====== Regex - Alpha-numeric and underscore ======

===== Letters, underscore, numbers =====

<code>
^\w+$
</code>

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===== Letters, underscore, numbers (2-5 in length) =====

<code>
^\w{2,5}
</code>

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===== Letters, underscore, numbers and Chinese characters =====

<code>
^[\u4E00-\u9FA5A-Za-z0-9_]+$
</code>

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