> For the complete documentation index, see [llms.txt](https://docs.ngsurvey.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.ngsurvey.com/projects/import-export/xlsform/survey-sheet/constraint.md).

# Constraint

The constraint column is used to validate user input. It prevents the user from continuing until the answer meets a condition you define.

If the answer doesn't satisfy the constraint, a message defined in `constraint_message` is shown to the user.

## 📝 How constraints work

The value of the question is represented by a dot (`.`), which means “the current answer.” You build a logic expression using that value.

Constraints are commonly used to:

* Set numeric ranges (e.g., age must be between 0 and 120)
* Require matching formats (e.g., ZIP codes, emails)
* Enforce dependencies (e.g., end date after start date)

| type    | name  | label         | constraint                         | constraint\_message           |
| ------- | ----- | ------------- | ---------------------------------- | ----------------------------- |
| integer | age   | Your age      | . >= 18 and . <= 99                | Age must be between 18 and 99 |
| text    | zip   | ZIP code      | regex(., '^\d{5}$')                | Enter a 5-digit ZIP code      |
| text    | email | Email address | regex(., '^\[^@]+@\[^@]+.\[^@]+$') | Enter a valid email           |
| date    | end   | End date      | . >= ${start}                      | End date must be after start  |

## ☑️ Using the dot (.) in constraint

The dot `.` represents the value entered by the user for the current question. Use it inside functions and expressions:

* `. >= 0` — valid if value is 0 or higher
* `regex(., '^\d{5}$')` — valid if it matches the pattern

## 🔠 Variable references

You can use `${question_name}` to reference answers from other questions in the form. Make sure referenced questions appear earlier in the form.

Example:

* `. > ${min_age}` — ensures current input is greater than a previously entered value

## 🔢 Operators and functions for constraints

Same as relevance, you can use:

\=, !=, >, <, >=, <= and, or, not()

| Function        | Description                          | Example                                |
| --------------- | ------------------------------------ | -------------------------------------- |
| regex()         | Validates the format of a text input | `regex(., '^\d{5}$')` for US ZIP codes |
| string-length() | Checks the number of characters      | `string-length(.) <= 50`               |
| selected()      | Checks if a choice was selected      | `selected(${choices}, 'option1')`      |
| today()         | Gets the current date                | `. <= today()`                         |
| now()           | Gets current date and time           | `. <= now()`                           |


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