This error is due to usage of < or > symbols. Do not use < or > symbols. Use &lt; and &gt; instead.

Example:

rex field=_raw "errorCode: (?<error_code>d+)"

The above usage in the query will throw an error due to usage of < and > symbols. See the following resolution to resolve this issue.

rex field=_raw "errorCode: (?<error_code>d+)"