"Rename file" fails silently #2

[stefanct]

The "Rename file" feature does not work for me (at least in some instances) in 4.3.1 and current 5.0 master (65a1a03). When that menu entry is clicked the file widget flashes shortly but the context menu remains as if no click happened, and more importantly the file is not renamed but an uncaught exception is thrown:

04:55:15.960 [JavaFX Application Thread] ERROR org.jabref.FallbackExceptionHandler - Uncaught exception occurred in Thread[JavaFX Application Thread,5,main]
java.lang.StringIndexOutOfBoundsException: String index out of range: -4
	at java.lang.String.substring(String.java:1967) ~[?:1.8.0_191]
	at org.jabref.logic.formatter.bibtexfields.RegexFormatter.<init>(RegexFormatter.java:39) ~[JabRef-5.0-dev--snapshot--2018-11-30--master--65a1a03b9.jar:?]
	at org.jabref.logic.formatter.Formatters.getFormatterForModifier(Formatters.java:94) ~[JabRef-5.0-dev--snapshot--2018-11-30--master--65a1a03b9.jar:?]
	at org.jabref.logic.bibtexkeypattern.BracketedPattern.applyModifiers(BracketedPattern.java:389) ~[JabRef-5.0-dev--snapshot--2018-11-30--master--65a1a03b9.jar:?]
	at org.jabref.logic.bibtexkeypattern.BracketedPattern.expandBrackets(BracketedPattern.java:129) ~[JabRef-5.0-dev--snapshot--2018-11-30--master--65a1a03b9.jar:?]
	at org.jabref.logic.util.io.FileUtil.createFileNameFromPattern(FileUtil.java:264) ~[JabRef-5.0-dev--snapshot--2018-11-30--master--65a1a03b9.jar:?]
	at org.jabref.logic.externalfiles.LinkedFileHandler.getSuggestedFileName(LinkedFileHandler.java:113) ~[JabRef-5.0-dev--snapshot--2018-11-30--master--65a1a03b9.jar:?]
	at org.jabref.logic.externalfiles.LinkedFileHandler.findExistingFile(LinkedFileHandler.java:128) ~[JabRef-5.0-dev--snapshot--2018-11-30--master--65a1a03b9.jar:?]
	at org.jabref.gui.fieldeditors.LinkedFileViewModel.performRenameWithConflictCheck(LinkedFileViewModel.java:210) ~[JabRef-5.0-dev--snapshot--2018-11-30--master--65a1a03b9.jar:?]
	at org.jabref.gui.fieldeditors.LinkedFileViewModel.rename(LinkedFileViewModel.java:203) ~[JabRef-5.0-dev--snapshot--2018-11-30--master--65a1a03b9.jar:?]
	at org.jabref.gui.fieldeditors.LinkedFilesEditor.lambda$createContextMenuForFile$393(LinkedFilesEditor.java:222) ~[JabRef-5.0-dev--snapshot--2018-11-30--master--65a1a03b9.jar:?]
[…]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_191]

The entry used for testing is the following:

@TechReport{Nie2017,
  author      = {Sen Nie and Ling Liu and Yuefeng Du and Wenkai Zhang},
  title       = {Over-the-Air: How we Remotely Compromised the Gateway, {BCM}, and Autopilot {ECU}s of Tesla Cars},
  institution = {Keen Security Lab of Tencent},
  year        = {2017},
  note        = {Blackhat 2018},
  file        = {:./us-18-Liu-Over-The-Air-How-We-Remotely-Compromised-The-Gateway-Bcm-And-Autopilot-Ecus-Of-Tesla-Cars-wp.pdf:PDF},
  url         = {http://i.blackhat.com/us-18/Thu-August-9/us-18-Liu-Over-The-Air-How-We-Remotely-Compromised-The-Gateway-Bcm-And-Autopilot-Ecus-Of-Tesla-Cars-wp.pdf},
}

No matter what I put into Preferences/Import/"File format pattern" in the settings (I tried various things like [year]-[auth]-[title:regex: :_], \year-\author-\format[Replace("\s,_")]{title} and year-author-title obviously not really knowing what I am doing because GUI and documentation issues (to put it mildly)) all with the same result.
This bug might also be related to #3920 and/or #3990 but probably not.

[bdcaf]

Might be a long shot - but I had issues with too long file names. See: #3582

[tobiasdiez]

The syntax for regular expressions in the pattern is the following [year]-[journal:regex("Organization","JabRef")]. This replaces Organization with JabRef.

It would be nice if you could improve the documentation in this regard by heading to https://github.com/JabRef/help.jabref.org/edit/gh-pages/en/BibtexKeyPatterns.md. Thanks!