allow a-z A-Z 0-9 and underscore in a variable used for username sign up

[jasonc]

i have the following that allows letters but i need it to also allow numbers and underscores and i can not work out what needs changing to get this to work.

 

!eregi("^[[:alpha:][:space:]]{3,30}$", $username )

 

i tried this...

 

!eregi("^[[:alpha:][:space:][0-9]]{3,30}$", $username )

 

to allow numbers but it did not work.

 

 

thanks in advance for your help.

[Orio]

eregi("^[a-z0-9_]+$", $str);

 

Orio