52 lines
1.9 KiB
Markdown
52 lines
1.9 KiB
Markdown
# Anchors: string start ^ and end $
|
|
|
|
The caret `pattern:^` and dollar `pattern:$` characters have special meaning in a regexp. They are called "anchors".
|
|
|
|
The caret `pattern:^` matches at the beginning of the text, and the dollar `pattern:$` -- at the end.
|
|
|
|
For instance, let's test if the text starts with `Mary`:
|
|
|
|
```js run
|
|
let str1 = "Mary had a little lamb";
|
|
alert( /^Mary/.test(str1) ); // true
|
|
```
|
|
|
|
The pattern `pattern:^Mary` means: "string start and then Mary".
|
|
|
|
Similar to this, we can test if the string ends with `snow` using `pattern:snow$`:
|
|
|
|
```js run
|
|
let str1 = "it's fleece was white as snow";
|
|
alert( /snow$/.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 `pattern:^...$` are often used to test whether or not a string fully matches the pattern. For instance, to check if the user input is in the right format.
|
|
|
|
Let's check whether or not a string is a time in `12:34` format. That is: two digits, then a colon, and then another two digits.
|
|
|
|
In regular expressions language that's `pattern:\d\d:\d\d`:
|
|
|
|
```js run
|
|
let goodInput = "12:34";
|
|
let badInput = "12:345";
|
|
|
|
let regexp = /^\d\d:\d\d$/;
|
|
alert( regexp.test(goodInput) ); // true
|
|
alert( regexp.test(badInput) ); // false
|
|
```
|
|
|
|
Here the match for `pattern:\d\d:\d\d` must start exactly after the beginning of the text `pattern:^`, and the end `pattern:$` must immediately follow.
|
|
|
|
The whole string must be exactly in this format. If there's any deviation or an extra character, the result is `false`.
|
|
|
|
Anchors behave differently if flag `pattern:m` is present. We'll see that in the next article.
|
|
|
|
```smart header="Anchors have \"zero width\""
|
|
Anchors `pattern:^` and `pattern:$` 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).
|
|
```
|