Allow negation (!, NOT operator) in \\begin{...} conditionals

CHANGELOG.md

@@ -74,6 +74,7 @@ We refer to [GitHub issues](https://github.com/JabRef/jabref/issues) by using `#
 - We added a context menu to the bib(la)tex-source-editor to copy'n'paste. [#5007](https://github.com/JabRef/jabref/pull/5007)
 - We added a bibliographic references search, for finding references in several LaTeX files. This tool scans directories and shows which entries are used, how many times and where.
 - We added an option in preferences to allow for integers in field "edition" when running database in bibtex mode. [#4680](https://github.com/JabRef/jabref/issues/4680)
+- We added the ability to use negation in export filter layouts. [#5138](https://github.com/JabRef/jabref/pull/5138)
 
 
 ### Fixed

src/main/java/org/jabref/logic/layout/LayoutEntry.java

@@ -251,15 +251,17 @@ private String handleOptionField(BibEntry bibtex, BibDatabase database) {
 
     private String handleFieldOrGroupStart(BibEntry bibtex, BibDatabase database) {
         Optional<String> field;
+        boolean negated = false;
         if (type == LayoutHelper.IS_GROUP_START) {
             field = bibtex.getResolvedFieldOrAlias(FieldFactory.parseField(text), database);
         } else if (text.matches(".*(;|(\\&+)).*")) {
             // split the strings along &, && or ; for AND formatter
             String[] parts = text.split("\\s*(;|(\\&+))\\s*");
             field = Optional.empty();
             for (String part : parts) {
-                field = bibtex.getResolvedFieldOrAlias(FieldFactory.parseField(part), database);
-                if (!field.isPresent()) {
+                negated = part.startsWith("!");
+                field = bibtex.getResolvedFieldOrAlias(FieldFactory.parseField(negated ? part.substring(1).trim() : part), database);
+                if (!(field.isPresent() ^ negated)) {
                     break;
                 }
             }
@@ -268,14 +270,15 @@ private String handleFieldOrGroupStart(BibEntry bibtex, BibDatabase database) {
             String[] parts = text.split("\\s*(\\|+)\\s*");
             field = Optional.empty();
             for (String part : parts) {
-                field = bibtex.getResolvedFieldOrAlias(FieldFactory.parseField(part), database);
-                if (field.isPresent()) {
+                negated = part.startsWith("!");
+                field = bibtex.getResolvedFieldOrAlias(FieldFactory.parseField(negated ? part.substring(1).trim() : part), database);
+                if (field.isPresent() ^ negated) {
                     break;
                 }
             }
         }
 
-        if ((!field.isPresent()) || ((type == LayoutHelper.IS_GROUP_START)
+        if ((!(field.isPresent() ^ negated)) || ((type == LayoutHelper.IS_GROUP_START)
                 && field.get().equalsIgnoreCase(LayoutHelper.getCurrentGroup()))) {
             return null;
         } else {

src/test/java/org/jabref/logic/layout/LayoutTest.java

@@ -100,6 +100,35 @@ void HTMLCharsWithDotlessIAndTiled() throws IOException {
                 layoutText);
     }
 
+    @Test
+    public void beginConditionals() throws IOException {
+        BibEntry entry = new BibEntry(StandardEntryType.Misc)
+                .withField(StandardField.AUTHOR, "Author");
+
+        // || (OR)
+        String layoutText = layout("\\begin{editor||author}\\format[HTMLChars]{\\author}\\end{editor||author}", entry);
+
+        assertEquals("Author", layoutText);
+
+        // && (AND)
+        layoutText = layout("\\begin{editor&&author}\\format[HTMLChars]{\\author}\\end{editor&&author}", entry);
+
+        assertEquals("", layoutText);
+
+        // ! (NOT)
+        layoutText = layout("\\begin{!year}\\format[HTMLChars]{(no year)}\\end{!year}", entry);
+
+        assertEquals("(no year)", layoutText);
+
+        // combined (!a&&b)
+        layoutText = layout(
+                "\\begin{!editor&&author}\\format[HTMLChars]{\\author}\\end{!editor&&author}" +
+                "\\begin{editor&&!author}\\format[HTMLChars]{\\editor} (eds.)\\end{editor&&!author}", entry);
+
+        assertEquals("Author", layoutText);
+
+    }
+
     /**
      * Test for http://discourse.jabref.org/t/the-wrapfilelinks-formatter/172 (the example in the help files)
      */