Email validation- characters length before @ and before dot

问题

I use the following regex pattern for validating the email address that works fine, BUT I need to validate the length of characters before @, which should NOT be less than 4 characters. The same rule I should put for the length of characters after @ and before dot ..

For example, this email address is NOT valid: a@b.c

However, this one should be valid: abcd@abcd.com

How can I do it?

Here is my current attempt:

<ui:define name="validation-tag">
    <f:validateRegex 
        pattern="([\w\.-]*[a-zA-Z0-9_]@[\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z])*"
        for="contactEmailAddress" />
</ui:define>

回答1:

We can impose length restrictions using positive look-aheads with anchors.

^(?=[^@]{4,}@)([\w\.-]*[a-zA-Z0-9_]@(?=.{4,}\.[^.]*$)[\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z])$

The ^ and $ will make the string match at start and end, then (?=[^@]{4,}@) will make sure we have at least 4 characters before the first @, and (?=.{4,}\.[^.]*$) will make sure the part before the last . is at least 4 symbols long.

See demo

回答2:

You can use {4,} after a sequence of valid characters to specify that you want at least for of them. So try the following pattern:

(([\w\.-]*[a-zA-Z0-9_]){4,}@([\w\.-]*[a-zA-Z0-9]){4,}\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z])*

回答3:

Be careful with too complicated regexes for email. The user can still make a mistake anyways (like tom@mycompny.com instead of tom@mycompany.com) and you risk to have false negatives.

Maybe something like /.*\@(.{2,}\.)+.{2,}/ (test it here) is good enough.

来源：https://stackoverflow.com/questions/30257463/email-validation-characters-length-before-and-before-dot