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                <!DOCTYPE html>
    <meta charset="utf-8">
    <title>E-mail format validation</title>
function checkEmailFormat() {
    var e_mail = theForm.e_mail.value;
    var regexFilter = /^([a-zA-Z0-9+_\-\.])+@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,6})$/;
    // The first and last backslash tell Javascript that it is a string with characters
    // that are otherwise 'illegal' in a string.
    // The string is also wrapped in ^....$. That means: test the whole to-test string
    // (the e_mail in this case) as one, and include possible spaces as well.
    // The other backslashes mean: escape the next character (take its value literally),
    // because in a regex that character has other values (meanings), too.
    // The plus sign means: multiple instances allowed, not 'plus what comes next'.
    // The {2,6} means: can be from 2 to 6 characters (a new domain is e.g. .museum).
    if ((regexFilter.test(e_mail)) == false) {
        alert('Not a valid e-mail format.');
        return false;
    else {
        alert('Valid format.'); // delete in production
        return true;
    <form action="whatever.php" id="theForm">
        <input type="text" name="e_mail" style="width:300px;">
        <input type="button" value="Check e-mail format" onclick="checkEmailFormat()">

    <p style="width:500px;">The validation criteria used in this script allow letters, numbers, +, -, _, and . in the pre-@ part. In the post-@ part they allow letters, numbers, -, and .. The allowed maximum number of characters in the very last part is 6. The criteria can be widened.</p>




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