diff --git a/mysql-test/r/func_regexp_pcre.result b/mysql-test/r/func_regexp_pcre.result
index 3430940401f91..85822a16e15e4 100644
--- a/mysql-test/r/func_regexp_pcre.result
+++ b/mysql-test/r/func_regexp_pcre.result
@@ -883,3 +883,12 @@ SELECT 1 FROM dual WHERE ('Alpha,Bravo,Charlie,Delta,Echo,Foxtrot,StrataCentral,
 1
 Warnings:
 Warning	1139	Got error 'pcre_exec: recursion limit of NUM exceeded' from regexp
+SELECT REGEXP_INSTR('a_kollision', 'oll');
+REGEXP_INSTR('a_kollision', 'oll')
+4
+SELECT REGEXP_INSTR('a_kollision', '(oll)');
+REGEXP_INSTR('a_kollision', '(oll)')
+4
+SELECT REGEXP_INSTR('a_kollision', 'o([lm])\\1');
+REGEXP_INSTR('a_kollision', 'o([lm])\\1')
+4
diff --git a/mysql-test/t/func_regexp_pcre.test b/mysql-test/t/func_regexp_pcre.test
index 8e3adad00377b..a98955069758f 100644
--- a/mysql-test/t/func_regexp_pcre.test
+++ b/mysql-test/t/func_regexp_pcre.test
@@ -430,3 +430,10 @@ SELECT CAST(0xE001 AS BINARY) REGEXP @regCheck;
 --echo # MDEV-12420: Testing recursion overflow
 --replace_regex /[0-9]+ exceeded/NUM exceeded/
 SELECT 1 FROM dual WHERE ('Alpha,Bravo,Charlie,Delta,Echo,Foxtrot,StrataCentral,Golf,Hotel,India,Juliet,Kilo,Lima,Mike,StrataL3,November,Oscar,StrataL2,Sand,P3,P4SwitchTest,Arsys,Poppa,ExtensionMgr,Arp,Quebec,Romeo,StrataApiV2,PtReyes,Sierra,SandAcl,Arrow,Artools,BridgeTest,Tango,SandT,PAlaska,Namespace,Agent,Qos,PatchPanel,ProjectReport,Ark,Gimp,Agent,SliceAgent,Arnet,Bgp,Ale,Tommy,Central,AsicPktTestLib,Hsc,SandL3,Abuild,Pca9555,Standby,ControllerDut,CalSys,SandLib,Sb820,PointV2,BfnLib,Evpn,BfnSdk,Sflow,ManagementActive,AutoTest,GatedTest,Bgp,Sand,xinetd,BfnAgentLib,bf-utils,Hello,BfnState,Eos,Artest,Qos,Scd,ThermoMgr,Uniform,EosUtils,Eb,FanController,Central,BfnL3,BfnL2,tcp_wrappers,Victor,Environment,Route,Failover,Whiskey,Xray,Gimp,BfnFixed,Strata,SoCal,XApi,Msrp,XpProfile,tcpdump,PatchPanel,ArosTest,FhTest,Arbus,XpAcl,MacConc,XpApi,telnet,QosTest,Alpha2,BfnVlan,Stp,VxlanControllerTest,MplsAgent,Bravo2,Lanz,BfnMbb,Intf,XCtrl,Unicast,SandTunnel,L3Unicast,Ipsec,MplsTest,Rsvp,EthIntf,StageMgr,Sol,MplsUtils,Nat,Ira,P4NamespaceDut,Counters,Charlie2,Aqlc,Mlag,Power,OpenFlow,Lag,RestApi,BfdTest,strongs,Sfa,CEosUtils,Adt746,MaintenanceMode,MlagDut,EosImage,IpEth,MultiProtocol,Launcher,Max3179,Snmp,Acl,IpEthTest,PhyEee,bf-syslibs,tacc,XpL2,p4-ar-switch,p4-bf-switch,LdpTest,BfnPhy,Mirroring,Phy6,Ptp'  REGEXP '^((?!\b(Strata|StrataApi|StrataApiV2)\b).)*$');
+
+#
+# MDEV-12942 REGEXP_INSTR returns 1 when using brackets
+#
+SELECT REGEXP_INSTR('a_kollision', 'oll');
+SELECT REGEXP_INSTR('a_kollision', '(oll)');
+SELECT REGEXP_INSTR('a_kollision', 'o([lm])\\1');
diff --git a/sql/item_cmpfunc.cc b/sql/item_cmpfunc.cc
index 4c17da46e1ece..369cdeda62da4 100644
--- a/sql/item_cmpfunc.cc
+++ b/sql/item_cmpfunc.cc
@@ -5518,7 +5518,7 @@ int Regexp_processor_pcre::pcre_exec_with_warn(const pcre *code,
 bool Regexp_processor_pcre::exec(const char *str, int length, int offset)
 {
   m_pcre_exec_rc= pcre_exec_with_warn(m_pcre, &m_pcre_extra, str, length, offset, 0,
-                                      m_SubStrVec, m_subpatterns_needed * 3);
+                                      m_SubStrVec, array_elements(m_SubStrVec));
   return false;
 }
 
@@ -5531,7 +5531,7 @@ bool Regexp_processor_pcre::exec(String *str, int offset,
   m_pcre_exec_rc= pcre_exec_with_warn(m_pcre, &m_pcre_extra,
                                       str->c_ptr_safe(), str->length(),
                                       offset, 0,
-                                      m_SubStrVec, m_subpatterns_needed * 3);
+                                      m_SubStrVec, array_elements(m_SubStrVec));
   if (m_pcre_exec_rc > 0)
   {
     uint i;