package lv.id.jc.piglatin.core;
   
   import static java.lang.annotation.ElementType.FIELD;
   import static java.lang.annotation.ElementType.METHOD;
   import static java.lang.annotation.ElementType.PARAMETER;
   import static java.lang.annotation.RetentionPolicy.RUNTIME;
   
   import java.lang.annotation.Documented;
   import java.lang.annotation.Retention;
  import java.lang.annotation.Target;
  
  import jakarta.validation.Constraint;
  import jakarta.validation.Payload;
  import jakarta.validation.constraints.NotBlank;
  import jakarta.validation.constraints.NotNull;
  import jakarta.validation.constraints.Pattern;
  
  /**
   * A single word.
   * <p>
   * A single word contains only Unicode letters and apostrophe (') in the middle.
   * Only one apostrophe is allowed.
   * <p>
   * Examples:
   * <ul>
   *     <li>word</li>
   *     <li>it's</li>
   *     <li>don't</li>
   *     <li>can't</li>
   *     <li>o'clock</li>
   * </ul>
   */
  @Documented
  @Retention(RUNTIME)
  @Constraint(validatedBy = {})
  @Target({PARAMETER, METHOD, FIELD})
  @NotNull
  @NotBlank
  @Pattern(regexp = """
      #
      # A single word
      # contains only Unicode letters and apostrophe (') in the middle
      # only one apostrophe is allowed
      #
      ^               # Start of the line
      (               # Start of the group
          \\p{L}      # Any Unicode letter
      |               # OR
          (?<=\\p{L}) # Positive lookbehind for a Unicode letter
          '           # An apostrophe
          (?=\\p{L})  # Positive lookahead for a Unicode letter
          (?!.*')     # Negative lookahead for another apostrophe
      )               # End of the group
      +               # One or more of the previous group
      $               # End of the line
      """,
      flags = {Pattern.Flag.COMMENTS, Pattern.Flag.CASE_INSENSITIVE}
  )
  public @interface Word {
      /**
       * The default error message if the validation fails.
       *
       * @return the error message
       */
      String message() default "must be a single word";
  
      /**
       * The groups the constraint belongs to.
       *
       * @return the groups
       */
      Class<?>[] groups() default {};
  
      /**
       * The payload that can be associated with a constraint violation.
       *
       * @return the payload
       */
      Class<? extends Payload>[] payload() default {};
  }