Fix RS_isRegis() to agree exactly with RS_compile()'s idea of what's a valid

Fix RS_isRegis() to agree exactly with RS_compile()'s idea of what's a valid
regis.  Correct the latter's oversight that a bracket-expression needs to be
terminated.  Reduce the ereports to elogs, since they are now not expected to
ever be hit (thus addressing Alvaro's original complaint).
In passing, const-ify the string argument to RS_compile.