Am tried this one ^[\w]{1}$
But its allowed only one character. more than one character failed.
That's because that is exactly what you told it to do: {1} means "exactly one of" - and is redundant, as is most of that Regex, which reduces to:
^\w$
If you want one or more, then try
^\w+$
If you want 4 then try
^\w{4}$
If you are going to play with regular expressions, get a copy of
Expresso[
^] - it's free, and it examines and generates Regexes.