Handle $ in IdentifierName

* Escape replacement characters in `Matcher#replaceAll`, since dollar signs in a replacement string are treated as references to captured subsequences.
* Add `$` to the list of accepted characters for the upper and lower underscore heuristics

Fixes #5515

PiperOrigin-RevId: 872826736

Author: cushon

core/src/main/java/com/google/errorprone/bugpatterns/IdentifierName.java

@@ -290,7 +290,7 @@ private static boolean isStaticVariable(Symbol symbol) {
     return symbol instanceof VarSymbol && isStatic(symbol);
   }
 
-  private static final Pattern LOWER_UNDERSCORE_PATTERN = Pattern.compile("[a-z0-9_]+");
-  private static final Pattern UPPER_UNDERSCORE_PATTERN = Pattern.compile("[A-Z0-9_]+");
+  private static final Pattern LOWER_UNDERSCORE_PATTERN = Pattern.compile("[a-z0-9$_]+");
+  private static final Pattern UPPER_UNDERSCORE_PATTERN = Pattern.compile("[A-Z0-9$_]+");
   private static final Splitter UNDERSCORE_SPLITTER = Splitter.on('_');
 }

core/src/main/java/com/google/errorprone/bugpatterns/IdentifierNames.java

@@ -22,6 +22,7 @@
 import static java.lang.Character.isDigit;
 
 import com.google.errorprone.ErrorProneFlags;
+import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import javax.inject.Inject;
 
@@ -68,7 +69,9 @@ String fixInitialismsIfNeeded(String input) {
   }
 
   static String fixInitialisms(String input) {
-    return PROBABLE_INITIALISM.matcher(input).replaceAll(r -> titleCase(r.group(1)) + r.group(2));
+    return PROBABLE_INITIALISM
+        .matcher(input)
+        .replaceAll(r -> Matcher.quoteReplacement(titleCase(r.group(1)) + r.group(2)));
   }
 
   private static String titleCase(String input) {

core/src/test/java/com/google/errorprone/bugpatterns/IdentifierNameTest.java

@@ -799,4 +799,17 @@ void test(Test this, int x) {}
             """)
         .doTest();
   }
+
+  @Test
+  public void dollarSign() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            class Test {
+              private static final String JSON_KEY_$SCHEMA = "$schema";
+            }
+            """)
+        .doTest();
+  }
 }