thinkphp Automatic validation of how it is implemented

How does the thinkphp auto-verification be implemented?
Today, when researching yourphp system code, I found that there is a sentence on the registration page:

HTML Code

Through this sentence, can achieve thinkphp automatic verification, such as less than 5 characters, he reminded, how is this implemented?

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Let's look at his development manual.
and analyze his code.
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Through the JS verification bar, look at the current page code lookup. JS look
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Validation in model
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Find out if this is a foreground or background verification.

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