Need regular expressions for username and password

I need help with two regular expressions 开发者_高级运维(ASP.NET MVC 2 app). This is what I need:

Username:

• must contain at least one letter; digits are allowed but not required
• cannot contain spaces
• no special characters are allowed, such as ~!@#$%^&
• underscore and "." (dot) is allowed
• cannot start with a space or underscore

Password:

• must contain at least one letter and one number
• cannot contain spaces
• are case sensitive

I tried with [a-zA-Z0-9]+[\w.][a-zA-Z]+[\w.] for username but it failed for "a123456"

Regex for username :

^[a-zA-Z]\w+|[0-9][0-9_]*[a-zA-Z]+\w*$

another regex for username , look ahead is used..

^(?=.*[a-zA-Z].*)[a-zA-Z0-9]\w*$

Paolo's regex for password is okay, but you should put ^ to the begining and $ to the end of the regex to specify begining and end of the capture..

UPDATE:

it is stated that username can contain dot character also. I have modified the regex , but I have assumed that username can not start dot character also. here is the modified regex;

^([a-zA-Z][\w.]+|[0-9][0-9_.]*[a-zA-Z]+[\w.]*)$

also you might build it here

http://gskinner.com/RegExr/

Thanks to Gursel, this regex should do for the username:

([a-zA-Z\d]+[\w\d]*|)[a-zA-Z]+[\w\d.]*

(Regexr here)

and this for the password:

([a-zA-Z]+[\d]+|[\d]+[a-zA-Z]+)[^\s]*

(Regexr here).