PHP does not verify the message of the Big God

PHP does not use regular verification email What's the big God
Condition 1: The first character must be English or numeric or underlined with which string function?
Condition 2: Must contain an @ STRING function Yes?
Condition 3: Contains more than one. Character
Conditional 4:@ characters before the.
Condition 5:[email Protected]
Condition 6: Length cannot be longer than 30 this knows with Mb_strlen
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References:

would like to know, such an application scenario, what is more appropriate than the regular?

Character-by-word analysis is simple, and regular expressions seem concise but inefficient

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