ereg function

I'm having problem to form an ereg function to check variable with the following criteria,

all characters are lower case ( only [a-z]),
numeric allow,
underscore allow,
no space,
minimum 6 characters,
maximum 20 characters

Is it possible to have all this in one ereg function?

Any help will be much appreciated!

The function that I've written is ereg("^{[0-9a-z_]{6,20}$",$var)}

But it allows the variable to start with underscore like '_xyz'.

Any idea to restrict the variable to start with an underscore but allow the underscore to appear anywhere in the variable after then ?

ereg("^{[^{][0-9a-z]{5,19}$",$var)}}

like this...

Andreas Bernhardsen

what is you want to have only one underscore allow ?