Initial implementation of <after ...>. Flips the target string and the AST (though probably missing some cases in the latter) then just parses as per normal - but backwards.

Author: jnthn

src/QRegex/Cursor.nqp

@@ -206,6 +206,19 @@ role NQPCursorRole {
         $cur;
     }
 
+    # Expects to get a regex whose syntax tree was flipped during the
+    # compile.
+    method after($regex) {
+        my $cur := self."!cursor_start"();
+        nqp::bindattr_s($cur, $?CLASS, '$!target', $!target.reverse());
+        nqp::bindattr_i($cur, $?CLASS, '$!from', nqp::chars($!target) - $!pos);
+        nqp::bindattr_i($cur, $?CLASS, '$!pos', nqp::chars($!target) - $!pos);
+        nqp::getattr_i($regex($cur), $?CLASS, '$!pos') >= 0 ??
+            $cur."!cursor_pass"($!pos, 'after') !!
+            $cur."!cursor_fail"();
+        $cur;
+    }
+
     method ws() {
         # skip over any whitespace, fail if between two word chars
         my $cur := self."!cursor_start"();

src/QRegex/P6Regex/Actions.nqp

@@ -358,7 +358,9 @@ class QRegex::P6Regex::Actions is HLL::Actions {
                 for $<arglist>[0].ast.list { $qast[0].push( $_ ) }
             }
             elsif $<nibbler> {
-                $qast[0].push(buildsub($<nibbler>[0].ast, :anon(1)));
+                $name eq 'after' ??
+                    $qast[0].push(buildsub(self.flip_ast($<nibbler>[0].ast), :anon(1))) !!
+                    $qast[0].push(buildsub($<nibbler>[0].ast, :anon(1)));
             }
         }
         make $qast;
@@ -516,6 +518,21 @@ class QRegex::P6Regex::Actions is HLL::Actions {
         $ast.subtype('capture');
     }
 
+    method flip_ast($qast) {
+        if $qast.rxtype eq 'literal' {
+            $qast[0] := $qast[0].reverse();
+        }
+        elsif $qast.rxtype eq 'concat' {
+            my @tmp;
+            while +@($qast) { @tmp.push(@($qast).shift) }
+            while @tmp      { @($qast).push(self.flip_ast(@tmp.pop)) }
+        }
+        elsif $qast.rxtype eq 'pastnode' {
+            # Don't go exploring these
+        }
+        else {
+            for @($qast) { self.flip_ast($_) }
+        }
+        $qast
+    }
 }
-
-