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

  server/apps/workflowengine/lib/Check/AbstractStringCheck.php

  Line 91 in 2a154f5

  throw new \UnexpectedValueException($this->l->t('The given regular expression is invalid'), 2);

• The defined regex patterns at

  server/apps/workflowengine/src/components/Checks/FileMimeType.vue

  Line 72 in 2a154f5

  pattern: 'httpd/unix-directory',

  and

  server/apps/workflowengine/src/components/Checks/FileMimeType.vue

  Line 87 in 2a154f5

  pattern: 'application/pdf',

  does not seem to be corret. These are passed to the serverside function mentioned above.

Currently there is also an open issue at R0Wi-DEV/workflow_ocr#32 where multiple users reported this problem.