Fix cudf::strings::replace_with_backrefs hang on empty match result

cpp/src/strings/replace/backref_re.cuh

@@ -61,14 +61,15 @@ struct backrefs_fn {
     auto out_ptr      = d_chars ? (d_chars + d_offsets[idx]) : nullptr;
     size_type lpos    = 0;                   // last byte position processed in d_str
     size_type begin   = 0;                   // first character position matching regex
-    size_type end     = nchars;              // last character position (exclusive)
+    size_type end     = -1;                  // match through the end of the string
 
     // copy input to output replacing strings as we go
-    while (prog.find(prog_idx, d_str, begin, end) > 0)  // inits the begin/end vars
+    while ((begin <= nchars) &&
+           (prog.find(prog_idx, d_str, begin, end) > 0))  // inits the begin/end vars
     {
-      auto spos = d_str.byte_offset(begin);             // get offset for the
-      auto epos = d_str.byte_offset(end);               // character position values;
-      nbytes += d_repl.size_bytes() - (epos - spos);    // compute the output size
+      auto spos = d_str.byte_offset(begin);               // get offset for the
+      auto epos = d_str.byte_offset(end);                 // character position values;
+      nbytes += d_repl.size_bytes() - (epos - spos);      // compute the output size
 
       // copy the string data before the matched section
       if (out_ptr) { out_ptr = copy_and_increment(out_ptr, in_ptr + lpos, spos - lpos); }
@@ -84,7 +85,7 @@ struct backrefs_fn {
           }
           // extract the specific group's string for this backref's index
           auto extracted = prog.extract(prog_idx, d_str, begin, end, backref.first - 1);
-          if (!extracted || (extracted.value().second <= extracted.value().first)) {
+          if (!extracted || (extracted.value().second < extracted.value().first)) {
             return;  // no value for this backref number; that is ok
           }
           auto spos_extract = d_str.byte_offset(extracted.value().first);   // convert
@@ -104,8 +105,8 @@ struct backrefs_fn {
 
       // setup to match the next section
       lpos  = epos;
-      begin = end;
-      end   = nchars;
+      begin = end + (begin == end);
+      end   = -1;
     }
 
     // finally, copy remainder of input string

cpp/tests/strings/replace_regex_tests.cpp

@@ -354,6 +354,21 @@ TEST_F(StringsReplaceRegexTest, ReplaceBackrefsRegexZeroIndexTest)
   CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(*results, expected);
 }
 
+// https://github.com/rapidsai/cudf/issues/13404
+TEST_F(StringsReplaceRegexTest, ReplaceBackrefsWithEmptyCapture)
+{
+  cudf::test::strings_column_wrapper input({"one\ntwo", "three\n\n", "four\r\n"});
+  auto sv = cudf::strings_column_view(input);
+
+  auto pattern       = std::string("(\r\n|\r)?$");
+  auto repl_template = std::string("[\\1]");
+
+  cudf::test::strings_column_wrapper expected({"one\ntwo[]", "three\n[]\n[]", "four[\r\n][]"});
+  auto prog    = cudf::strings::regex_program::create(pattern);
+  auto results = cudf::strings::replace_with_backrefs(sv, *prog, repl_template);
+  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(*results, expected);
+}
+
 TEST_F(StringsReplaceRegexTest, ReplaceBackrefsRegexErrorTest)
 {
   cudf::test::strings_column_wrapper strings({"this string left intentionally blank"});