Let's analyze this regex:

/^[5]+$/

• ^ indicates the pattern anchors to the beginning of the string.
• [...] is a character class. It allows you to create an alternate of choice where a single character in the target string matches any one of the characters within the class.
• [5] is a character class comprised of only one character, which isn't very useful in this case. You're using a "match any of these" construct but asserting that "these" are any of the following: "5". You could do away with the character class and just put a 5 there.
• + is a quantifier. It says match one or more of the previous atom. In this case, match [5] one or more times.
• $ is an anchor to the end of string.

Taken in full, you're saying starting from the beginning and ending at the end of the target string, match one or more 5 characters, and nothing else. You didn't allow for the trailing digits. Since you anchor to the end of the string, you're asserting that the string can only contain a bunch of 5s.

If you want to allow for eight trailing digits that are not 5s, you need to specify that too:

/^5\d{8}$/

Now you're saying the target has to start with a 5 and then must match eight more digits, at which point the target string must end.

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