How can I concatenate regex literals in JavaScript?

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.