# Adding a Validation Pattern

When creating a text field in Umbraco Forms, a validation pattern in the form of a regular expression can be applied. Default patterns can be removed or re-ordered, and custom ones created and added.

## Provided patterns

Umbraco Forms ships with three patterns: number, email, and URL. The class names are `Number`, `Email`, and `Url` respectively, and all are found in the `Umbraco.Forms.Core.Providers.ValidationPatterns` namespace.

## Creating a custom validation pattern

To create a custom format function, create a class that implements `IValidationPattern`. You will need to initialize five properties:

* `Alias` - an alias that should be unique across the patterns and is typically camel-cased with no spaces.
* `Name` - the name of the pattern that will be visible in the backoffice.
* `LabelKey` - as an alternative to providing a name, a translation key can be provided. This will be used to look-up the name in the correct language for the backoffice user.
* `Pattern` - the regular expression pattern.
* `ReadOnly` - a flag indicating whether the pattern can be edited in the backoffice.

The following example shows the implementation of a pattern for a United Kingdom postcode (credit for the [pattern](https://stackoverflow.com/a/69806181/489433) to [Mecanik](https://stackoverflow.com/users/6583298/mecanik) at StackOverflow).

```csharp
using Umbraco.Forms.Core.Interfaces;
namespace Umbraco.Forms.TestSite.Business.ValidationPatterns
{
    public class UkPostCode : IValidationPattern
    {
        public string Alias => "ukPostCode";
        public string Name => "UK Post Code";
        public string LabelKey => string.Empty;
        public string Pattern => @"^([a-zA-Z]{1,2}[a-zA-Z\d]{1,2})\s(\d[a-zA-Z]{2})$";
        public bool ReadOnly => true;
    }
}
```

## Registering the validation pattern

As with other provider types, the validation pattern needs to be registered. There are options to add, remove, and re-order patterns.

An example registration using the `IUmbracoBuilder` is shown below:

```csharp
public static IUmbracoBuilder AddCustomProviders(this IUmbracoBuilder builder)
{
    builder.FormsValidationPatterns()
        .Append<UkPostCode>();
    return builder;
}
```

## Using the pattern

With the pattern registered it will be available for selection by editors in the backoffice when they create validation for fields supporting this feature.

![Validation pattern](https://1470284034-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FHN4dErU7ghf8hOdcQpSs%2Fuploads%2Fgit-blob-e90ec8642dc47b19782794ef5e20c5ceb1f57c0b%2Fvalidation-pattern.png?alt=media)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.umbraco.com/umbraco-forms/13.latest/developer/extending/adding-a-validation-pattern.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.