Bootstrap 4 doesn't include icons (glyphicons are gone), and there are now just 2 validation states (is-valid and is-invalid) that control display of the valid-feedback and invalid-feedback text.
With a little extra CSS, you can position an icon inside the input (to the right), and control its' display using is-valid or is-invalid on the form-control input. Use a font lib like fontawesome for the icons. I created a new feedback-icon class that you can add to the valid/invalid-feedback.
.valid-feedback.feedback-icon,
.invalid-feedback.feedback-icon {
position: absolute;
width: auto;
bottom: 10px;
right: 10px;
margin-top: 0;
}
HTML
<div class="form-group position-relative">
<label for="input2">Valid with icon</label>
<input type="text" class="form-control is-valid" id="input2">
<div class="valid-feedback feedback-icon">
<i class="fa fa-check"></i>
</div>
<div class="invalid-feedback feedback-icon">
<i class="fa fa-times"></i>
</div>
</div>
Demo of input validation icons
Demo with working validation
.valid-feedback.feedback-icon,
.invalid-feedback.feedback-icon {
position: absolute;
width: auto;
bottom: 10px;
right: 10px;
margin-top: 0;
}
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container">
<div class="form-group position-relative">
<label for="input2">Valid with icon</label>
<input type="text" class="form-control is-valid" id="input2">
<div class="valid-feedback feedback-icon">
<i class="fa fa-check"></i>
</div>
<div class="invalid-feedback feedback-icon">
<i class="fa fa-times"></i>
</div>
</div>
</div>
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