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HTML 

              
                <label for="phone-numer">Phone Number</label>
<div class="phone-number-input">
  <input id="phone-number" type="text" placeholder="(555) 555-5555">
  <label class="valid" for="phone-number">
    <svg width="28px" height="28px" viewBox="0 0 28 28" version="1.1">
      <g stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
        <g transform="translate(-890.000000, -623.000000)" fill="#FFFFFF" opacity="0.9">
          <g transform="translate(524.000000, 468.000000)">
            <g transform="translate(366.000000, 155.000000)">
              <path d="M9.97374888,20.6610254 L11.2477328,21.9350092 L22.4350092,10.7477328 L20.3071035,8.61982707 L11.2477328,17.6791979 L7.62790569,14.0593708 L5.5,16.1872765 L9.97374888,20.6610254 Z M13.8863076,27.3315173 C21.3118918,27.3315173 27.3315173,21.3118918 27.3315173,13.8863076 C27.3315173,6.46072329 21.3118918,0.441097862 13.8863076,0.441097862 C6.46072329,0.441097862 0.441097862,6.46072329 0.441097862,13.8863076 C0.441097862,21.3118918 6.46072329,27.3315173 13.8863076,27.3315173 Z"></path>
            </g>
          </g>
        </g>
      </g>
    </svg>
  </label>
  <label class="invalid" for="phone-number">
    <svg width="28px" height="28px" viewBox="0 0 28 28" version="1.1">
      <g stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
        <g transform="translate(-930.000000, -623.000000)" fill="#FFFFFF" opacity="0.9">
          <g transform="translate(524.000000, 468.000000)">
            <g transform="translate(406.000000, 155.000000)">
              <path d="M16.0142133,13.8863076 L20.5438986,9.35662218 L18.415993,7.22871648 L13.8863076,11.7584019 L9.35662218,7.22871648 L7.22871648,9.35662218 L11.7584019,13.8863076 L7.22871648,18.415993 L9.35662218,20.5438986 L13.8863076,16.0142133 L18.415993,20.5438986 L20.5438986,18.415993 L16.0142133,13.8863076 Z M13.8863076,27.3315173 C21.3118918,27.3315173 27.3315173,21.3118918 27.3315173,13.8863076 C27.3315173,6.46072329 21.3118918,0.441097862 13.8863076,0.441097862 C6.46072329,0.441097862 0.441097862,6.46072329 0.441097862,13.8863076 C0.441097862,21.3118918 6.46072329,27.3315173 13.8863076,27.3315173 Z"></path>
            </g>
          </g>
        </g>
      </g>
    </svg>
  </label>
</div>
!

CSS 

              
                input:not([data-validation="valid"]) ~ label.valid,
input:not([data-validation="invalid"]) ~ label.invalid {
  display: none;
}

// NON-ARTICLE STYLES

* {
  box-sizing: border-box;
}

html, body {
  height: 100%;
  font-size: 16px;
}

body {
  display: flex;
  align-items: center;
  justify-content: center;
  flex-direction: column;
  
  font-family: 'Open Sans', sans-serif;
  line-height: 1.5rem;
  
  background: linear-gradient(to bottom, #0CC8A1, #383194);
}

label {
  margin-bottom: 0.25rem;
  
  color: transparentize(white, 0.1);
  font-size: 1rem;
  line-height: 1.5rem;
  font-weight: 600;
}

input {
  width: 100%;
  display: block;
  
  font-size: 1.65rem;
  line-height: 2rem;
  
  border-radius: 0.25rem;
  outline: none;
  padding: 0.375rem;
  
  background-color: transparentize(white, 0.9);
  border: 2px solid transparentize(white, 0.5);
  color: transparentize(white, 0.1);
  font-weight: 300;
  text-align: center;
  
  &::placeholder {
    color: transparentize(white, 0.7);
  }
  
  &:focus::placeholder {
    color: transparent;
  }
}

.phone-number-input {
  width: 22rem;
  max-width: 100%;
  position: relative;
  
  // Add a little extra padding to the bottom of the input to vertically center it.
  padding-bottom: 1.5rem;
  
  .valid, .invalid {
    position: absolute;
    top: calc(0.625rem + 2px);
    right: calc(0.625rem + 2px);
    border-radius: 100%;
    width: 1.75rem;
    height: 1.75rem;
  }
}

.valid {
  color: green;
}

.invalid {
  color: red;
}
!

JS 

              
                $("input").on("input blur", function(event) {
  var value = $(this).val();
  
  if (isPhoneNumber(value)) {
    $(this).attr({ "data-validation": "valid" });
    return;
  }
  
  if (event.type === "blur" && value !== "") {
    $(this).attr({ "data-validation": "invalid" });
  }
  else {
    $(this).removeAttr("data-validation");
  }
});

function isPhoneNumber(string) {
  return /^1?([2-9]\d\d){2}\d{4}$/.test(string.replace(/\D/g, ""));
}
!
999px

Console