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# Fix pattern parse errors when running rules

When using a targeted language's reserved words in rules, you may see the following error:

```console theme={null}
[ERROR] Pattern parse error in rule
```

## Background

Each programming language has a list of reserved words that cannot be used as identifiers, such as the names of variables or functions. If you write a rule that results in the following error when run, you are triggering a reserved word conflict:

```console theme={null}
[ERROR] Pattern parse error in rule ruleName:
 Invalid pattern for JavaScript:
--- pattern ---
delete
--- end pattern ---
Pattern error: Stdlib.Parsing.Parse_error
```

## Resolution

Using a reserved word in your rule leads to parsing errors, so if you see this error, determine if the words cited in the error are reserved words. If they are, you can replace your `metavariable-pattern` with `metavariable-regex`.

This substitution works because `metavariable-pattern` tries to match the pattern within the captured metavariable, which is going to be affected by how reserved keywords are parsed, while `metavariable-regex` runs a regex on the text range associated with the metavariable, ignoring how its content would be parsed and bypassing the issue.

### Example

The following rule would elicit the "\[ERROR] Pattern parse error in rule" response:

```yaml theme={null}
patterns:
- pattern-inside: app.$FUNC(...)
- pattern-not-regex: .(middleware.csrf.validate).
- metavariable-pattern:
       metavariable: $FUNC
patterns:
- pattern-either:
- pattern: post=
- pattern: put
- pattern: delete
- pattern: patch
```

To fix the error, replace

```yaml theme={null}
- metavariable-pattern:
       metavariable: $FUNC
```

with

```yaml theme={null}
- metavariable-regex:
    metavariable: $FUNC
    regex: ^(post|put|delete|patch)$
```