/[A-Z0-9]+[A-Z0-9]+[A-Z0-9]{1}/i

##</code><code>##

/[A-Z0-9]{3,}/i

##</code><code>##

/\w{3,}/

##</code><code>##

/^            # match the start of the line
 [A-Z0-9]{3,} # match at least 3 alnums, beginning at the start of the line
 $            # match the end of the line -- we've only matched alnums
 /xi;

##</code><code>##

/^[A-Z0-9]{3,}$/i;