Correctly parse decimals between 0 and -1.

The negative sign was being lost because stoll("-0") is 0.
Also permits parsing decimals between -1 and 1 without the leading 0.

libgnucash/engine/gnc-numeric.cpp

@@ -120,7 +120,7 @@ using boost::smatch;
 using boost::regex_search;
 GncNumeric::GncNumeric(const std::string& str, bool autoround)
 {
-    static const std::string numer_frag("(-?[0-9]+)");
+    static const std::string numer_frag("(-?[0-9]*)");
     static const std::string denom_frag("([0-9]+)");
     static const std::string hex_frag("(0x[a-f0-9]+)");
     static const std::string slash( "[ \\t]*/[ \\t]*");
@@ -175,10 +175,13 @@ GncNumeric::GncNumeric(const std::string& str, bool autoround)
     }
     if (regex_search(str, m, decimal))
     {
-        GncInt128 high(stoll(m[1].str()));
+        auto neg = (m[1].length() && m[1].str()[0] == '-');
+        GncInt128 high((neg && m[1].length() > 1) || (!neg && m[1].length()) ?
+                       stoll(m[1].str()) : 0);
         GncInt128 low(stoll(m[2].str()));
         int64_t d = powten(m[2].str().length());
-        GncInt128 n = high * d + (high >= 0 ? low : -low);
+        GncInt128 n = high * d + (neg ? -low : low);
+
         if (!autoround && n.isBig())
         {
             std::ostringstream errmsg;

libgnucash/engine/test/gtest-gnc-numeric.cpp

@@ -179,6 +179,12 @@ TEST(gncnumeric_constructors, test_string_constructor)
                  std::out_of_range);
     EXPECT_THROW(GncNumeric bad_string("Four score and seven"),
                  std::invalid_argument);
+    GncNumeric neg_decimal_frac("-0.12345");
+    EXPECT_EQ(-12345, neg_decimal_frac.num());
+    EXPECT_EQ(100000, neg_decimal_frac.denom());
+    GncNumeric neg_decimal_frac_nozero("-.12345");
+    EXPECT_EQ(-12345, neg_decimal_frac_nozero.num());
+    EXPECT_EQ(100000, neg_decimal_frac_nozero.denom());
 }
 
 TEST(gncnumeric_output, string_output)