Author: drkrtz
[Added to the original trac issue at 10.03pm, Monday, 16th September 2013]

*Capitol city. Sorry for the spelling error. :)

Author: lonvia
[Added to the original trac issue at 10.21pm, Monday, 16th September 2013]

You seem to have found the one variant that does not work. ;) "washington d.c." works fine, as does "washington dc" and "washington, dc".

Basically, Nominatim does not know that "d.c." is an abbreviation for the [http://nominatim.openstreetmap.org/details.php?place_id=98170241 District of Columbia] as there is only "DC" given as reference. It should be a bit more clever in handling these optional full stops, might be doable with some regex magic in the normalization stage.

Author: drkrtz
[Added to the original trac issue at 10.44pm, Monday, 16th September 2013]

Can "Wasghington, D.C." be added as an alternate name for the node? Short of spelling out "Washington, Disctrict of Columbia", this really is the most accurate spelling so we probably shouldn't rely on regex magic to return the correct result.
http://www.openstreetmap.org/browse/node/158368533

Looking at the node details, it seems like all the other punctuation variants are captured by various language translations. Perhaps simply changing one of the translation results to "Wasghington, D.C.", or updating the stated "alt_name" to this spelling, will fix the issue.

I've added a note to the map as well. Feel free to delete/resolve along with this bug report.
http://www.openstreetmap.org/?note=43163

Best,
Derek

Author: lonvia
[Added to the original trac issue at 6.15am, Tuesday, 17th September 2013]

This will unfortunately not work either because of bug #4449 (no commas allowed in names) that is more or less a wontfix. You might get away with adding alt_name=D.C. to the District of Columbia.