Addressed review comments (thanks @jpountz !)

Reordered constructor args and added arg validation.

lucene/core/src/java/org/apache/lucene/search/RegexpQuery.java

@@ -96,14 +96,14 @@ public RegexpQuery(Term term, int flags, int maxDeterminizedStates) {
    * Constructs a query for terms matching <code>term</code>.
    * 
    * @param term regular expression.
-   * @param maxDeterminizedStates maximum number of states that compiling the
    * @param syntax_flags optional RegExp syntax features from {@link RegExp}
    *  automaton for the regexp can result in.  Set higher to allow more complex
    *  queries and lower to prevent memory exhaustion.
    * @param match_flags boolean 'or' of match behavior options such as case insensitivity
+   * @param maxDeterminizedStates maximum number of states that compiling the
    */
-  public RegexpQuery(Term term,  int maxDeterminizedStates, int syntax_flags, int match_flags) {
-    this(term, defaultProvider, maxDeterminizedStates, syntax_flags, match_flags);
+  public RegexpQuery(Term term, int syntax_flags, int match_flags, int maxDeterminizedStates) {
+    this(term, syntax_flags, match_flags, defaultProvider, maxDeterminizedStates);
   }
 
   /**
@@ -118,22 +118,22 @@ public RegexpQuery(Term term,  int maxDeterminizedStates, int syntax_flags, int
    */
   public RegexpQuery(Term term, int syntax_flags, AutomatonProvider provider,
       int maxDeterminizedStates) {
-    this(term, provider, maxDeterminizedStates, syntax_flags, 0);
+    this(term, syntax_flags, 0, provider, maxDeterminizedStates);
   }
 
   /**
    * Constructs a query for terms matching <code>term</code>.
    * 
    * @param term regular expression.
    * @param syntax_flags optional RegExp features from {@link RegExp}
+   * @param match_flags boolean 'or' of match behavior options such as case insensitivity
    * @param provider custom AutomatonProvider for named automata
    * @param maxDeterminizedStates maximum number of states that compiling the
    *  automaton for the regexp can result in.  Set higher to allow more complex
    *  queries and lower to prevent memory exhaustion.
-   * @param match_flags boolean 'or' of match behavior options such as case insensitivity
    */
-  public RegexpQuery(Term term, AutomatonProvider provider,
-      int maxDeterminizedStates, int syntax_flags, int match_flags) {
+  public RegexpQuery(Term term, int syntax_flags, int match_flags, AutomatonProvider provider,
+      int maxDeterminizedStates) {
     super(term,
           new RegExp(term.text(), syntax_flags, match_flags).toAutomaton(
                        provider, maxDeterminizedStates), maxDeterminizedStates);

lucene/core/src/java/org/apache/lucene/util/automaton/RegExp.java

@@ -521,8 +521,9 @@ public RegExp(String s, int syntax_flags) throws IllegalArgumentException {
    *              regular expression
    */
   public RegExp(String s, int syntax_flags, int match_flags) throws IllegalArgumentException {    
-    // (for BWC reasons we don't validate invalid bits, just trim instead)
-    syntax_flags  = syntax_flags & 0xff;
+    if (syntax_flags >  ALL) {
+      throw new IllegalArgumentException("Illegal syntax flag");
+    }
 
     if (match_flags > 0 && match_flags <= ALL) {
       throw new IllegalArgumentException("Illegal match flag");

lucene/core/src/test/org/apache/lucene/search/TestRegexpQuery.java

@@ -74,8 +74,8 @@ private long regexQueryNrHits(String regex) throws IOException {
   }
 
   private long caseInsensitiveRegexQueryNrHits(String regex) throws IOException {
-    RegexpQuery query = new RegexpQuery(newTerm(regex), Operations.DEFAULT_MAX_DETERMINIZED_STATES, 
-        RegExp.ALL, RegExp.ASCII_CASE_INSENSITIVE);
+    RegexpQuery query = new RegexpQuery(newTerm(regex), RegExp.ALL, RegExp.ASCII_CASE_INSENSITIVE,
+        Operations.DEFAULT_MAX_DETERMINIZED_STATES);
     return searcher.count(query);
   }