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HTML 

              
                <html>

<head>
  <title>Simple JavaScript Template</title>
</head>

<body>
  <header>
    <h1>@learnMern</h1>
    <h3>Simple JavaScript Template</h3>
    <p>Everything below this line is created with JavaScript</p>
  </header>

  <div class="container">

    <div class="container container__main">
      <div class="container container__header">
        <i class="fas fa-phone fa-3x"></i>
        <h1>Validator</h1>
      </div>

      <input id="userPhoneNum" type="text" placeholder="Enter phone number">
      <button class="btn btn__primary" onclick="submit()">Validate</button>
      <button class="btn btn__secondary" onclick="clearSubmit()">Clear</button>

      <div id="content" class="content">
        <h3>Phone Number Validation Return</h3>
        <p id="content__phoneNumber"></p>
      </div>
    </div>

  </div>

</body>

</html>
!

CSS 

              
                :root {
  --primary-light: #8abdff;
  --primary: #6d5dfc;
  --primary-dark: #5b0eeb;
  --white: #ffffff;
  --greyLight-1: #e4ebf5;
  --greyLight-2: #c8d0e7;
  --greyLight-3: #bec8e4;
  --greyDark: #9baacf;
}

$shadow: 0.3rem 0.3rem 0.6rem var(--greyLight-2),
  -0.2rem -0.2rem 0.5rem var(--white);
$inner-shadow: inset 0.2rem 0.2rem 0.5rem var(--greyLight-2),
  inset -0.2rem -0.2rem 0.5rem var(--white);

html {
  box-sizing: border-box;

  overflow-y: scroll;
  background: var(--greyLight-1);
  color: var(--primary);
  @media screen and (min-width: 900px) {
    font-size: 75%;
  }
}

h3 {
  color: rgba(109, 93, 252, 0.89);
}

p {
  color: rgba(109, 93, 252, 0.6);
}

header {
  border-bottom: 1px solid rgba(255, 255, 255, 0.6);
}

.container {
  width: 100%;

  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  align-items: center;
  font-family: "Poppins", sans-serif;
  background: var(--greyLight-1);
  &__main {
    width: 50%;
    min-height: 200px;
    border-radius: 1rem;
    margin: 4rem;
    padding: 2rem;
    box-shadow: 0.8rem 0.8rem 1.4rem var(--greyLight-2),
      -0.2rem -0.2rem 1.8rem var(--white);
  }
  &__header {
    width: 100%;
    justify-content: space-between;
    padding: 2rem;
    box-shadow: $shadow;
    border-radius: 1rem;
    color: rgba(109, 93, 252, 0.89);
  }
}

.content {
  display: none;
  flex-direction: column;
  border-radius: 1rem;
  box-shadow: 0.8rem 0.8rem 1.4rem var(--greyLight-2),
    -0.2rem -0.2rem 1.8rem var(--white);
  padding: 2rem;
  width: 100%;
  margin: 2rem 0;
}

span {
  color: #ff073a;
  font-weight: bold;
}

input {
  width: 100%;
  margin: 2rem 0 1rem 0;
  height: 3rem;
  border: none;
  border-radius: 1rem;
  font-size: 1rem;
  padding-left: 1rem;
  box-shadow: $inner-shadow;
  background: none;
  font-family: inherit;
  color: var(--greyDark);

  &::placeholder {
    color: var(--greyLight-3);
  }
  &:focus {
    outline: none;
    box-shadow: $shadow;

    + .search__icon {
      color: var(--primary);
    }
  }
}

.btn {
  width: 100%;
  height: 3rem;
  margin: 2rem 0 1rem 0;
  font-family: inherit;
  font-size: 1rem;
  border: 0;
  padding: 0;
  border-radius: 1rem;
  box-shadow: $shadow;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  transition: 0.3s ease;

  &__primary {
    background: var(--primary);
    box-shadow: inset 0.2rem 0.2rem 1rem var(--primary-light),
      inset -0.2rem -0.2rem 1rem var(--primary-dark), $shadow;
    color: var(--greyLight-1);
    &:hover {
      color: var(--white);
    }
    &:active {
      box-shadow: inset 0.2rem 0.2rem 1rem var(--primary-dark),
        inset -0.2rem -0.2rem 1rem var(--primary-light);
    }
  }
  &__secondary {
    color: var(--greyDark);
    &:hover {
      color: var(--primary);
    }
    &:active {
      box-shadow: $inner-shadow;
    }
  }
}

@media only screen and (min-width: 768px) {
  input {
    width: 30%;
  }
  .btn {
    width: 20%;
  }

  .container__main {
    justify-content: space-between;
  }

  .btn__primary {
    position: relative;
    left: 10%;
  }
}
!

JS 

              
                // DOM variables
const userPhoneNum = document.getElementById("userPhoneNum");
const content = document.getElementById("content");
const content__phoneNumber = document.getElementById("content__phoneNumber");

// regex variable to match valid 10 digit phone numbers 555-555-5555, (555)555-5555, (555) 555-5555, 555 555 5555, 5555555555, 1 555 555 5555
const regex = /^1?\s?(\(\d{3}\)|\d{3})(\s|-)?\d{3}(\s|-)?\d{4}$/gm;

// logs an explanation of the regex
const regexSummary = (regex) => {
  console.log(`regex: ${regex} \n
    **** ^1? **** matches strings that start with 1 and 1 is optional.
      ^ anchor matches the beginning of the string.
      ? quantifier is for optional\n
    **** \\s? **** followed by a space and space being optional 
      \\s character class matches whitespace; spaces, tabs, and line breaks.\n
    **** (\\(\\d{3}\\) **** followed by open parentheses and three digits followed by closed parentheses. \n
      \\d matches any digit 0-9
      {3} matches three digits \n
    **** | **** or \n
      | acts as a boolean Or and matches the expression befor or after the | \n
    **** \\d{3})(\\s|-)? **** followed by three digits followed by an optional space or hyphen. \n    
    **** | **** or \n
    **** \\d{3}(\\s|-)? **** followed by three digits and an optional space or hyphen. \n
    **** \\d{4}$ **** followed by four digits found at the end of the string. \n
    $ matches the end of the string.

    `);
};

// validates a phone number
const validatePhone = (string) => {
  regexSummary(regex);

  if (string.match(regex)) {
    console.log(true);
    return true;
  } else {
    console.log(false);
    return false;
  }
};

// handler functions
const submit = () => {
  let phoneNumber = userPhoneNum.value;

  if (validatePhone(phoneNumber)) {
    content.style.display = "flex";
    content__phoneNumber.innerHTML = `The phone number <span id="phoneNumber__span">${phoneNumber}</span> is valid!!!`;
    document.getElementById("phoneNumber__span").style.color = "#49fb35";
    userPhoneNum.value = "";
  } else {
    content.style.display = "flex";
    content__phoneNumber.innerHTML = `The phone number <span id="phoneNumber__span">${phoneNumber}</span> is NOT valid!!!, please enter a valid 10 digit number`;
    document.getElementById("phoneNumber__span").style.color = "#ff073a";
    userPhoneNum.value = "";
  }
};

const clearSubmit = () => {
  content.style.display = "none";
  content__phoneNumber.innerHTML = "";
  console.clear();
};
!
999px

Console