regex: disallow literal backspace [\b] inside java

RegExpSupport/src/org/intellij/lang/regexp/RegExpLanguageHost.java

@@ -38,6 +38,10 @@ public interface RegExpLanguageHost {
     return true;
   }
 
+  default boolean supportsLiteralBackspace(RegExpChar aChar) {
+    return true;
+  }
+
   boolean isValidCategory(@NotNull String category);
   @NotNull
   String[][] getAllKnownProperties();

RegExpSupport/src/org/intellij/lang/regexp/RegExpLanguageHosts.java

@@ -91,6 +91,11 @@ public final class RegExpLanguageHosts extends ClassExtension<RegExpLanguageHost
     }
   }
 
+  public boolean supportsLiteralBackspace(@Nullable RegExpChar regExpChar) {
+    final RegExpLanguageHost host = findRegExpHost(regExpChar);
+    return host != null && host.supportsLiteralBackspace(regExpChar);
+  }
+
   public boolean supportsNamedGroupSyntax(@Nullable final RegExpGroup group) {
     final RegExpLanguageHost host = findRegExpHost(group);
     return host != null && host.supportsNamedGroupSyntax(group);

RegExpSupport/src/org/intellij/lang/regexp/validation/RegExpAnnotator.java

@@ -133,7 +133,7 @@ public final class RegExpAnnotator extends RegExpElementVisitor implements Annot
   @Override
   public void visitRegExpChar(final RegExpChar ch) {
     final Character value = ch.getValue();
-    if (value == null) {
+    if (value == null || (value == '\b' && !myLanguageHosts.supportsLiteralBackspace(ch))) {
       switch (ch.getType()) {
         case CHAR:
           myHolder.createErrorAnnotation(ch, "Illegal/unsupported escape sequence");

RegExpSupport/testData/RETest.xml

@@ -434,6 +434,10 @@
       <pattern><![CDATA[<weak_warning descr="Redundant character escape">\-</weak_warning>[<weak_warning descr="Redundant character escape">\*</weak_warning>\-\[\]\\<weak_warning descr="Redundant character escape">\+]</weak_warning>]]></pattern>
       <expected>OK</expected>
     </test>
+    <test host="com.intellij.psi.impl.JavaRegExpHost">
+      <pattern>[\b]</pattern>
+      <expected>ERR</expected>
+    </test>
   </charclasses>
 
   <groups>

java/java-impl/src/com/intellij/psi/impl/JavaRegExpHost.java

@@ -101,6 +101,11 @@ public class JavaRegExpHost implements RegExpLanguageHost {
     }
   }
 
+  @Override
+  public boolean supportsLiteralBackspace(RegExpChar aChar) {
+    return false;
+  }
+
   private static boolean hasAtLeastJdkVersion(PsiElement element, JavaSdkVersion version) {
     final JavaSdkVersion version1 = getJavaVersion(element);
     return version1 != null && version1.isAtLeast(version);