Statistics collection question

A couple of questions about the "most_common_vals" stuff in pg_stats
for a high traffic table:

1. Can I tell the stats collector to collect only values of a column
where a certain regex is matched? It is currently collecting the 500
values where most of them are values that I don't want, so it's
polluted with unwanted vals and therefore useless.

2. Secondly, for a unique column in the table, will the
"most_common_vals" always be -1? I guess this could make sense, but I
was wondering if the stats collector could somehow collect at least
1000 unique values to improve at least some performance.

TIA!