Here is my code at the moment, allowing ONLY a-z and 0-9. If I wanted a similar regExp that will NOT allow any string containing a dot (.) how would I have to modify it?
if ( preg_match('/^[a-z0-9_]+$/',$myExtraField) == false ) {
This is inside a symfony2 form class, not sure whether the regExp is the same as flat PHP hence I included my present regExp... never had the time to dive into regExp and hardly ever use it.