# `RegularExpression`

A pattern for matching text: `[A-Z]{2}[0-9]{2}`.

Use it when a person sets a pattern while your app runs. A type's own `pattern:` is checked when you compile, and suits a fixed rule better.

## Shape

One value, held as text in one set form, with no parts: `\p{Lu}\w*`.

## Example

```jadpo
RegularExpression("[A-Z]{2}[0-9]{2}")
```

`RegularExpression(raw)` checks the text, and text that isn't one breaks the `format` rule.

`text.parse(raw, as: RegularExpression)` reads one from text that comes from outside. Text that isn't one at all fails `malformed`, such as `(a+)\1`, `(?<name>a)\k<name>` or `foo(?=bar)`.

## Good to know

- Backreferences such as `\1` are refused, which keeps every pattern quick to run.
- `==` tells you whether two are the same. `<` and `list.sort_by` don't take it.

## In JSON

A string, in a request, a response or `json.write`:

```json
"[A-Z]{2}[0-9]{2}"
"(?i)grey|gray"
"\\p{Lu}\\w*"
```

## Details

<details>
<summary>Text form, storage, how it is shown, and operations</summary>

`RegularExpression` is in the group Software, and has been in Jadpo since 0.1.0.

### Text form

`text.from` writes one standard text form, the same form `text.parse` reads back: `[A-Z]{2}[0-9]{2}`, `(?i)grey\|gray` and `\p{Lu}\w*`.

### How it is stored

It is stored in one column named for the field, of type `text`. `(?i)grey\|gray` is stored as `'(?i)grey\|gray'`.

### How it is shown to people

`text.format(value, locale:)` shows it to people in their locale.

With `locale: Locale.en_gb`:

| Value | Shown |
|---|---|
| `[A-Z]{2}[0-9]{2}` | `[A-Z]{2}[0-9]{2}` |
| `(?i)grey\|gray` | `(?i)grey\|gray` |
| `\p{Lu}\w*` | `\p{Lu}\w*` |

### Operations

It also takes `text.parse`, `text.from`, `text.format`, `json.read`, `json.write` and `==`. [How every standard type behaves](/docs/every-standard-type-meets-one-contract.md) says how they work.

</details>