> For the complete documentation index, see [llms.txt](https://docs.internetobject.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.internetobject.org/schema-definition-language/data-types/string.md).

# String Types

The **`string`** type validates text. It has two **predefined shortcuts** that are `string` with a built-in pattern: [Email](/schema-definition-language/data-types/string/email.md) and [URL](/schema-definition-language/data-types/string/url.md).

> `date`, `time`, and `datetime` are **not** string subtypes — they are their own types with their own values. See [Date and Time](/schema-definition-language/data-types/date-and-time.md).
>
> For how strings are *written* (open, quoted, raw), see [Strings](/structure-and-syntax/values/string.md).

## The string family

| Type     | Is                                          |
| -------- | ------------------------------------------- |
| `string` | any text                                    |
| `email`  | `string` validated against an email pattern |
| `url`    | `string` validated against a URL pattern    |

```ruby
contact: email, site: url
---
~ a@b.com, 'https://example.com'    # ✓
~ notanemail, 'https://example.com' # ✗ invalid-email
```

> **Quote values containing `:` or spaces** (URLs, times-of-day, "Last, First"). An unquoted `https://x.com` is misread because the open string ends at `:`.

## TypeDef

A `string` MemberDef accepts only the options below. Any other key is invalid.

| Option        | Type            | Description                                                     |
| ------------- | --------------- | --------------------------------------------------------------- |
| `type`        | string          | `string`, `email`, or `url`. First positional value.            |
| `default`     | string          | Value used when the member is omitted. Second positional value. |
| `choices`     | array of string | Restricts the value to a fixed set. Third positional value.     |
| `pattern`     | string          | A regular expression the value must match.                      |
| `flags`       | string          | Regex flags for `pattern` (e.g. `i`).                           |
| `len`         | int ≥ 0         | Exact length in characters.                                     |
| `minLen`      | int ≥ 0         | Minimum length.                                                 |
| `maxLen`      | int ≥ 0         | Maximum length.                                                 |
| `format`      | string          | Serialization form: `auto` (default), `regular`, `raw`.         |
| `encloser`    | string          | Quote character used when serializing: `"` (default) or `'`.    |
| `escapeLines` | bool            | Whether to escape line breaks on serialization.                 |
| `optional`    | bool            | If `true`, the member may be omitted. Shorthand: `?` suffix.    |
| `null`        | bool            | If `true`, the member may be `null`. Shorthand: `*` suffix.     |

> **`len` precedence.** When `len` is set, `minLen` and `maxLen` are ignored.

## Constraints

### minLen / maxLen / len

```ruby
name: { string, minLen: 5, maxLen: 20 }
---
~ Ethan              # ✓
~ Alexandra Daddario # ✓
~ Leo                # ✗ invalid-min-length
```

### pattern

A regular expression. Use a [raw string](/structure-and-syntax/values/string/raw-strings.md) (`r'…'`) to avoid escaping backslashes.

```ruby
ssn: { string, pattern: r'^[0-9]{3}-[0-9]{2}-[0-9]{4}$' }
---
~ '123-45-6789'   # ✓
~ '12345678'      # ✗ invalid-pattern
```

### choices

```ruby
dept: { string, choices: [cs, mech, civil] }
---
~ cs     # ✓
~ art    # ✗ invalid-choice
```

> Quote choices that look like numbers or contain commas, e.g. `["19.02, 72.85"]`, so they are treated as strings.

## Optional, nullable & defaults

```ruby
nickname?*: { string, anonymous }   # optional + nullable, default "anonymous"
---
~ {}      # ✓ → "anonymous" (omitted, default applies)
~ N       # ✓ → null
~ John    # ✓ → "John"
```

| Input                   | Result                                                         |
| ----------------------- | -------------------------------------------------------------- |
| valid text              | the string                                                     |
| fails a constraint      | `invalid-*` error (`invalid-min-length`, `invalid-pattern`, …) |
| `N`, nullable (`*`)     | `null`                                                         |
| `N`, not nullable       | `null-not-allowed` error                                       |
| omitted, `default` set  | the default                                                    |
| omitted, optional (`?`) | absent                                                         |
| omitted, required       | `value-required` error                                         |

> **Use the `*` suffix for nullability** — keyed `null:` is not yet honored (see below).

## Implementation status (beta)

* Keyed `optional:` works; keyed `null:` is not yet honored — use the `*` suffix.

## See Also

* [Strings (value syntax)](/structure-and-syntax/values/string.md)
* [Email](/schema-definition-language/data-types/string/email.md) · [URL](/schema-definition-language/data-types/string/url.md)
* [Date and Time](/schema-definition-language/data-types/date-and-time.md)
* [TypeDef](/schema-definition-language/advanced-schema-concepts/typedef.md) · [MemberDef](/schema-definition-language/advanced-schema-concepts/memberdef.md)


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