# Line boundary in JavaScript regexes

There are special characters that indicate the beginning `'^'` of or the end `'$'` of a string. Let's see how they work with examples.

## Example

In this example, the search pattern is: replace `'aaa'` with `'!'` only if it is at the beginning of the string:

`let str = 'aaa aaa aaa'; let res = str.replace(/^aaa/g, '!');`

As a result, the following will be written to the variable:

`'! aaa aaa'`

## Example

In this example, the search pattern is: replace `'aaa'` with `'!'` only if it is at the end of the string:

`let str = 'aaa aaa aaa'; let res = str.replace(/aaa$/g, '!');`

As a result, the following will be written to the variable:

`'aaa aaa !'`

## Example

When `'^'` is at the beginning of the regex, and `'$'` is at the end, then in this way we check the entire string for compliance with the regex.

In the following example, the search pattern is: letter `'a'` is repeated one or more times, replace the whole string with `'!'` if it consists of only letters `'a'`.

`let str = 'aaa'; let res = str.replace(/^a+$/g, '!');`

As a result, the following will be written to the variable:

`'!'`

## Practical tasks

Given a string:

`let str = 'abc def xyz';`

Write a regex that will find the first substring of letters.

Given a string:

`let str = 'abc def xyz';`

Write a regex that will find the last substring of letters.

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