diff --git a/core/org.eclipse.cdt.ui.tests/ui/org/eclipse/cdt/ui/tests/text/contentassist/CompletionProposalComputerPreferenceParserTest.java b/core/org.eclipse.cdt.ui.tests/ui/org/eclipse/cdt/ui/tests/text/contentassist/CompletionProposalComputerPreferenceParserTest.java
new file mode 100644
index 00000000000..7afe5feff54
--- /dev/null
+++ b/core/org.eclipse.cdt.ui.tests/ui/org/eclipse/cdt/ui/tests/text/contentassist/CompletionProposalComputerPreferenceParserTest.java
@@ -0,0 +1,81 @@
+/*******************************************************************************
+ * Copyright (c) 2020 Kichwa Coders Canada Inc. and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ */
+package org.eclipse.cdt.ui.tests.text.contentassist;
+
+import static java.util.Arrays.asList;
+import static org.eclipse.cdt.internal.ui.text.contentassist.CompletionProposalComputerPreferenceParser.parseCategoryOrder;
+import static org.eclipse.cdt.internal.ui.text.contentassist.CompletionProposalComputerPreferenceParser.parseExcludedCategories;
+import static org.junit.Assert.assertEquals;
+
+import java.text.ParseException;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+
+import org.junit.Test;
+
+public class CompletionProposalComputerPreferenceParserTest {
+
+	@Test
+	public void testParseExcludedCategories() throws ParseException {
+		assertEquals(asSet(), parseExcludedCategories("\0"));
+		assertEquals(asSet(), parseExcludedCategories("\0\0"));
+		assertEquals(asSet("cat1"), parseExcludedCategories("cat1\0"));
+		assertEquals(asSet("cat1", "cat2"), parseExcludedCategories("cat1\0cat2"));
+		assertEquals(asSet("cat1"), parseExcludedCategories("cat1${0x0}"));
+		assertEquals(asSet("cat1", "cat2"), parseExcludedCategories("cat1${0x0}cat2"));
+		assertEquals(asSet("cat1"), parseExcludedCategories("cat1$$$${0x0}"));
+		assertEquals(asSet("cat1", "cat2"), parseExcludedCategories("cat1$$$${0x0}cat2"));
+		assertEquals(asSet("cat1", "cat2", "cat3", "cat4"),
+				parseExcludedCategories("cat1$$$${0x0}cat2${0x0}cat3\0cat4\0"));
+	}
+
+	@Test
+	public void testParseCategoryOrder() throws ParseException {
+		assertEquals(asMap("cat1", 1), parseCategoryOrder("cat1:1\0"));
+		assertEquals(asMap("cat1", 1000), parseCategoryOrder("cat1:1000\0\0"));
+		assertEquals(asMap("cat1", 1000, "cat2", 2000), parseCategoryOrder("cat1:1000\0cat2:2000"));
+		assertEquals(asMap("cat1", 1), parseCategoryOrder("cat1:1${0x0}"));
+		assertEquals(asMap("cat1", 1000, "cat2", 2000), parseCategoryOrder("cat1:1000${0x0}cat2:2000"));
+		assertEquals(asMap("cat1", 1), parseCategoryOrder("cat1:1$$$$${0x0}"));
+		assertEquals(asMap("cat1", 1000, "cat2", 2000), parseCategoryOrder("cat1:1000$$$$${0x0}cat2:2000"));
+		assertEquals(asMap("cat1", 1000, "cat2", 2000, "cat3", 3000, "cat4", 4000),
+				parseCategoryOrder("cat1:1000$$$$${0x0}cat2:2000${0x0}cat3:3000\0cat4:4000\0"));
+	}
+
+	@Test(expected = ParseException.class)
+	public void testParseIntFailsGracefully1() throws ParseException {
+		assertEquals(asMap(), parseCategoryOrder("cat1:this is not a number\0"));
+	}
+
+	@Test(expected = ParseException.class)
+	public void testParseIntFailsGracefully2() throws ParseException {
+		assertEquals(asMap(), parseCategoryOrder("cat1 missing number\0"));
+	}
+
+	@Test(expected = ParseException.class)
+	public void testParseIntFailsGracefully3() throws ParseException {
+		assertEquals(asMap(), parseCategoryOrder("cat1:0:extra field\0"));
+	}
+
+	private Set<String> asSet(String... elem) {
+		return new HashSet<>(asList(elem));
+	}
+
+	private Map<String, Integer> asMap(Object... elem) {
+		HashMap<String, Integer> map = new HashMap<>();
+		for (int i = 0; i < elem.length; i += 2) {
+			map.put((String) elem[i + 0], (Integer) elem[i + 1]);
+		}
+		return map;
+	}
+}
diff --git a/core/org.eclipse.cdt.ui.tests/ui/org/eclipse/cdt/ui/tests/text/contentassist/ContentAssistTestSuite.java b/core/org.eclipse.cdt.ui.tests/ui/org/eclipse/cdt/ui/tests/text/contentassist/ContentAssistTestSuite.java
index c95b6f6fb69..290d67c52a4 100644
--- a/core/org.eclipse.cdt.ui.tests/ui/org/eclipse/cdt/ui/tests/text/contentassist/ContentAssistTestSuite.java
+++ b/core/org.eclipse.cdt.ui.tests/ui/org/eclipse/cdt/ui/tests/text/contentassist/ContentAssistTestSuite.java
@@ -22,6 +22,8 @@ import org.junit.runners.Suite;
 
 		ContentAssistTests.class,
 
+		CompletionProposalComputerPreferenceParserTest.class,
+
 })
 public class ContentAssistTestSuite {
 }
diff --git a/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/preferences/CodeAssistAdvancedConfigurationBlock.java b/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/preferences/CodeAssistAdvancedConfigurationBlock.java
index 86927a8c57f..2f286342a2f 100644
--- a/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/preferences/CodeAssistAdvancedConfigurationBlock.java
+++ b/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/preferences/CodeAssistAdvancedConfigurationBlock.java
@@ -14,6 +14,7 @@
  *******************************************************************************/
 package org.eclipse.cdt.internal.ui.preferences;
 
+import java.text.ParseException;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.Comparator;
@@ -21,10 +22,12 @@ import java.util.HashMap;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 
 import org.eclipse.cdt.internal.ui.dialogs.IStatusChangeListener;
 import org.eclipse.cdt.internal.ui.dialogs.StatusInfo;
 import org.eclipse.cdt.internal.ui.text.contentassist.CompletionProposalCategory;
+import org.eclipse.cdt.internal.ui.text.contentassist.CompletionProposalComputerPreferenceParser;
 import org.eclipse.cdt.internal.ui.text.contentassist.CompletionProposalComputerRegistry;
 import org.eclipse.cdt.internal.ui.util.Messages;
 import org.eclipse.cdt.internal.ui.util.SWTUtil;
@@ -230,22 +233,25 @@ final class CodeAssistAdvancedConfigurationBlock extends OptionsConfigurationBlo
 		}
 
 		private boolean readInclusionPreference(CompletionProposalCategory cat) {
-			String[] ids = getTokens(getValue(PREF_EXCLUDED_CATEGORIES), SEPARATOR);
-			for (String id : ids) {
-				if (id.equals(cat.getId()))
-					return false;
+			String value = getValue(PREF_EXCLUDED_CATEGORIES);
+			try {
+				Set<String> parseExcludedCategories = CompletionProposalComputerPreferenceParser
+						.parseExcludedCategories(value);
+				return !parseExcludedCategories.contains(cat.getId());
+			} catch (ParseException e) {
+				return true;
 			}
-			return true;
 		}
 
 		private int readOrderPreference(CompletionProposalCategory cat) {
-			String[] sortOrderIds = getTokens(getValue(PREF_CATEGORY_ORDER), SEPARATOR);
-			for (String sortOrderId : sortOrderIds) {
-				String[] idAndRank = getTokens(sortOrderId, COLON);
-				if (idAndRank[0].equals(cat.getId()))
-					return Integer.parseInt(idAndRank[1]);
+			String categoryOrderPref = getValue(PREF_CATEGORY_ORDER);
+			try {
+				Map<String, Integer> parseCategoryOrder = CompletionProposalComputerPreferenceParser
+						.parseCategoryOrder(categoryOrderPref);
+				return parseCategoryOrder.getOrDefault(cat.getId(), LIMIT + 1);
+			} catch (ParseException e) {
+				return LIMIT + 1;
 			}
-			return LIMIT + 1;
 		}
 
 		public void update() {
diff --git a/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/text/contentassist/CompletionProposalComputerPreferenceParser.java b/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/text/contentassist/CompletionProposalComputerPreferenceParser.java
new file mode 100644
index 00000000000..428baf98a6c
--- /dev/null
+++ b/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/text/contentassist/CompletionProposalComputerPreferenceParser.java
@@ -0,0 +1,78 @@
+/*******************************************************************************
+ * Copyright (c) 2020 Kichwa Coders Canada Inc. and others.
+ *
+ * This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License 2.0
+ * which accompanies this distribution, and is available at
+ * https://www.eclipse.org/legal/epl-2.0/
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ */
+package org.eclipse.cdt.internal.ui.text.contentassist;
+
+import java.text.ParseException;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+
+import org.eclipse.cdt.ui.PreferenceConstants;
+
+/**
+ * Parses the Completion Proposal Computer Preferences.
+ * <p>
+ * See org.eclipse.cdt.internal.ui.preferences.CodeAssistAdvancedConfigurationBlock.PreferenceModel
+ * for the write side of the preferences.
+ */
+public class CompletionProposalComputerPreferenceParser {
+	/**
+	 * Parses the {@link PreferenceConstants#CODEASSIST_EXCLUDED_CATEGORIES} value and
+	 * converts to a set of categories that are excluded.
+	 * @param preferenceValue as stored in {@link PreferenceConstants#CODEASSIST_EXCLUDED_CATEGORIES}
+	 * @return set of excluded categories
+	 * @throws ParseException if the value cannot be parsed
+	 */
+	public static Set<String> parseExcludedCategories(String preferenceValue) throws ParseException {
+		Set<String> disabled = new HashSet<>();
+		String[] disabledArray = splitOnNulls(preferenceValue);
+		disabled.addAll(Arrays.asList(disabledArray));
+		return disabled;
+	}
+
+	/**
+	 * Parses the {@link PreferenceConstants#CODEASSIST_CATEGORY_ORDER} value and
+	 * converts to a map of category ids to sort rank number
+	 * @param preferenceValue as stored in {@link PreferenceConstants#CODEASSIST_CATEGORY_ORDER}
+	 * @return map of category id to rank order
+	 * @throws ParseException if the value cannot be parsed
+	 */
+	public static Map<String, Integer> parseCategoryOrder(String preferenceValue) throws ParseException {
+		Map<String, Integer> ordered = new HashMap<>();
+		String[] orderedArray = splitOnNulls(preferenceValue);
+		for (String entry : orderedArray) {
+			String[] idRank = entry.split(":"); //$NON-NLS-1$
+			if (idRank.length != 2) {
+				throw new ParseException(entry, 0);
+			}
+			String id = idRank[0];
+			int rank;
+			try {
+				rank = Integer.parseInt(idRank[1]);
+			} catch (NumberFormatException e) {
+				throw new ParseException(entry, 0);
+			}
+			ordered.put(id, Integer.valueOf(rank));
+		}
+		return ordered;
+	}
+
+	/**
+	 * See Bug 558809. Oomph seems to have failed at times to encode/decode nul character '\0'
+	 * from the format it is stored in Oomph setup files. We can have ${0x0} instead of \0, infact
+	 * there can be multiple $, so $$$$${0x0} is a valid split too.
+	 */
+	private static String[] splitOnNulls(String preference) {
+		return preference.split("\\000|(\\$+\\{0x0\\})"); //$NON-NLS-1$
+	}
+}
diff --git a/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/text/contentassist/CompletionProposalComputerRegistry.java b/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/text/contentassist/CompletionProposalComputerRegistry.java
index 12c66d662e1..33843a56842 100644
--- a/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/text/contentassist/CompletionProposalComputerRegistry.java
+++ b/core/org.eclipse.cdt.ui/src/org/eclipse/cdt/internal/ui/text/contentassist/CompletionProposalComputerRegistry.java
@@ -14,6 +14,7 @@
  *******************************************************************************/
 package org.eclipse.cdt.internal.ui.text.contentassist;
 
+import java.text.ParseException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -24,7 +25,6 @@ import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
-import java.util.StringTokenizer;
 
 import org.eclipse.cdt.internal.ui.util.Messages;
 import org.eclipse.cdt.ui.CUIPlugin;
@@ -272,19 +272,19 @@ public final class CompletionProposalComputerRegistry {
 
 	private List<CompletionProposalCategory> getCategories(List<IConfigurationElement> elements) {
 		IPreferenceStore store = CUIPlugin.getDefault().getPreferenceStore();
-		String preference = store.getString(PreferenceConstants.CODEASSIST_EXCLUDED_CATEGORIES);
-		Set<String> disabled = new HashSet<>();
-		StringTokenizer tok = new StringTokenizer(preference, "\0"); //$NON-NLS-1$
-		while (tok.hasMoreTokens())
-			disabled.add(tok.nextToken());
-		Map<String, Integer> ordered = new HashMap<>();
-		preference = store.getString(PreferenceConstants.CODEASSIST_CATEGORY_ORDER);
-		tok = new StringTokenizer(preference, "\0"); //$NON-NLS-1$
-		while (tok.hasMoreTokens()) {
-			StringTokenizer inner = new StringTokenizer(tok.nextToken(), ":"); //$NON-NLS-1$
-			String id = inner.nextToken();
-			int rank = Integer.parseInt(inner.nextToken());
-			ordered.put(id, Integer.valueOf(rank));
+		Set<String> disabled = Collections.emptySet();
+		Map<String, Integer> ordered = Collections.emptyMap();
+		boolean parseFailed = false;
+		try {
+			disabled = CompletionProposalComputerPreferenceParser
+					.parseExcludedCategories(store.getString(PreferenceConstants.CODEASSIST_EXCLUDED_CATEGORIES));
+			ordered = CompletionProposalComputerPreferenceParser
+					.parseCategoryOrder(store.getString(PreferenceConstants.CODEASSIST_CATEGORY_ORDER));
+		} catch (ParseException e) {
+			// Failed to parse user setting, clear all settings
+			// and display error message to user allowing them to
+			// reset on first use
+			parseFailed = true;
 		}
 
 		List<CompletionProposalCategory> categories = new ArrayList<>();
@@ -296,13 +296,20 @@ public final class CompletionProposalComputerRegistry {
 
 					CompletionProposalCategory category = new CompletionProposalCategory(element, this);
 					categories.add(category);
-					category.setIncluded(!disabled.contains(category.getId()));
-					Integer rank = ordered.get(category.getId());
-					if (rank != null) {
-						int r = rank.intValue();
-						boolean separate = r < 0xffff;
-						category.setSeparateCommand(separate);
-						category.setSortOrder(r);
+					if (parseFailed) {
+						// When parse has failed we do the same thing as if the user had disabled
+						// off ever proposal category. This causes a pop-up on first completion
+						// attempt with the option of resetting defaults
+						category.setIncluded(false);
+					} else {
+						category.setIncluded(!disabled.contains(category.getId()));
+						Integer rank = ordered.get(category.getId());
+						if (rank != null) {
+							int r = rank.intValue();
+							boolean separate = r < 0xffff;
+							category.setSeparateCommand(separate);
+							category.setSortOrder(r);
+						}
 					}
 				}
 			} catch (CoreException x) {