Limit group separator to one single character in Prefs->Entry Keyword Separator

CHANGELOG.md

@@ -13,6 +13,8 @@ Note that this project **does not** adhere to [Semantic Versioning](https://semv
 
 ### Changed
 
+- We changed the setting of the keyword separator to accept a single character only. [#177](https://github.com/koppor/jabref/issues/177)
+
 ### Fixed
 
 ### Removed

src/main/java/org/jabref/gui/preferences/entry/EntryTab.java

@@ -1,9 +1,12 @@
 package org.jabref.gui.preferences.entry;
 
+import java.util.function.UnaryOperator;
+
 import javafx.fxml.FXML;
 import javafx.scene.control.Button;
 import javafx.scene.control.CheckBox;
 import javafx.scene.control.TextField;
+import javafx.scene.control.TextFormatter;
 
 import org.jabref.gui.actions.ActionFactory;
 import org.jabref.gui.actions.StandardActions;
@@ -48,6 +51,17 @@ public void initialize() {
 
         keywordSeparator.textProperty().bindBidirectional(viewModel.keywordSeparatorProperty());
 
+        // Use TextFormatter to limit the length of the Input of keywordSeparator to 1 character only.
+        UnaryOperator<TextFormatter.Change> singleCharacterFilter = change -> {
+            if (change.getControlNewText().length() <= 1) {
+                return change;
+            }
+            return null; // null means the change is rejected
+        };
+        TextFormatter<String> formatter = new TextFormatter<>(singleCharacterFilter);
+
+        keywordSeparator.setTextFormatter(formatter);
+
         resolveStrings.selectedProperty().bindBidirectional(viewModel.resolveStringsProperty());
         resolveStringsForFields.textProperty().bindBidirectional(viewModel.resolveStringsForFieldsProperty());
         nonWrappableFields.textProperty().bindBidirectional(viewModel.nonWrappableFieldsProperty());