Return True if string is valid

def is_valid(self, qstr=None):
        """Return True if string is valid"""
        if not self.help.source_is_console():
            return True
        if qstr is None:
            qstr = self.currentText()
        if not re.search(r'^[a-zA-Z0-9_\.]*$', str(qstr), 0):
            return False
        objtxt = to_text_string(qstr)
        shell_is_defined = False
        if self.help.get_option('automatic_import'):
            shell = self.help.internal_shell
            if shell is not None:
                shell_is_defined = shell.is_defined(objtxt, force_import=True)
        if not shell_is_defined:
            shell = self.help.get_shell()
            if shell is not None:
                try:
                    shell_is_defined = shell.is_defined(objtxt)
                except socket.error:
                    shell = self.help.get_shell()
                    try:
                        shell_is_defined = shell.is_defined(objtxt)
                    except socket.error:
                        # Well... too bad!
                        pass
        return shell_is_defined