[Help-bash] bash 4.3 escaping : (breaking script), unlike 4.2. Huh?

[B. S.]

In kubuntu 12.04, bash (4.2.25) line of:

tmpstr="{ md5sum - ; echo \$TAR_REALNAME ; } | sed -e 
':a;N;\$!ba;s/-\\n//g'"

when used in a tar --to-command call:

tar blah --to-command="${tmpstr}" --extract

worked fine.


In kubuntu 14.04, bash (4.3.11), same code, causes tar (--to-command 
subshell) to report:

tar: { md5sum - ; echo $TAR_REALNAME ; } | sed -e 
'\:a;N;$!ba;s/-\\n//g': Cannot exec: No such file or directory
tar: Error is not recoverable: exiting now


i.e. the colon is now getting escaped.

- what has changed 4.2 to 4.3 (i.e. what magic search term will take me 
directly to an explanation of what's going on / changed)?


- how do I stop the : from being escaped?

TIA!