Anchors - Regular expressions

3ZvZ...ckJh

20 Apr 2024

Anchors: string start ^ and end $

The caret ^ and dollar $ characters have special meaning in a regexp. They are called “anchors”.
The caret ^ matches at the beginning of the text, and the dollar $ – at the end.

For instance, let’s test if the text starts with Mary:

let str1 = "Mary had a little lamb";
alert( /^Mary/.test(str1) ); // true

The pattern ^Mary means: “string start and then Mary”.

Similar to this, we can test if the string ends with snow using snow$:

let str1 = "its fleece was white as snow";
alert( /snow$/.test(str1) ); // true

In these particular cases we could use string methods startsWith/endsWith instead. Regular expressions should be used for more complex tests.

Testing for a full match

Both anchors together ^...$ are often used to test whether or not a string fully matches the pattern. For instance, to check if the user input is in the right format.
Let’s check whether or not a string is a time in 12:34 format. That is: two digits, then a colon, and then another two digits.

In regular expressions language that’s \d\d:\d\d:

let goodInput = "12:34";
let badInput = "12:345";

let regexp = /^\d\d:\d\d$/;
alert( regexp.test(goodInput) ); // true
alert( regexp.test(badInput) ); // false

Here the match for \d\d:\d\d must start exactly after the beginning of the text ^, and the end $ must immediately follow.
The whole string must be exactly in this format. If there’s any deviation or an extra character, the result is false.
Anchors behave differently if flag m is present. We’ll see that in the next article.

Anchors have “zero width”

Anchors ^ and $ are tests. They have zero width.
In other words, they do not match a character, but rather force the regexp engine to check the condition (text start/end).