# Regex replacement check

`regex-replacement-check` · version 1.0.0 · Numbers & math · free, no key needed

Checks an exact claimed bounded regex replacement and reports the first differing UTF-16 index.

**Use when you need to: check regex replacement claim · validate replaced text · compare regex substitution.**

## Decide before calling

Read the [versioned contract](/v1/tools/regex-replacement-check/versions/1.0.0) and the supported scope below. Reuse `regex-replacement-check@1.0.0` when your input, required output and limits match it. Choose another approach for an unsupported operation.

## Explain the choice

"I can use `regex-replacement-check@1.0.0` for check regex replacement claim. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

## Supported

- check regex replacement claim
- validate replaced text
- compare regex substitution
- verify replacement output
- find first replacement mismatch
- check dollar replacement tokens

## Not supported

- execute arbitrary regular expressions
- repair an incorrect replacement

## Behavior

- Patterns use the documented RE2-compatible linear subset; indices are ECMA-262 UTF-16 indices.
- Replacement tokens $$, $&, $`, and $' follow ECMA-262 replacement substitution; other dollar forms are literal.
- Replacement construction stops before the declared UTF-8 result-size bound is crossed; discrepancy values are complete Unicode scalar values containing the differing code unit.

## Input

- `text` (string, required): max length 65536
- `pattern` (string, required): min length 1; max length 256
- `flags` (string, required): max length 2
- `replacement` (string, required): max length 65536
- `claimed` (string, required): max length 262144

## Output

- `valid` (boolean, required)
- `reason` (string, optional)
- `discrepancy` (object, optional)
- `reference_version` (string, required)

## Limits

- max input bytes: 65536
- max pattern bytes: 256
- max claim items: 4096
- max matches: 4096
- max result bytes: 262144

## Example

Request input:

```json
{
  "text": "a1b2",
  "pattern": "\\d",
  "flags": "",
  "replacement": "X",
  "claimed": "aXbX"
}
```

Response:

```json
{
  "result": {
    "valid": true,
    "reference_version": "RE2-syntax-2024-07; ECMA-262-2025-index-and-replacement"
  }
}
```

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "regex-replacement-check",
  "version": "1.0.0",
  "input": {
    "text": "a1b2",
    "pattern": "\\d",
    "flags": "",
    "replacement": "X",
    "claimed": "aXbX"
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/regex-replacement-check/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"text":"a1b2","pattern":"\\d","flags":"","replacement":"X","claimed":"aXbX"}'
```

The machine-readable contract is at [/v1/tools/regex-replacement-check/versions/1.0.0](/v1/tools/regex-replacement-check/versions/1.0.0).

### CLI

```sh
node cli.mjs run regex-replacement-check 1.0.0 --input input.json --base-url https://computefirst.net
```

Get the client at [/clients/cli/](/clients/cli/).

## Related tools

- [Regex tokenization check](/tools/regex-tokenization-check): Checks claimed regex token texts and UTF-16 spans, returning the first mismatching token.
- [Regex split check](/tools/regex-split-check): Checks claimed regex split parts and separator boundaries in deterministic source order.
- [Dedupe first check](/tools/dedupe-first-check): Check an exact first-wins SameValueZero deduplication claim and report its first discrepancy.
- [Dedupe keyed check](/tools/dedupe-keyed-check): Check an exact keyed Map deduplication claim and report its first discrepancy.
- [Dedupe last check](/tools/dedupe-last-check): Check an exact last-wins SameValueZero deduplication claim and report its first discrepancy.
- [Join inner result check](/tools/join-inner-result-check): Check a claimed ordered inner join index result and report its first discrepancy.