Workflowengine "The given regular expression is invalid"

Steps to reproduce

1. Create a new flow (concrete workflow app doesn't matter)
2. Choose File MIME type
3. Choose matches (or does not match)
4. Choose PDF documents (or Folder)

Note: the filters for Images and Office Documents are working.

Expected behaviour

Workflow can be created

Actual behaviour

If i try to save the workflow an error message is raised "The given regular expression is invalid"

Server configuration

Operating system: Ubuntu 20.04

Web server: Apache2

Database: MySql 8.0

PHP version: 7.4

Nextcloud version: 20.0.1

Additional information

• I think this only applies to Nextcloud 20 or later (because like i saw the filter "matches" is not available in NC 19)
• The error message comes from https://github.com/nextcloud/server/blob/2a154f57ceeae89b15dfe166aff3a7916afde94c/apps/workflowengine/lib/Check/AbstractStringCheck.php#L91
• The defined regex patterns at https://github.com/nextcloud/server/blob/2a154f57ceeae89b15dfe166aff3a7916afde94c/apps/workflowengine/src/components/Checks/FileMimeType.vue#L72 and https://github.com/nextcloud/server/blob/2a154f57ceeae89b15dfe166aff3a7916afde94c/apps/workflowengine/src/components/Checks/FileMimeType.vue#L87 does not seem to be corret. These are passed to the serverside function mentioned above.

Currently there is also an open issue at https://github.com/R0Wi/workflow_ocr/issues/32 where multiple users reported this problem.

Start with the regex definitions in apps/workflowengine/src/components/Checks/FileMimeType.vue, then inspect validation in apps/workflowengine/lib/Check/AbstractStringCheck.php around line 91. Reproduce the File MIME type matches and does-not-match cases for PDF documents and Folder. Done means these filters use valid patterns and the workflow can be saved without the invalid-regex error.