Regular Expressions...

[Quevas]

I've a textbox which must contain only alfanumeric characters (a-z A-Z 0-9).
Someone told me that I've to use a regular expression to check the text.
I've tried with
if (preg_match("[a-z A-Z 0-9]",$text) ) echo "ok";
but it doesn't work.
Anyone could help me please?

[Quevas]

Now I'm trying with
ereg("[a-z A-Z 0-9]",$text)

and It seems to work until I insert a mixed text like aSd"£.

[Quevas]

I've found a solution:

for ($i=0;$i<strlen($text);$i++) 
{
if (!ereg("[a-z A-Z 0-9]",$text[$i]))
die("Error in text!");
}