> ## Documentation index
> Fetch the complete documentation index at: https://arize.com/llms.txt
> Ask it with GET https://arize.com/ask?q=<keywords or question>. Short keywords rank best.
> Use these to discover all available pages before exploring further.

# Regex Match

> Pass when the output matches a regular expression pattern.

Compiles a regex and reports a binary `match` / `mismatch` against the model's output — handy for asserting format invariants like phone numbers, currency strings, UUIDs, or identifier prefixes without paying for a model call.

For a zero-code option, see the pre-built [Regex Match](/docs/phoenix/evaluation/server-evals/pre-built-metrics/regex) metric. Use the version below when you want to extend the logic — capture groups, multiple alternative patterns, richer explanations.

## Code

<Tabs>
<Tab title="Python" icon="python">
```python
import re


def evaluate(output, pattern):
    if output is None:
        return {
            "label": "missing",
            "score": 0.0,
            "explanation": "Output is missing.",
        }

    try:
        compiled = re.compile(pattern)
    except re.error as exc:
        return {
            "label": "invalid_pattern",
            "score": 0.0,
            "explanation": f"Failed to compile regex {pattern!r}: {exc}",
        }

    match = compiled.search(str(output))
    if match is None:
        return {
            "label": "mismatch",
            "score": 0.0,
            "explanation": f"Output does not match /{pattern}/.",
        }
    return {
        "label": "match",
        "score": 1.0,
        "explanation": f"Matched substring: {match.group(0)!r}",
    }
```
</Tab>
<Tab title="TypeScript" icon="js">
```typescript
function evaluate({ output, pattern }: EvaluatorParams) {
  if (output == null) {
    return {
      label: "missing",
      score: 0,
      explanation: "Output is missing.",
    };
  }

  let regex: RegExp;
  try {
    regex = new RegExp(pattern as string);
  } catch (err) {
    return {
      label: "invalid_pattern",
      score: 0,
      explanation: `Failed to compile regex ${JSON.stringify(pattern)}: ${
        (err as Error).message
      }`,
    };
  }

  const match = String(output).match(regex);
  if (match === null) {
    return {
      label: "mismatch",
      score: 0,
      explanation: `Output does not match /${pattern}/.`,
    };
  }
  return {
    label: "match",
    score: 1,
    explanation: `Matched substring: ${JSON.stringify(match[0])}`,
  };
}
```
</Tab>
</Tabs>

Both `re.search` and `String.match` find a match anywhere in the string. If you need the *entire* output to match, switch to `re.fullmatch` in Python, or anchor the pattern with `^...$` in TypeScript.

## Input mapping

| Parameter | Bind to |
|-----------|---------|
| `output` | The model output, usually `output`. |
| `pattern` | Use a **literal** regex string (e.g. `^[A-Z]{3}-\d{4}$`) when every example shares the same pattern, or a dataset path (e.g. `metadata.expected_pattern`) when patterns vary per example. |

## Output configuration

Categorical labels:

| Label | Score |
|-------|-------|
| `match` | `1.0` |
| `mismatch` | `0.0` |
| `missing` | `0.0` |
| `invalid_pattern` | `0.0` |

Optimization direction: **maximize**.

## Runtime requirements

| Setting | Value |
|---------|-------|
| Sandbox | Any — works in the in-process **WebAssembly** (Python) or **Deno** (TypeScript) backends. |
| Dependencies | None — uses `re` / built-in `RegExp`. |
| Internet access | Not required. |
| Environment variables | None. |

<Warning>
Untrusted regex patterns can be vulnerable to catastrophic backtracking (ReDoS). If `pattern` is sourced from per-example dataset values rather than a fixed literal, prefer patterns from a trusted column you control.
</Warning>