Filter option in custom format. Default value is empty or any.
Available formats:

• any pull requests applied for this project: any, * or empty string
• filtered by destination branch: my-branch or more complex reg-ex filter r:^master (must be started with r: and case insensitive match).
• filtered by source and destination branches: s:source-branch d:dest-branch
• filtered by source and destination branches with regex: s:r:^feature d:r:master$
• filtered by many destination/source branches: s:one s:two s:three d:master d:r:master$
• filtered by many sources branches: s:one s:two s:r:^three d:

When you using format with source branch filter s or destination filter d, you must specify great than one source and destination filter, eg s:1 s:2 s:... d:.
Any sources and any destinations for pull request:

• filter string: *
• filter string: s: d: