bash in RHEL5 fails with: syntax error near `@(6'

Issue

Consider following script:

#!/bin/bash

DATE=$(/bin/date +%m%d%Y)
CP=/bin/cp
MV=/bin/mv
AWK=/bin/awk
RELEASE=$(sed 's/.* \([0-9]\{1\}\.\?[0-9]*\) .*/\1/' /etc/redhat-release)
[[  $RELEASE == @(6)* ]] && CONF=/etc/rsyslog.conf || CONF=/etc/syslog.conf
[[  $RELEASE == @(6)* ]] && INIT=/etc/init.d/rsyslog || INIT=/etc/init.d/syslog
[[ -f $CONF ]] && $CP -p $CONF $CONF.${DATE} || exit 127
$AWK '$1 ~ /authpriv\.\*/{print"\nauthpriv.*\t\t\t\t\t\t@example.com"};{print}' $CONF > $CONF.bak
mv $CONF.bak $CONF
$INIT restart

The script works fine with bash on RHEL 6 but fails on RHEL 5. Works with ksh on RHEL 5 and RHEL 6.

This is the error with bash on RHEL5:

./x: line 7: syntax error in conditional expression: unexpected token `('
./x: line 7: syntax error near `@(6'
./x: line 7: `[[ $RELEASE == @(6)* ]] && CONF=/etc/rsyslog.conf || CONF=/etc/syslog.conf'

So what is the difference with bash?