> ## Documentation Index
> Fetch the complete documentation index at: https://docs.neuraltrust.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Regex

The Regex Evaluator is a specialized tool designed to validate text responses against regular expression patterns. It provides a binary evaluation where the response must match the specified regex pattern to pass.

## Purpose

The Regex Evaluator is particularly useful when you need to:

* Validate text formats and patterns
* Check for specific text structures
* Verify data formats (emails, phone numbers, etc.)
* Ensure responses follow a particular pattern
* Test for specific text content requirements

## How It Works

The evaluator uses a binary scoring system based on regex pattern matching:

* **Score: 1**: The response matches the specified regex pattern
* **Score: 0**: The response does not match the specified regex pattern

The evaluation uses Python's `re.search()` function to check if the response contains any substring that matches the provided regex pattern.

## Usage Example

```python theme={null}
import asyncio

from trusttest.evaluation_contexts import ExpectedResponseContext
from trusttest.evaluators import RegexEvaluator


async def evaluate():
    # Create evaluator with an email pattern
    evaluator = RegexEvaluator(
        pattern=r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
    )
    result = await evaluator.evaluate(
        response="user@example.com",
        context=ExpectedResponseContext()
    )
    print(result)

if __name__ == "__main__":
    asyncio.run(evaluate())
```

The evaluator returns a tuple containing:

* A binary score (0 or 1) indicating pattern match status
* A list of explanations including:
  * Success message with the pattern if matched
  * Failure message with the pattern if not matched

## When to Use

Use the Regex Evaluator when you need to:

* Validate email addresses
* Check phone number formats
* Verify date formats
* Ensure specific text patterns
* Test URL formats
* Validate structured data formats
* Check for specific text content
* Verify code snippets or commands