[CentOS] that ever puzzling special chars escaping + rdiff-backup

hi everyone

a good basher around here?

I try in a script:

_rdiffBack="rdiff-backup -v5 --tempdir /tmp/ --no-eas 
--exclude-other-filesystems --exclude-symbolic-links"
_rdffiExclude="--exclude '**/~*' --exclude '**.tmp' 
--exclude-regexp '(.glusterfs|.trashcan|temp)'"
_execCom=${!2}
_sourceDir=${1}
_backupTo=${3}

__backMeUp() {
   for _sourceDir in ${_sourceDirs[@]}; do
     echo \$ ${_execCom} ${_rdffiExclude} ${_sourceDir}/ 
${_backupTo} \> ${_logFile} | ${_copyToLog}
     ${_execCom} ${_rdffiExclude} ${_sourceDir}/ 
${_backupTo} > ${_logFile} 2>&1
   done
}

case ${1} in
   rdiffThis)
   declare -a _sourceDirs=( /0-ALL.DATA/rdiffThis ); 
__backMeUp ${@}
   ;;
esac

this is a snippet which may look obscure but obvious is: 
rdiff-backup some data, just that.
And when I run it as a script

~]$ runme.sh rdiffThis _rdiffBack toHere

I see:
Fatal Error: Fatal Error: The file specification
     ''**'
cannot match any files in the base directory

You notice that the script echos what it is meant to 
execute, which suffice I copy&paste that echo in bash 
prompt... and it works!? No "Fatal Error"
And the problem seems to specifically be a shell pattern for 
--exclude. Take two --exclude out, having:

_rdffiExclude="--exclude-regexp '(.glusterfs|.trashcan|temp)'"

and script works when invoked, fine!
I've been fiddling around but failed to make rdiff-backup 
1.2.8 play along.
Must be something trivial, right?
b.w.
L