diff --git a/regen/embed.pl b/regen/embed.pl
index 5bd4cd4a20f6..684b50f7343c 100755
--- a/regen/embed.pl
+++ b/regen/embed.pl
@@ -104,6 +104,27 @@ BEGIN
                    pod/perlreapi.pod
                  );
 
+# A regular expression that matches names that are externally visible, but
+# Perl reserves for itself.  Generally, we want things to be delimitted on
+# both sides to show it isn't part of a larger word such as 'hyperlink',
+# 'perlustrate', or 'properly'.  Underscores delimit besides the typical ^ or
+# \b.  All caps PERL has looser rules to accommodate the many existing symbols
+# where everything is jammed together, and the less likelihood that something
+# with all caps is innocently referring to something unrelated to Perl.
+my $names_reserved_for_perl_use_re =
+                         qr/  ^ (  PL_ \w+ \b
+                                 | perl_        # The underscore delimits
+                                 | Perl [_A-Z]  # Uppercase delimits here too
+                                 | PERL [A-Z]+ [[:alpha:]] ( \b | _ )
+                                )
+
+                              # The \d is for PERL5, for example
+                            | ( _ | \b )  PERL ( _ | \b | \d+ )
+
+                              # This is for obsolete and deprecated uses
+                            | ( _ | \b ) CPERL (arg | scope) ( _ | \b )
+                          /x;
+
 # This is a list of symbols that are not documented to be available for
 # modules to use, but are nevertheless currently not kept by embed.h from
 # being visible to the world.
@@ -111,7 +132,7 @@ BEGIN
 # Strive to make this list empty.
 #
 # The list does not include symbols that we have documented as being reserved
-# for perl's use, namely those that begin with 'PL_' or contain qr/perl/i.
+# for perl's use, namely those that match the pattern just above.
 # There are two parts of the list; the second part contains the symbols which
 # have a trailing underscore; the first part those without.
 #
@@ -1336,6 +1357,8 @@ BEGIN
     is_XDIGIT_high
     isXDIGIT_LC_utf8
     isXDIGIT_uni
+    is_XPERLSPACE_cp_high
+    is_XPERLSPACE_high
     IV_DIG
     IV_MAX_P1
     JE_OLD_STACK_HWM_restore
@@ -4729,9 +4752,7 @@ sub find_undefs {
                 # Just the symbol, no arglist nor definition
                 $name =~ s/ (?: \s | \( ) .* //x;
 
-                # These are reserved for Perl's use, so not a problem.
-                next if $name =~ / ^ PL_ /x;
-                next if $name =~ /perl/i;
+                next if $name =~ $names_reserved_for_perl_use_re;
 
                 next unless $line->reduce_conds($constraints_re,
                                                 \%constraints);