diff --git a/MANIFEST b/MANIFEST
index 401e6c2fa8dc..72b20c026c4d 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -5190,6 +5190,7 @@ ext/XS-APItest/t/sv_numeq.t			Test sv_numeq
 ext/XS-APItest/t/sv_streq.t			Test sv_streq
 ext/XS-APItest/t/svcat.t			Test sv_catpvn
 ext/XS-APItest/t/svcatpvf.t			Test sv_catpvf argument reordering
+ext/XS-APItest/t/svcow.t			Test COW
 ext/XS-APItest/t/sviscow.t			Test SvIsCOW
 ext/XS-APItest/t/svpeek.t			XS::APItest extension
 ext/XS-APItest/t/svpv.t				More generic SvPVbyte and SvPVutf8 tests
diff --git a/embed.fnc b/embed.fnc
index 0332f63875ec..2a9a61b445fd 100644
--- a/embed.fnc
+++ b/embed.fnc
@@ -4144,7 +4144,7 @@ RTp	|MEM_SIZE|malloc_good_size					\
 #endif
 #if defined(PERL_ANY_COW)
 : Used in regexec.c
-EXpx	|SV *	|sv_setsv_cow	|NULLOK SV *dsv 			\
+EXdpx	|bool	|sv_setsv_cow	|NN SV **pdsv				\
 				|NN SV *ssv
 #endif
 #if defined(PERL_CORE)
diff --git a/ext/XS-APItest/APItest.xs b/ext/XS-APItest/APItest.xs
index 69561db6a106..469f7646fcec 100644
--- a/ext/XS-APItest/APItest.xs
+++ b/ext/XS-APItest/APItest.xs
@@ -3158,6 +3158,17 @@ sv_setsv_cow_hashkey_core()
 bool
 sv_setsv_cow_hashkey_notcore()
 
+void
+sv_grow(SV *sv, UV len)
+    CODE:
+        sv_force_normal(sv);
+        SvGROW(sv, len);
+
+void
+sv_force_normal(SV *sv)
+    CODE:
+        sv_force_normal(sv);
+
 void
 sv_set_deref(SV *sv, SV *sv2, int which)
     CODE:
diff --git a/ext/XS-APItest/t/svcow.t b/ext/XS-APItest/t/svcow.t
new file mode 100644
index 000000000000..5bd927424613
--- /dev/null
+++ b/ext/XS-APItest/t/svcow.t
@@ -0,0 +1,46 @@
+#!perl
+use strict;
+use warnings;
+use XS::APItest;
+use B;
+
+use Test::More tests => 11;
+
+{
+    # github #21877
+    # the regexp engine would COW an SV that had a large
+    # SvLEN() in cases where sv_setsv() wouldn't.
+    # This led to some surprises.
+    # - On cywgin this produced some strange performance problems
+    # - In general it meant the (large) buffer of the SV remained
+    #   allocated for longer than it otherwise would.
+    # Also, since the SV became CoW, further copies would also
+    # be CoW, for example, code like:
+    #
+    # while (<>) { # sv_getsv() currently allocates a large-ish buffer
+    #    /regex that (captures)/; # CoW large buffer
+    #    push @save, $_; # copy in @save still has that large buffer
+    # }
+    my $x = "Something\n" x 1000;
+    cmp_ok(length $x, '>=', 1250,
+           "need to be at least 1250 to be COWed");
+    sv_grow($x, 1_000_000);
+    my $ref = B::svref_2object(\$x);
+    cmp_ok($ref->LEN, '>=', 1_000_000,
+           "check we got it longer");
+    ok(!SvIsCOW($x), "not cow before");
+    is($ref->REFCNT, 1, "expected reference count");
+    ok($x =~ /me(.)hing/, "match");
+    ok(!SvIsCOW($x), "not cow after");
+
+    # make sure reasonable SVs are COWed
+    my $y = "Something\n" x 1000;
+    sv_force_normal($y);
+    cmp_ok(length $y, '>=', 1250,
+           "need to be at least 1250 to be COWed");
+    my $ref2 = B::svref_2object(\$y);
+    ok(!SvIsCOW($y), "not cow before");
+    is($ref2->REFCNT, 1, "expected reference count");
+    ok($y =~ /me(.)hing/, "match");
+    ok(SvIsCOW($y), "is cow after");
+}
diff --git a/proto.h b/proto.h
index e132956f8ac8..6eaa2106a054 100644
--- a/proto.h
+++ b/proto.h
@@ -6047,10 +6047,10 @@ Perl_uvuni_to_utf8(pTHX_ U8 *d, UV uv)
 
 #endif /* !defined(NO_MATHOMS) */
 #if defined(PERL_ANY_COW)
-PERL_CALLCONV SV *
-Perl_sv_setsv_cow(pTHX_ SV *dsv, SV *ssv);
+PERL_CALLCONV bool
+Perl_sv_setsv_cow(pTHX_ SV **pdsv, SV *ssv);
 # define PERL_ARGS_ASSERT_SV_SETSV_COW          \
-        assert(ssv)
+        assert(pdsv); assert(ssv)
 
 #endif
 #if defined(PERL_CORE)
diff --git a/regexec.c b/regexec.c
index 7c1e246537c0..f10762a4160f 100644
--- a/regexec.c
+++ b/regexec.c
@@ -3559,7 +3559,9 @@ S_reg_set_capture_string(pTHX_ REGEXP * const rx,
             else {
                 /* create new COW SV to share string */
                 RXp_MATCH_COPY_FREE(prog);
-                RXp_SAVED_COPY(prog) = sv_setsv_cow(RXp_SAVED_COPY(prog), sv);
+                /* sv_setsv_cow() might not COW for some reason */
+                if (!sv_setsv_cow(&RXp_SAVED_COPY(prog), sv))
+                    goto didnt_cow;
             }
             RXp_SUBBEG(prog) = (char *)SvPVX_const(RXp_SAVED_COPY(prog));
             assert (SvPOKp(RXp_SAVED_COPY(prog)));
@@ -3569,6 +3571,9 @@ S_reg_set_capture_string(pTHX_ REGEXP * const rx,
         } else
 #endif
         {
+#ifdef PERL_ANY_COW
+        didnt_cow: ;
+#endif
             SSize_t min = 0;
             SSize_t max = strend - strbeg;
             SSize_t sublen;
diff --git a/sv.c b/sv.c
index e83fd763591d..17f3a03bdb99 100644
--- a/sv.c
+++ b/sv.c
@@ -4902,10 +4902,33 @@ Perl_sv_setsv_mg(pTHX_ SV *const dsv, SV *const ssv)
     SvSETMAGIC(dsv);
 }
 
+/*
+=for apidoc sv_setsv_cow
+
+Attempt to make a COW copy of C<ssv> in C<*pdsv>.
+
+C<*pdsv> must be NULL or a valid SV, if NULL it will be filled in with
+a valid SV on success.
+
+C<ssv> must be a POK, pPOK SV.
+
+Returns true if the copy succeeds, false if a CoW copy cannot be made
+for some reason.
+
+sv_setsv_cow() is used by the regular expression engine to attempt to
+make a COW copy of the matched against string for use in reporting
+C<$1> etc.
+
+If this fails the regular expression engine instead makes a non-SV
+copy of a subset of the matched against string.
+
+=cut
+*/
+
 #ifdef PERL_ANY_COW
 #  define SVt_COW SVt_PV
-SV *
-Perl_sv_setsv_cow(pTHX_ SV *dsv, SV *ssv)
+bool
+Perl_sv_setsv_cow(pTHX_ SV **pdsv, SV *ssv)
 {
     STRLEN cur = SvCUR(ssv);
     STRLEN len = SvLEN(ssv);
@@ -4916,6 +4939,8 @@ Perl_sv_setsv_cow(pTHX_ SV *dsv, SV *ssv)
 #endif
 
     PERL_ARGS_ASSERT_SV_SETSV_COW;
+
+    SV *dsv = *pdsv;
 #ifdef DEBUGGING
     if (DEBUG_C_TEST) {
         PerlIO_printf(Perl_debug_log, "Fast copy on write: %p -> %p\n",
@@ -4925,6 +4950,13 @@ Perl_sv_setsv_cow(pTHX_ SV *dsv, SV *ssv)
                     sv_dump(dsv);
     }
 #endif
+    if (!SvIsCOW(ssv) &&
+        (!CHECK_COWBUF_THRESHOLD(cur, len)
+         || ! CHECK_COW_THRESHOLD(cur, len))) {
+        DEBUG_C(PerlIO_printf(Perl_debug_log,
+                              "Fast copy on write: Sizes %zu/%zu not appropriate to COW\n", cur, len));
+        return FALSE;
+    }
     if (dsv) {
         if (SvTHINKFIRST(dsv))
             sv_force_normal_flags(dsv, SV_COW_DROP_PV);
@@ -4970,6 +5002,7 @@ Perl_sv_setsv_cow(pTHX_ SV *dsv, SV *ssv)
     sv_buf_to_ro(ssv);
 
   common_exit:
+    *pdsv = dsv;
     SvPV_set(dsv, new_pv);
     SvFLAGS(dsv) = new_flags;
     if (SvUTF8(ssv))
@@ -4980,7 +5013,7 @@ Perl_sv_setsv_cow(pTHX_ SV *dsv, SV *ssv)
     if (DEBUG_C_TEST)
                 sv_dump(dsv);
 #endif
-    return dsv;
+    return TRUE;
 }
 #endif