I need a regexp that fails in every character is outside [a-zA-Z0-9_] or if the string is only numbers

The following JavaScript code returns false if the string contains only numbers or characters other than [a-zA-Z0-9_].

function validate(n) {
    return (!/^[\d]+$/.test(n) && /^[\w]+$/.test(n));
}

Is it possible to do this with just a regexp? I want to take a开发者_运维问答dvantage of pattern attributes on the input tag for validation.

<input type="text" pattern="...">

Edit: I also want it to be between 5 and 20 characters

Like this: ^[a-zA-Z0-9_]*[a-zA-Z_]+[a-zA-Z0-9_]*$

This regex matches zero or more characters including numbers, at least one character that isn't a number, followed by another zero or more characters including numbers.