How to restrict specific characters in a text box?

I’m trying to restrict single and double quotations within the text box of a form ( ” or ‘). I’ve researched and seen JS to only allow alphanumeric and so on, but nothing to restrict a specific character exactly.

I’m fine with it not allowing it to be typed or to use an alert pop-up upon validation. Both work for me.

See code Below:

function validateForm() {
  let x = document.forms["commentform"]["commentbox"].value;
  if (x == "") {
    alert("form must be filled out");
    return false;
  }
}

<div align="center">
  <h2>Comment Expiration Demand</h2>
  <form name="commentform" action="/action_page.php" onsubmit="return validateForm()" method="post">
    <input type="text" size="50" maxlength="40" name="commentbox" id="commentbox">
    <br> 40 characters max length, no single or double quotations ( " or ' )
    <br>
    <br>
    <input type="submit" value="submit" id="submit">
  </form>
</div>

Answer

First you are directly comparing the value of the input field. Instead you want to search for a string in that value.

Also, "" is an empty string. You need to use "'" for ' and '"' for ".

function validateForm() {
    let x = document.forms["commentform"]["commentbox"].value;
    if (x.indexOf("'") > -1 || x.indexOf('"') > -1) {
        alert("form must be filled out");
        return false;
    }
}

<body>
    <div align="center">
        <h2>Comment Expiration Demand</h2>
        <form name="commentform" action="/action_page.php" onsubmit="return validateForm()" method="post">
            <input type="text" size="50" maxlength="40" name="commentbox" id="commentbox">
            <br>
            40 characters max length, no single or double quotations ( " or ' )
            <br>
            <br>
            <input type="submit" value="submit" id="submit"> 
        </form>
    </div>
</body>

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Answer