Your current pattern repeats a group 8 or more times. That group by itself matches 1 or more chars a-z followed by 1 or more digits.

That means that the minimum string length to match is 16 chars in pairs of at least 2. So for example a string like a1aa1a1a1a1a1a1a1 would match.

You could write the pattern using 2 lookahead assertions to assert a length of at least 8 and assert at least a char a-z.

Then match at least a digit. Using a case insensitive match:

^(?=[a-z\d]{8,}$)(?=\d*[a-z])[a-z]*\d[a-z\d]*$

In parts, the pattern matches:

• ^ Start of string
• (?=[a-z\d]{8,}$) Positive lookahead, assert 8 or more chars a-z or digits till end of string
• (?=\d*[a-z]) Positive lookahead to assert at least a char a-z
• [a-z]* Match optional chars a-z
• \d Match at least a single digit
• [a-z\d]* Match optional chars a-z or digits
• $ End of string

Regex demo

const regex = /^(?=[a-z\d]{8,}$)(?=\d*[a-z])[a-z]*\d[a-z\d]*$/i;
[
  "AdfhGg432",
  "Abc1aaa"
].forEach(s =>
  console.log(`Match "${s}": ${regex.test(s)}`)
)