Merge Allow any characters after expected MM header

This merge allows any characters (esp. `\r`) after the expected matrix market header. Additionally, if the regex search failed, the description line is also printed.

Related PR: #1557

core/base/mtx_io.cpp

@@ -679,9 +679,9 @@ class mtx_io {
 
         std::smatch match{};
         GKO_CHECK_MATCH(
-            regex_match(
+            regex_search(
                 description_line, match,
-                std::regex("%%matrixmarket matrix "
+                std::regex("^%%matrixmarket matrix "
                            "(coordinate|array) "
                            "(real|integer|complex|pattern) "
                            "(general|symmetric|skew-symmetric|hermitian)")),
@@ -693,7 +693,9 @@ class mtx_io {
             "    <LAYOUT-TYPE>     is one of: coordinate, array\n"
             "    <VALUE-TYPE>      is one of: real, integer, complex, pattern\n"
             "    <LAYOUT-MODIFIER> is one of: general, symmetric, "
-            "skew-symmetric, hermitian\n");
+            "skew-symmetric, hermitian\n"
+            "Found the following header instead: " +
+                description_line);
 
         data.layout = layout_map.at(match[1]);
         data.entry = format_map.at(match[2]);

core/test/base/mtx_io.cpp

@@ -390,6 +390,22 @@ TEST(MtxReader, ReadsSparseComplexHermitianMtx)
 }
 
 
+TEST(MtxReader, ReadIgnoresExtraCharacters)
+{
+    using tpl = gko::matrix_data<double, gko::int32>::nonzero_type;
+    std::istringstream iss(
+        "%%MatrixMarket matrix array real general ??? whateve\r\n"
+        "1 1\n"
+        "0.1\n");
+
+    auto data = gko::read_raw<double, gko::int32>(iss);
+
+    ASSERT_EQ(data.size, gko::dim<2>(1, 1));
+    auto& v = data.nonzeros;
+    ASSERT_EQ(v[0], tpl(0, 0, 0.1));
+}
+
+
 std::array<gko::uint64, 20> build_binary_complex_data()
 {
     gko::uint64 int_val{};