# Substitute

| type   | returns  |
| ------ | -------- |
| Method | `string` |

## Arguments

| name        | type                               | description                           |
| ----------- | ---------------------------------- | ------------------------------------- |
| value       | regex \| string                    | The regex to perform replacements on  |
| replacement | string \| `(...matches) -> string` | Replacement for each matches location |

## Explanation

Matches all occurences of `value`, with `replacement`.

### `value`

* If value is a string. It is matched as appears in the string gloablly, it is not converted to a regular expression.
* A regular expression's capture groups are passed to the replacement if applicable.

### String replacements

If the replacement is a string, the following charcater sequences have special behavior:

| name     | explanation                                              | example               |
| -------- | -------------------------------------------------------- | --------------------- |
| `$\$`    | Inserts a literal `$`                                    | `"$\$59.99"`          |
| `$&`     | Refers to the entire matches string                      | `"( $& )"`            |
| `$n`     | Where `n` is a number, refers to the `n`th capture group | `$2, $1`              |
| `` $` `` | Refers to the match the precedes the given match         | `` encountered: $` `` |
| `$'`     | Refers to the match that follows the given match         | `upcoming: $'`        |

### Function replacements

If the replacement is a function, the capture groups are passed in the format `( c1, c2, ..., cn )`. The function should return string, otherwise an error will be thrown

## Examples

```swift
"1 + 1 = 2".sub(/\d+ ([+*/-]) \d/g, "$1") == "+ = 2"
"1 + 1 - 2".sub(/(\d+ )?([+-]) \d+/g, "$'") == " - 2 "
"Hi".sub("i", "ello") == "Hello"
```


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