not inside another)
const s = "restart start up";

console.log("has start word?", /\\bstart\\b/.test(s));    // true
console.log("matches restart?", /\\brestart\\b/.test(s));  // true
console.log("start inside restart?", /start/.test("restart")); // true (partial)

検証を安全に組み合わせる

// Small, beginner-friendly email-like check (not RFC-complete)
// Strategy: small regex + extra length checks
function isEmailLite(text) {
  if (typeof text !== "string") return false;
  if (text.length > 254) return false;          // cap length
  const re = /^[^\\s@]+@[^\\s@]+\\.[^\\s@]+$/;  // basic shape
  return re.test(text.trim());
}

console.log("ok:", isEmailLite("a@b.co"));
console.log("bad space:", isEmailLite("a @b.co"));
console.log("too basic:", isEmailLite("abc"));

グローバルフラグの注意点

// Using /g with test() changes lastIndex and can surprise you
const r = /a/g;

console.log("1st:", r.test("a")); // true
console.log("2nd:", r.test("a")); // false (cursor moved)
console.log("3rd:", r.test("a")); // true again

// Safer: avoid /g when repeatedly calling test() on separate strings
function hasA(text) {
  return /a/.test(text); // no g flag
}
console.log("safe:", hasA("a"), hasA("a"));

検証でのアンカーのクイズ

まとめ