正则表达式，如何添加带有结束字符的前置字符

I am using Regular Expressions to code the last characters in an SQL statement, for example the operator - e.g if the SQL statement consist of WHERE keyword LIKE 100, then it must search for LIKE. But now I want to add DISTINCT in front of the field name, for example DISTINCT keyword, in the same regex. I am having difficulties with the front character that must be DISTINCT. Ant help please?

$operator = preg_match("/(^DISTINCT\s)\s(NOT|LIKE|NOT LIKE|\<|\>|\!\=)$/", $field, $matches);

$oplength = strlen($matches[0]);

    if(strpos($field, $matches[0])) {
        $field = trim(substr($field, 0, -$oplength));
    }
    else {
        $matches[0] = '=';
    }