diff --git a/.spec/math/fraction_spec.lua b/.spec/math/fraction_spec.lua
index e67fba1..469de74 100644
--- a/.spec/math/fraction_spec.lua
+++ b/.spec/math/fraction_spec.lua
@@ -13,6 +13,10 @@ describe("Fraction", function()
 		{ base = 2, str = "0.(10)", val = frac(2, 3) },
 		{ base = nil, str = "3.(3)", val = frac(10, 3) }, -- test default base
 		{ base = 10, str = "1.2(3)", val = frac(12, 10) + frac(1, 30) },
+		{ base = 10, str = "1", val = frac(1, 1) },
+		{ base = 10, str = "-2", val = frac(-2, 1) },
+		{ base = 10, str = "-1.25", val = frac(-5, 4) },
+		{ base = 10, str = "-1.3(51)", val = frac(-223, 165) },
 		{ base = 16, str = "1.(45d17)", val = frac(42, 33) },
 	}
 
diff --git a/src/math/fraction.lua b/src/math/fraction.lua
index 88311fb..2eeafce 100644
--- a/src/math/fraction.lua
+++ b/src/math/fraction.lua
@@ -70,8 +70,8 @@ local function read_base_param(base)
 	return base
 end
 
-function fraction.from_float_string(
-	str, -- <digit>{<digit>}.{digit}[(<digit>{digit})], ex.: `-1.2(3)`
+local function parse_positive_double_string(
+	str, -- EBNF: digit { digit } "." { digit } [ "(" digit { digit } ")" ], ex.: `1.2(3)`
 	base -- integer from 2 to 36, defaults to 10 (decimal)
 )
 	base = read_base_param(base)
@@ -81,6 +81,7 @@ function fraction.from_float_string(
 
 	local integer, fractional = str:match("^([0-9a-zA-Z][0-9a-zA-Z]-)%.([0-9a-zA-Z%(%)]+)")
 	if not fractional then
+		assert(str:match("[0-9a-zA-Z]+"))
 		return new(read_number(str), 1)
 	end
 
@@ -96,6 +97,16 @@ function fraction.from_float_string(
 	return read_number(integer) + after_dot / base ^ #pre_period
 end
 
+function fraction.from_float_string(
+	str, -- EBNF: [ "-" ] digit { digit } "." { digit } [ "(" digit { digit } ")" ], ex.: `-1.2(3)`
+	base -- integer from 2 to 36, defaults to 10 (decimal)
+)
+	if str:sub(1, 1) == "-" then
+		return -parse_positive_double_string(str:sub(2), base)
+	end
+	return parse_positive_double_string(str, base)
+end
+
 -- Conversions
 
 function metatable:__tostring()