你可以用pattern attribute美元.required attribute也是必需的，否则将从constraint validation中排除具有空值的输入字段.
<input pattern=".{3,}" required title="3 characters minimum">
<input pattern=".{5,10}" required title="5 to 10 characters">
如果要创建将图案用于"空或最小长度"的选项，可以执行以下操作:
<input pattern=".{0}|.{5,10}" required title="Either 0 OR (5 to 10 chars)">
<input pattern=".{0}|.{8,}" required title="Either 0 OR (8 chars minimum)">