regex - Email validation- characters length before @ and before dot

Question

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>

Answer 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