firstLocaleTag function

String? firstLocaleTag(String? header)

Parses the first valid BCP-47 language tag from an Accept-Language header string.

Extracts the leading entry from header, ignoring wildcard tokens ("*") and discarding quality weighting parameters (";q=..."). Returns null if header is empty or contains no valid language tag.

final tag = firstLocaleTag('fr-CH, fr;q=0.9, en;q=0.8');
// tag: "fr-CH"

See also:

• parseAcceptLanguage, for parsing and sorting all entries in a header.
• resolveLocale, for matching candidate tags against supported catalogs.

Implementation

String? firstLocaleTag(String? header) {
  if (header == null) return null;
  final firstPart = header.split(',').firstOrNull;
  if (firstPart == null) return null;

  final tag = firstPart.trim().split(';').firstOrNull?.trim();
  if (tag == null || tag.isEmpty || tag == '*') {
    return null;
  }

  return _isValidLanguageTag(tag) ? tag : null;
}