我需要一个(PHP)正则表达式来匹配雅虎的用户名规则：
Use 4 to 32 characters and start with a letter. You may use letters,numbers,underscores,and one dot (.).
/^[A-Za-z](?=[A-Za-z0-9_.]{3,31}$)[a-zA-Z0-9_]*\.?[a-zA-Z0-9_]*$/
还是稍短一点
/^[a-z](?=[\w.]{3,31}$)\w*\.?\w*$/i
