规则：字段必须至少包含2个非空格字母数字字符。
请任何人能够指导我们如何写定定期的表述
提前致谢。
匹配一个字母数字字符，其后跟零个或多个字符，后跟另一个字母数字字符：
w.*w
如果验证自动添加 ^ 和 $，则你还需要匹配前后的字符：
.*w.*w.*
w代码匹配 A-Z、a-z、0-9和_。如果您对字母数字字符有不同的定义，您可以使用一组字符来代替，例如：
[A-Za-z0-9].*[A-Za-z0-9]
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