Fix regex parsing logic handling of nested quantifiers (#16798)

Fixes the libcudf regex parsing logic when handling nested fixed quantifiers. The logic handles fixed quantifiers by simple repeating the previous instruction. If the previous item is a group (capture or non-capture) that group may also contain an internal fixed quantifier as well.
Found while working on #16730

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Bradley Dice (https://github.com/bdice)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #16798

cpp/src/strings/regex/regcomp.cpp

@@ -716,13 +716,13 @@ class regex_parser {
       if (item.type != COUNTED && item.type != COUNTED_LAZY) {
         out.push_back(item);
         if (item.type == LBRA || item.type == LBRA_NC) {
-          lbra_stack.push(index);
+          lbra_stack.push(out.size() - 1);
           repeat_start_index = -1;
         } else if (item.type == RBRA) {
           repeat_start_index = lbra_stack.top();
           lbra_stack.pop();
         } else if ((item.type & ITEM_MASK) != OPERATOR_MASK) {
-          repeat_start_index = index;
+          repeat_start_index = out.size() - 1;
         }
       } else {
         // item is of type COUNTED or COUNTED_LAZY
@@ -731,26 +731,39 @@ class regex_parser {
         CUDF_EXPECTS(repeat_start_index >= 0, "regex: invalid counted quantifier location");
 
         // range of affected item(s) to repeat
-        auto const begin = in.begin() + repeat_start_index;
-        auto const end   = in.begin() + index;
+        auto const begin = out.begin() + repeat_start_index;
+        auto const end   = out.end();
+
         // count range values
         auto const n = item.d.count.n;  // minimum count
         auto const m = item.d.count.m;  // maximum count
-
         assert(n >= 0 && "invalid repeat count value n");
         // zero-repeat edge-case: need to erase the previous items
-        if (n == 0) { out.erase(out.end() - (index - repeat_start_index), out.end()); }
-
-        // minimum repeats (n)
-        for (int j = 1; j < n; j++) {
-          out.insert(out.end(), begin, end);
+        if (n == 0 && m == 0) { out.erase(begin, end); }
+
+        std::vector<regex_parser::Item> repeat_copy(begin, end);
+        // special handling for quantified capture groups
+        if ((n > 1) && (*begin).type == LBRA) {
+          (*begin).type = LBRA_NC;  // change first one to non-capture
+          // add intermediate groups as non-capture
+          std::vector<regex_parser::Item> ncg_copy(begin, end);
+          for (int j = 1; j < (n - 1); j++) {
+            out.insert(out.end(), ncg_copy.begin(), ncg_copy.end());
+          }
+          // add the last entry as a regular capture-group
+          out.insert(out.end(), repeat_copy.begin(), repeat_copy.end());
+        } else {
+          // minimum repeats (n)
+          for (int j = 1; j < n; j++) {
+            out.insert(out.end(), repeat_copy.begin(), repeat_copy.end());
+          }
         }
 
         // optional maximum repeats (m)
         if (m >= 0) {
           for (int j = n; j < m; j++) {
             out.emplace_back(LBRA_NC, 0);
-            out.insert(out.end(), begin, end);
+            out.insert(out.end(), repeat_copy.begin(), repeat_copy.end());
           }
           for (int j = n; j < m; j++) {
             out.emplace_back(RBRA, 0);
@@ -760,8 +773,9 @@ class regex_parser {
           // infinite repeats
           if (n > 0) {  // append '+' after last repetition
             out.emplace_back(item.type == COUNTED ? PLUS : PLUS_LAZY, 0);
-          } else {  // copy it once then append '*'
-            out.insert(out.end(), begin, end);
+          } else {
+            // copy it once then append '*'
+            out.insert(out.end(), repeat_copy.begin(), repeat_copy.end());
             out.emplace_back(item.type == COUNTED ? STAR : STAR_LAZY, 0);
           }
         }

cpp/tests/strings/contains_tests.cpp

@@ -474,6 +474,20 @@ TEST_F(StringsContainsTests, FixedQuantifier)
   }
 }
 
+TEST_F(StringsContainsTests, NestedQuantifier)
+{
+  auto input   = cudf::test::strings_column_wrapper({"TEST12 1111 2222 3333 4444 5555",
+                                                     "0000 AAAA 9999 BBBB 8888",
+                                                     "7777 6666 4444 3333",
+                                                     "12345 3333 4444 1111 ABCD"});
+  auto sv      = cudf::strings_column_view(input);
+  auto pattern = std::string(R"((\d{4}\s){4})");
+  cudf::test::fixed_width_column_wrapper<bool> expected({true, false, false, true});
+  auto prog    = cudf::strings::regex_program::create(pattern);
+  auto results = cudf::strings::contains_re(sv, *prog);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(*results, expected);
+}
+
 TEST_F(StringsContainsTests, QuantifierErrors)
 {
   EXPECT_THROW(cudf::strings::regex_program::create("^+"), cudf::logic_error);

cpp/tests/strings/extract_tests.cpp

@@ -19,7 +19,6 @@
 #include <cudf_test/base_fixture.hpp>
 #include <cudf_test/column_utilities.hpp>
 #include <cudf_test/column_wrapper.hpp>
-#include <cudf_test/debug_utilities.hpp>
 #include <cudf_test/table_utilities.hpp>
 
 #include <cudf/detail/iterator.cuh>
@@ -240,6 +239,21 @@ TEST_F(StringsExtractTests, SpecialNewLines)
   CUDF_TEST_EXPECT_COLUMNS_EQUAL(results->view().column(0), expected);
 }
 
+TEST_F(StringsExtractTests, NestedQuantifier)
+{
+  auto input   = cudf::test::strings_column_wrapper({"TEST12 1111 2222 3333 4444 5555",
+                                                     "0000 AAAA 9999 BBBB 8888",
+                                                     "7777 6666 4444 3333",
+                                                     "12345 3333 4444 1111 ABCD"});
+  auto sv      = cudf::strings_column_view(input);
+  auto pattern = std::string(R"((\d{4}\s){4})");
+  auto prog    = cudf::strings::regex_program::create(pattern);
+  auto results = cudf::strings::extract(sv, *prog);
+  // fixed quantifier on capture group only honors the last group
+  auto expected = cudf::test::strings_column_wrapper({"4444 ", "", "", "1111 "}, {1, 0, 0, 1});
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(results->view().column(0), expected);
+}
+
 TEST_F(StringsExtractTests, EmptyExtractTest)
 {
   std::vector<char const*> h_strings{nullptr, "AAA", "AAA_A", "AAA_AAA_", "A__", ""};