Java Regex validate username length

You can use:

String pattern = "^[A-Za-z_][A-Za-z0-9_]{7,29}$";

^[A-Za-z_] ensures input starts with an alphabet or underscore and then [A-Za-z0-9_]{7,29}$ makes sure there are 7 to 29 of word characters in the end making total length 8 to 30.

Or you can shorten it to:

String pattern = "^[A-Za-z_]\\w{7,29}$";

You regex is trying to match 8 to 30 instances of ([A-Za-z_][A-Za-z0-9_]*) which means start with an alphabet or underscore followed by a word char of any length.