SQL & PL/SQL :: Finding Double Vowels

I am reading Section 4-8 (page 42/216) in the Oracle Database 2-day Developer Guide from here:

[Code]....

It reads:

Suppose that you want to select every employee whose last name has a double vowel(two adjacent occurrences of the same vowel).

Example 4.9 shows how you can do this.

The regular expression ([AEIOU]) represents any vowel. The metacharacter 1 represents the first (and in this case, only) regular expression. The third function parameter, 'i', specifies that the match is case-insensitive.

Example 4.9 Selecting All Employees Whose Last Names Have Double Vowels

SELECT FIRST_NAME, LAST_NAME
FROM EMPLOYEES
WHERE REGEXP_LIKE(LAST_NAME, '([AEIOU])1', 'i');

Result is similar to:

FIRST_NAME LAST_NAME
-------------------- -------------------------
Harrison Bloom
Lex De Haan
Kevin Feeney
Ki Gee
Nancy Greenberg
Danielle Greene
Alexander Khoo
David Lee

8 rows selected.

I don't understand how [AEIOU])1 would find a double vowel(two adjacent occurrences of the same vowel).