Anchors and Boundaries

Understanding Anchors

Anchors are special characters in Regex that match positions in a string rather than actual characters. They are useful for specifying where a match must occur in the string.

Common Anchors

^ : Matches the start of a string
$ : Matches the end of a string

Example: Using Anchors

Let’s look at some examples of how these anchors work:

Caret (^) Anchor

javascript

	const regexStart = /^hello/;
	console.log(regexStart.test('hello world')); // true (matches 'hello' at the start)
	console.log(regexStart.test('world hello')); // false (no match because 'hello' is not at the start)

Dollar Sign ($) Anchor

javascript

	const regexEnd = /world$/;
	console.log(regexEnd.test('hello world')); // true (matches 'world' at the end)
	console.log(regexEnd.test('world hello')); // false (no match because 'world' is not at the end)

Combining Anchors

You can combine ^ and $ to match an exact string:

javascript

	const regexExact = /^hello world$/;
	console.log(regexExact.test('hello world')); // true (matches the entire string)
	console.log(regexExact.test('hello')); // false (no match because the entire string does not match)
	console.log(regexExact.test('world hello')); // false (no match because the entire string does not match)

Word Boundaries

Word boundaries \b are used to match positions where a word starts or ends. They are useful for matching whole words.

\b : Matches a word boundary
\B : Matches a non-word boundary

Example: Using Word Boundaries

Word Boundary (\b)

javascript

	const regexWordBoundary = /\bhello\b/;
	console.log(regexWordBoundary.test('hello world')); // true (matches 'hello' as a whole word)
	console.log(regexWordBoundary.test('say hello!')); // true (matches 'hello' as a whole word)
	console.log(regexWordBoundary.test('helloworld')); // false (no match because 'hello' is not a whole word)

Non-Word Boundary (\B)

javascript

	const regexNonWordBoundary = /\Bworld\B/;
	console.log(regexNonWordBoundary.test('helloworld')); // true (matches 'world' within a word)
	console.log(regexNonWordBoundary.test('hello world')); // false (no match because 'world' is not within a word)

Exercises

Write a regular expression that matches a string starting with the word “JavaScript”.
Create a Regex pattern that matches a string ending with the word “developer”.
Write a regular expression that matches the word “code” as a whole word.

Anchors and boundaries are essential for specifying the position of matches within strings and ensuring that your patterns match exactly what you intend.