In this blog, you're going to learn how to concatenate regex literals in JavaScript. We'll explore two methods to achieve this and understand why it's important for dynamic pattern matching.

Approach 1: Using the RegExp Constructor

Concatenating regex literals can be done by creating a new RegExp object and passing concatenated strings as its argument. This method allows for dynamic pattern building.

<script>
  const regexPart1 = /[a-z]/;
  const regexPart2 = /[0-9]/;
  const concatenatedRegex = new RegExp(regexPart1.source + regexPart2.source);
</script>

Explanation

  1. Declare two regex literals as separate variables.
  2. Use the source property to access the pattern string of each regex.
  3. Concatenate the pattern strings using the + operator.
  4. Create a new RegExp object with the concatenated pattern.

Example

<form id="passwordForm">
  <label for="password">Password:</label>
  <input type="password" id="password" name="password">
  <button type="submit">Submit</button>
</form>
<script>
  document.getElementById('passwordForm').addEventListener('submit', function(event) {
  const password = document.getElementById('password').value;
  const regexPart1 = /[a-z]/;
  const regexPart2 = /[0-9]/;
  const concatenatedRegex = new RegExp(regexPart1.source + regexPart2.source);
  
  if (!concatenatedRegex.test(password)) {
    alert('Password must contain at least one lowercase letter and one digit.');
    event.preventDefault();
  }
});
</script>

Approach 2: Using String Concatenation

Another approach is to concatenate regex literals directly within a string using string concatenation or template literals.

<script>
 const concatenatedRegex = /[a-z]/.source + /[0-9]/.source;
</script>

Explanation

  1. Concatenate regex literals directly within a string using the source property.
  2. Use the + operator for string concatenation.