erAck on Nostr: Because the empty match is the first match for any "zero or more" pattern if the ...

Because the empty match is the first match for any "zero or more" pattern if the string does not start with it (and it's not a blank but an empty string). It's the same for the pattern 'x*' and this ">335" string. The second possible match for '[0-9]*' is 335. If you want it to match only the digits then instead use '[0-9]+', or restrict match to the second occurrence, like

=REGEX(">335";"[0-9]*";;2)

'[0-9]{1,3}' matches one to three digits, as many times as possible.