JS RegExp Assertions

JavaScript RegExp Assertions and Boundaries

Assertions in regular expressions do not match characters themselves. Instead, they assert that a specific condition is true at a particular position in the string (like checking if we are at the beginning of a word, or the end of a line).

1. String Boundaries (^ and $)

These two assertions anchor your search pattern to the exact beginning or end of the string.

let text = "The cat is in the hat";

// Does the string START with "The"?
console.log(/^The/.test(text)); // true
// Does the string END with "hat"?
console.log(/hat$/.test(text)); // true
// Does the string END with "cat"?
console.log(/cat$/.test(text)); // false

Form Validation: Developers constantly use ^ and $ together to validate exact inputs. For example, /^\d{5}$/ ensures the user's input is exactly 5 digits from start to finish, with no extra characters before or after.

2. Word Boundaries (\b)

The \b assertion matches a word boundary—the exact position where a word character (\w) meets a non-word character (\W), or the start/end of the string.

It is incredibly useful when you want to search for a whole word, and not a fragment of another word!

let text = "I am looking for a plan and a plane. Not a planet.";

// We want to replace the word "plan" with "IDEA".
// BAD: This replaces "plan" inside "plane" and "planet" too!
console.log(text.replace(/plan/g, "IDEA")); 
// GOOD: The \b asserts it must be an independent word
console.log(text.replace(/\bplan\b/g, "IDEA")); 

(Note: The uppercase \B matches a non-word boundary, ensuring the pattern is found inside another word).

3. Lookahead Assertions ((?=))

Lookaheads allow you to assert that a pattern is followed by another pattern, without actually including that second pattern in the match result.

let text = "100 USD and 50 EUR and 200 USD";

// Find numbers that are immediately followed by " USD"
let match = text.match(/\d+(?= USD)/g);
// It matched the numbers, but DID NOT include " USD" in the result!
console.log(match); // ["100", "200"]

let text = "100 USD and 50 EUR and 200 USD";

// Find numbers that are NOT followed by " USD"
let match = text.match(/\d+(?! USD)/g);
console.log(match); // ["50"]