Step 1 - Email check (practical, not RFC-complete)

extension EmailCheck on String {
  bool get isEmail {
    final trimmed = trim();
    final at = trimmed.indexOf('@');
    if (at <= 0 || at == trimmed.length - 1) return false;
    return !trimmed.contains(' ');
  }
}

void main() {
  print('a@b.com'.isEmail);
  print('not-an-email'.isEmail);
}

Step 2 - Currency-style string

extension CurrencyFormat on num {
  String toCurrency({String symbol = '\$', int fractionDigits = 2}) {
    final fixed = toStringAsFixed(fractionDigits);
    return '$symbol$fixed';
  }
}

void main() {
  print((19.5).toCurrency());
  print((1000).toCurrency(symbol: 'BDT ', fractionDigits: 0));
}

Step 3 - Combine in a tiny UI string

String priceLine(String email, num price) {
  if (!email.isEmail) return 'bad email';
  return 'Receipt to $email: ${price.toCurrency()}';
}

Practice tasks

• Tighten isEmail with a single RegExp you document as “demo-only”.
• Add extension on String { String get normalizedEmail => trim().toLowerCase(); } and use it before isEmail.
• Add num.toPercent() returning "12%" style strings from a 0.12 input.