java 39 lines · 8 steps

Normalizing email addresses in Java

A validator that canonicalizes emails, applying Gmail's dot and plus-tag rules so equivalent addresses collapse to one form.

Explained by highlit

Intermediate validation regex normalization string-processing immutability

1public final class EmailNormalizer {
2 
  private static final Pattern EMAIL_PATTERN = Pattern.compile(
      "^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,}$"
  );
6 
  private static final Set<String> GMAIL_DOMAINS = Set.of("gmail.com", "googlemail.com");
8 
  public String normalize(String rawEmail) {
      if (rawEmail == null || rawEmail.isBlank()) {
          throw new InvalidEmailException("Email must not be empty");
      }
13 
      String trimmed = rawEmail.strip().toLowerCase(Locale.ROOT);
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      if (!EMAIL_PATTERN.matcher(trimmed).matches()) {
          throw new InvalidEmailException("Malformed email: " + rawEmail);
      }
19 
      int at = trimmed.lastIndexOf('@');
      String localPart = trimmed.substring(0, at);
      String domain = trimmed.substring(at + 1);
23 
      if (GMAIL_DOMAINS.contains(domain)) {
          int plus = localPart.indexOf('+');
          if (plus >= 0) {
              localPart = localPart.substring(0, plus);
          }
          localPart = localPart.replace(".", "");
          domain = "gmail.com";
      }
32 
      if (localPart.isEmpty()) {
          throw new InvalidEmailException("Email local part is empty after normalization");
      }
36 
      return localPart + "@" + domain;
  }
39}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Normalizing to a canonical form lets you treat differently-typed inputs as one identity.
2Validate structure with a compiled pattern before slicing a string into meaningful parts.
3Provider-specific rules like Gmail's dot-insensitivity belong behind an explicit domain check.

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