diff --git a/Lib/_pydecimal.py b/Lib/_pydecimal.py
index ff23322ed5..d8e94a727d 100644
--- a/Lib/_pydecimal.py
+++ b/Lib/_pydecimal.py
@@ -547,7 +547,7 @@ def __new__(cls, value="0", context=None):
         # From a string
         # REs insist on real strings, so we can too.
         if isinstance(value, str):
-            m = _parser(value.strip().replace("_", ""))
+            m = _parser(value.strip())
             if m is None:
                 if context is None:
                     context = getcontext()
@@ -6114,20 +6114,20 @@ def _convert_for_comparison(self, other, equality_op=False):
 # lookahead expression '(?=\d|\.\d)' checks this.
 
 import re
-_parser = re.compile(r"""        # A numeric string consists of:
+_parser = re.compile(r"""              # A numeric string consists of:
 #    \s*
-    (?P<sign>[-+])?              # an optional sign, followed by either...
+    (?P<sign>[-+])?                    # an optional sign, followed by either...
     (
-        (?=\d|\.\d)              # ...a number (with at least one digit)
-        (?P<int>\d*)             # having a (possibly empty) integer part
-        (\.(?P<frac>\d*))?       # followed by an optional fractional part
-        (E(?P<exp>[-+]?\d+))?    # followed by an optional exponent, or...
+        (?=\d|\.\d)                    # ...a number (with at least one digit)
+        (?P<int>\d*|\d+(_\d+)*)        # having a (possibly empty) integer part
+        (\.(?P<frac>\d*|\d+(_\d+)*))?  # followed by an optional fractional part
+        (E(?P<exp>[-+]?\d+(_\d+)*))?   # followed by an optional exponent, or...
     |
-        Inf(inity)?              # ...an infinity, or...
+        Inf(inity)?                    # ...an infinity, or...
     |
-        (?P<signal>s)?           # ...an (optionally signaling)
-        NaN                      # NaN
-        (?P<diag>\d*)            # with (possibly empty) diagnostic info.
+        (?P<signal>s)?                 # ...an (optionally signaling)
+        NaN                            # NaN
+        (?P<diag>\d*|\d+(_\d+)*)       # with (possibly empty) diagnostic info.
     )
 #    \s*
     \Z