diff --git a/Lib/fractions.py b/Lib/fractions.py
index 64a8959d7d..f182246fb6 100644
--- a/Lib/fractions.py
+++ b/Lib/fractions.py
@@ -21,17 +21,17 @@
 _PyHASH_INF = sys.hash_info.inf
 
 _RATIONAL_FORMAT = re.compile(r"""
-    \A\s*                      # optional whitespace at the start, then
-    (?P<sign>[-+]?)            # an optional sign, then
-    (?=\d|\.\d)                # lookahead for digit or .digit
-    (?P<num>\d*)               # numerator (possibly empty)
-    (?:                        # followed by
-       (?:/(?P<denom>\d+))?    # an optional denominator
-    |                          # or
-       (?:\.(?P<decimal>\d*))? # an optional fractional part
-       (?:E(?P<exp>[-+]?\d+))? # and optional exponent
+    \A\s*                        # optional whitespace at the start, then
+    (?P<sign>[-+]?)              # an optional sign, then
+    (?=\d|\.\d)                  # lookahead for digit or .digit
+    (?P<num>[_\d]*)              # numerator (possibly empty)
+    (?:                          # followed by
+       (?:/(?P<denom>\d[_\d]*))? # an optional denominator
+    |                            # or
+       (?:\.(?P<decimal>\d*))?   # an optional fractional part
+       (?:E(?P<exp>[-+]?\d+))?   # and optional exponent
     )
-    \s*\Z                      # and optional whitespace to finish
+    \s*\Z                        # and optional whitespace to finish
 """, re.VERBOSE | re.IGNORECASE)
 
 
diff --git a/Lib/test/test_fractions.py b/Lib/test/test_fractions.py
index 949ddd9072..9cc8d34527 100644
--- a/Lib/test/test_fractions.py
+++ b/Lib/test/test_fractions.py
@@ -173,6 +173,8 @@ def testFromString(self):
         self.assertEqual((-12300, 1), _components(F("-1.23e4")))
         self.assertEqual((0, 1), _components(F(" .0e+0\t")))
         self.assertEqual((0, 1), _components(F("-0.000e0")))
+        self.assertEqual((123, 1), _components(F("1_2_3")))
+        self.assertEqual((41, 107), _components(F("1_2_3/3_2_1")))
 
         self.assertRaisesMessage(
             ZeroDivisionError, "Fraction(3, 0)",