<validators>
	<field name="username">
		<field-validator type="requiredstring">
			<param name="trim">true</param>
			<message>用户名不能为空</message>
		</field-validator>
		<field-validator type="regex">
			 <param name="regexExpression"><![CDATA[(\w{4,20})]]></param>
			<message>长度为4-25</message>
		</field-validator>
	</field>


	<field name="password">
		<field-validator type="requiredstring">
			<param name="trim">true</param>
			<message>密码不能为空</message>
		</field-validator>
		<field-validator type="regex">
			 <param name="regexExpression"><![CDATA[(\w{4,20})]]></param>
			<message>密码4-25</message>
		</field-validator>
	</field>
</validators>

修改：

<param name="expression"><![CDATA[(\w{4,25})]]></param>
参数名字改为regexExpression了
<param name="regexExpression"><![CDATA[(\w{4,25})]]></param>

Struts2 validation.xml 正则表达式不起作用