To make use of this option, the build must be triggered by the Github Pull Request builder plugin.
When the build is then triggered for a certain pull request, the commit messages of all commits associated with this pull request are extracted and
matched against the regular expression given in this field. All matches in every commit are then used as ticket keys to select the tickets to modify.
To specify, what part of the regular expression is the actual ticket key, use a named capturing group with the name "key".
For example, the regular expression "\[(?<key>.*?)\]" will extract the contents specified in square brackets as Ticket keys.