Answer by Wiktor Stribiżew for masking of email address in java

Your look-ahead (?=[^@]*?.@) requires at least 1 character to be there in front of @ (see the dot before @).

If you remove it, you will get all the expected symbols replaced:

(?<=.{3}).(?=[^@]*?@)

Here is the regex demo (replace with *).

However, the regex is not a proper regex for the task. You need a regex that will match each character after the first 3 characters up to the first @:

(^[^@]{3}|(?!^)\G)[^@]

See another regex demo, replace with $1*. Here, [^@] matches any character that is not @, so we do not match addresses like abc@example.com. Only those emails will be masked that have 4+ characters in the username part.

See IDEONE demo:

String s = "nileshkemse@gmail.com";System.out.println(s.replaceAll("(^[^@]{3}|(?!^)\\G)[^@]", "$1*"));