feat: allow custom sub-addressing delimiter character #239

lib/sub-addressing.ts

@@ -12,12 +12,21 @@ const TAG_REGEX = /^[a-zA-Z0-9-]{1,30}$/;
 
 export const DEFAULT_SUB_ADDRESS_DELIMITER = '+';
 export const SUPPORTED_SUB_ADDRESS_DELIMITERS = ['+', '-', '.', '='] as const;
-export type SubAddressDelimiter = (typeof SUPPORTED_SUB_ADDRESS_DELIMITERS)[number];
+export type SubAddressDelimiterPreset = (typeof SUPPORTED_SUB_ADDRESS_DELIMITERS)[number];
 
-export function isSupportedSubAddressDelimiter(value: string): value is SubAddressDelimiter {
+export function isSupportedSubAddressDelimiter(value: string): value is SubAddressDelimiterPreset {
   return (SUPPORTED_SUB_ADDRESS_DELIMITERS as readonly string[]).includes(value);
 }
 
+// RFC 5321 atext "special" characters, minus alphanumerics and "@". A custom
+// delimiter must be exactly one of these — they're safe to embed in a local
+// part and unambiguously separate the user from the tag.
+const VALID_DELIMITER_REGEX = /^[!#$%&'*+\-./=?^_`{|}~]$/;
+
+export function isValidSubAddressDelimiter(value: unknown): value is string {
+  return typeof value === 'string' && VALID_DELIMITER_REGEX.test(value);
+}
+
 export type TagValidationErrorCode =
   | 'EMPTY'
   | 'TOO_LONG'