<apex:pageBlock id="myblock">
       Email  <apex:inputText value="{!email}" id="email"/><br/><br/>
        <apex:commandButton value="Click me!"  action="{!checkEmail}"/>
 </apex:pageBlock>

public void checkEmail()
    {
        if(!Pattern.matches('[a-zA-Z0-9._-]+@[a-zA-Z]+.[a-zA-Z]{2,4}', email))
        {
            ApexPages.addmessage(new ApexPages.message(ApexPages.severity.Error, 'Check your email'));
        }
    }