Only one of the various possible syntaxes for backrefs within a regex seem to be working.
should in theory find all forms consisting of the same character twice. However, it's not working. Neither are the following, which according to man pcre should be equivalent:
The following, however, DOES work, even though it SHOULD be identical to the preceding:
I suspect this is due to the regex optimiser and its lack of full PCRE-awareness (in cl/regopt.c) -- i.e. it is doing an incorrect optimisation and doing simple string matching on the first three but not on the fourth -- but cannot be sure without further investigation.