variable containing syntax error in conditional expression reports no diagnostics

[g . branden . robinson]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-unknown-linux-gnu' 
-DCONF_VENDOR='unknown' -DLOCALEDIR='/users/brarobin/share/locale' 
-DPACKAGE='bash' -DSHELL -DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib   -g -O2
uname output: Linux cc-dev-2 2.6.18-404.el5 #1 SMP Sat Mar 7 04:14:13 EST 2015 
x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-unknown-linux-gnu

Bash Version: 4.3
Patch Level: 30
Release Status: release

Description:
        I get a [[ ]] test failure with no error diagnostic, tricking me
        into thinking the test legitimately failed when the culprit was
        really a bad regex.

Repeat-By:
        Constrast:

        $ [[ foo =~ ) ]] && echo match || echo no match
        bash: syntax error in conditional expression: unexpected token `)'
        bash: syntax error near `)'

        $ REGEX=')'
        $ $ [[ foo =~ $REGEX ]] && echo match || echo no match
        no match

end