Related to bug 652211...

This expression worked on f<13, but was b0rk by removal of the cond-rmatch patch:

( echo $BASH_VERSION ; b=foo ; [[ $b-42 =~ "^$b-[0-9][0-9]\$" ]] && echo right || echo wrong )
4.0.38(1)-release
right

( echo $BASH_VERSION ; b=foo ; [[ $b-42 =~ "^$b-[0-9][0-9]\$" ]] && echo right || echo wrong )
4.1.7(1)-release
wrong

So, apparently the fix is to remove the quotes. But the only way to get the trailing $ to properly match EOL is to also not escape it.

( b=foo ; [[ $b-42 =~ ^$b-[0-9][0-9]\$ ]] && echo right || echo wrong )
wrong

Which still leaves some ambiguity over what purpose the $ symbol has in that context.

( b=foo ; [[ $b-42 =~ ^$b-[0-9][0-9]$ ]] && echo right || echo wrong )
right