Regex: implement non-capturing groups

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

test/Regex.java

@@ -56,5 +56,6 @@ public class Regex {
     expectFind(" +", "Hello  ,   world! ", "  ", "   ", " ");
     expectMatch("[0-9A-Fa-f]+", "08ef");
     expectNoMatch("[0-9A-Fa-f]+", "08@ef");
+    expectGroups("(?:a)", "a");
   }
 }

test/regex/Compiler.java

@@ -140,8 +140,14 @@ class Compiler implements PikeVMOpcodes {
   }
 
   private class Group extends Expression {
+    private final boolean capturing;
+
     private ArrayList<Expression> list = new ArrayList<Expression>();
 
+    public Group(boolean capturing) {
+      this.capturing = capturing;
+    }
+
     public void push(Expression expr) {
       list.add(expr);
     }
@@ -160,22 +166,27 @@ class Compiler implements PikeVMOpcodes {
     }
 
     protected void writeCode(Output output) {
-      int groupIndex = ++ output.groupCount;
+      int groupIndex = -1;
+      if (capturing) {
+        groupIndex = ++ output.groupCount;
         output.add(SAVE_OFFSET);
         output.add(2 * groupIndex);
+      }
       for (Expression expr : list) {
         expr.writeCode(output);
       }
+      if (capturing) {
         output.add(SAVE_OFFSET);
         output.add(2 * groupIndex + 1);
       }
     }
+  }
 
   private class Group0 extends Expression {
     private final Group group;
 
     public Group0() {
-      group = new Group();
+      group = new Group(true);
     }
 
     public void writeCode(Output output) {
@@ -227,13 +238,20 @@ class Compiler implements PikeVMOpcodes {
           c == '+' ? 1 : 0, c == '?' ? 1 : -1, greedy));
         continue;
       }
-      case '(':
+      case '(': {
+        boolean capturing = true;
         if (index + 1 < array.length && array[index + 1] == '?') {
+          if (index + 2 < array.length && array[index + 2] == ':') {
+            index += 2;
+            capturing = false;
+          } else {
             throw new UnsupportedOperationException("Not yet supported: "
               + regex.substring(index));
           }
-        current.push(groups.push(new Group()));
+        }
+        current.push(groups.push(new Group(capturing)));
         continue;
+      }
       case ')':
         if (groups.size() < 2) {
           throw new RuntimeException("Invalid group close @" + index + ": "