Easy way to strip out non alphanumeric characters?

[rgrne]

I need to strip out non alphanumeric characters from usernames.  Just stripping them out would be OK, but replacing them with some other character would be better.  Is there any easy way to do this, like an equivalent of strip_tags for html characters?  I'm looking into eregi and eregi_replace, but it seems like I'd have to list all possible non-alphanumeric characters and go through letter by letter. Am I missing something?

[fert]

$str=preg_replace("/^[a-zA-z]/","a",$str);

this code will replace all non letters with the letter a, but I'm not sure if it'll work because the ^ character has two uses in regex.

[Glyde]

Should be:

$cleanString = preg_replace("/[^a-z0-9]/is", "", "$%#teststring34");

[neel_basu]

Replace \W