Edit in JSFiddle

function doUpdate() {
    intendedLength = $(".counter_string_length").val() * 1;
    mySep = $(".separator").val();
    if (mySep.length == 0) { mySep = '_'};
    repeatedStr = $(".repeatedStr").val();
    repeatedPrefix = $(".repeatedPrefix").val();
     
    myPrefix = $(".prefix").val();
    mySuffix = $(".suffix").val();
    myPrefix = myPrefix.replace(/\s/g, mySep);
    result = "";
    counter = intendedLength;
    if (mySuffix.length > 0) {
        result = mySuffix;
        counter = intendedLength - result.length - repeatedStr.length - repeatedPrefix;
    }

    while (result.length < intendedLength) {
        if (counter <= 1 + repeatedStr.length + repeatedPrefix.length) {
            result = mySep.repeat(counter) + result;
        } else {
            result = repeatedPrefix + counter + repeatedStr +  mySep + result;
        }
        counter = intendedLength - result.length;
    }

    // only use prefix if it can fit
    var logMsg = ""
    var lengthOfLastNumber = ('' + intendedLength).length
    if ((myPrefix.length > 1) && (myPrefix.length < (intendedLength - lengthOfLastNumber - 1))) {
        var regex = new RegExp("^([^" + mySep + "]*)");
        remainingResult = result.substring(myPrefix.length);
        matchPart = regex.exec(remainingResult);
        matchPart = matchPart[0].replace(/./g, mySep);
        result = myPrefix + matchPart + remainingResult.substring(matchPart.length);
    }

    $('#result').val(result);
    updateLengthCount();
}

function updateLengthCount() {
    var len = $('#result').val();
    $('#actualLength').text(len.length);
}

$(function () {

    $(".input input").on("change keyup click", function () {
        doUpdate();
    });

    $("#result").on("click", function () {
        $(this).select(); // make it easier to copy the result by selecting it when they click inside.
    });

    $('#result').on("change keypress keyup paste", function () {
        setTimeout(function () {
            updateLengthCount();
        }, 100);
    });

    $('.counter_string_length').bind("keydown", function (e) {
        var input = $('.counter_string_length');

        var addend = 0;
        var multiplier = 1;
        let currentVal = parseInt(input.val(), 10);
        if (e.keyCode === 38) /*up key*/ {
            if (e.shiftKey) {
                input.val(currentVal + 1);
            } else {
                input.val(currentVal * 2);
            }
        } else if (e.keyCode === 40) /*down key*/ {
            if (e.shiftKey) {
                input.val(currentVal - 1);
            } else {
                input.val(Math.round(currentVal / 2));
            }
        }
    });

    $('[name=setupEmail]').on("click", setupEmail);
    $('[name=setupSimple]').on("click", setupSimple);

    doUpdate();
});

function setupEmail() {
    $(".counter_string_length").val('255').focus();
    $(".separator").val("_");
    $(".repeatedStr").val("");
    $(".prefix").val("Test against RFC email max by exceeding 254 characters");
    $(".suffix").val("test@example.com");
    // see https://stackoverflow.com/questions/386294/what-is-the-maximum-length-of-a-valid-email-address
    doUpdate();
}

function setupSimple() {
    $(".counter_string_length").val('256').focus();
    $(".separator").val(".");
    $(".repeatedStr").val("");
    $(".prefix").val("");
    $(".suffix").val("");
    // see https://stackoverflow.com/questions/386294/what-is-the-maximum-length-of-a-valid-email-address
    doUpdate();
}

<b> This generates self-referential counter strings.  Anywhere in the string, the separator character is at the position indicated by the number to its left. </b>

<br/>This is useful for testing string length limits in form fields for all kinds of applications.
<br/>
<br/>
<div class="defaults">Default setups:
    <input type="button" name="setupEmail" value="Email 254+" />
    <input type="button" name="setupSimple" value="Simple 256" />
</div>
<div class="input">
    <label for="counter_string">Length</label>
    <input type='text' class='counter_string_length' name='counter_string_length' value="256" /> 
    <span class="hint">Use up/down &amp; shift-up/down to change.</span>
</div>
<div class="input">
    <label for="prefix">Prefix</label>
    <input type='text' class='prefix' name="prefix" value="Self-referential counting string" />
</div>
<div class="input">
    <label for="separator">Separator</label>
    <input type='text' class='separator' name="separator" value="." maxlength="1" />
</div>
<div class="input">
    <label for="repeatedPrefix">Before each number</label>
    <input type='text' class='repeatedPrefix' name="repeatedPrefix" value="" />
</div>
<div class="input">
    <label for="repeatedStr">After each number</label>
    <input type='text' class='repeatedStr' name="repeatedStr" value="->" />
</div>
<div class="input">
    <label for="suffix">Suffix</label>
    <input type='text' class='suffix' name="suffix" value="" />
</div>
<br/>Result to copy to clipboard (or paste your own text to measure its length):
<br/>
<textarea id='result' style="width: 90%" rows="10">(please change the text boxes above to see a result)</textarea>
<div>Actual length: <span id="actualLength">?</span> (paste text above to measure its length)</div>

label {
    width: 12em;
}
.input {
  padding: 3px;
}
.input input {
    width: 200px;
    left: 12em;
    position: absolute;
}
.input .hint {
    left: calc(12em + (3px * 4) + 200px);
    position: absolute;
    color: gray;
    font-size: small;
}
.defaults {
    float: right;
    clear: none;
    margin-right: 10em;
}
.defaults input {
    display: block;
}