Regular expression mentioned below is used to validate a user input in Java.
username.matches("^\\p{L}+[\\p{L}\\p{Z}.']+")
The regular expression is working for more than one character input, but fails for single character input.
As '+' denotes one and more than one characters, I confused how to support one character input as valid input.
That's because both parts in your regex are requiring at least one character each (see the + almost at the end of the regex). If you want that part to be optional, it should be * instead.
The regex you have will match 2 or more symbols. The reason is, this is symbol one (or more):
\\p{L}+
And this is symbol 2 (or more):
[\\p{L}\\p{Z}.']+
Most likely you want the last part to be "0 or more", like this:
"^\\p{L}+[\\p{L}\\p{Z}.']*"
Your regex requires a minimum of 2 characters.
"^\p{L}+" - minimum of 1
"[\p{L}\p{Z}.']+" - minimum of 1
The "+" does denote one or more characters.
