Validate user-submitted username string for qualifying length and character constraints

The regex below is to validate a username.

• Must be between 4-26 characters long

• Start with atleast 2 letters

• Can only contain numbers and one underscore and one dot

I have this so far, but isn't working.

$username = $_POST['username'];
if (!eregi("^([a-zA-Z][0-9_.]){4,26}$",$username)) {
    return false;
} else {
    echo "username ok";
}

You could use the regex

/^(?=[a-z]{2})(?=.{4,26})(?=[^.]*\.?[^.]*$)(?=[^_]*_?[^_]*$)[\w.]+$/iD

as in

<?php

$username=$_POST['username'];

if (!preg_match('/^(?=[a-z]{2})(?=.{4,26})(?=[^.]*\.?[^.]*$)(?=[^_]*_?[^_]*$)[\w.]+$/iD',
                $username))
{
 return false;
}
else
{
 echo "username ok";
}

?>

• The ^(?=[a-z]{2}) ensure the string "Start with atleast 2 letters".
• The (?=.{4,26}) ensure it "Must be between 4-26 characters long".
• The (?=[^.]*\.?[^.]*$) ensures the following characters contains at most one . until the end.
• Similarly (?=[^_]*_?[^_]*$) ensures at most one _.
• The [\w.]+$ commits the match. It also ensures only alphanumerics, _ and . will be involved.

(Note: this regex assumes hello_world is a valid user name.)