The User Interface's "Search" box accepts predicates of the form "[-]operator:value[:value]".
These predicates may be separated by 'and' or 'or' keywords, or spaces which mean the same
as 'and'. Expressions like this may be grouped with parenthesis. Grouped expressions are 
evaluated first. Grouped expressions may be negated.
An 'and' besides an 'or' is evaluated first. This means for example that

tag:foo or is:pano tag:bar 

will return all images having tag foo together with the panorama images having tag bar.

Negation of a predicate is achieved by prepending a minus sign: -is:landscape will match
with pictures of not landscape ratio.

For example

    -(after:"2010-01-01" before:"2010-03-02T12:33:44") or loc:"Amsterdam" 

will return all images having "modtime" outside the specified period, joined with
all images taken in Amsterdam.

When you need to match a value containing a space, you need to use double quotes around
the value only. For example: tag:"Three word tagname" and not "tag:Three word tagname".
If your value contains double quotes you can use backslash escaping.
For example: attr:bar:"He said: \"Hi\"" 

Usable operators:
    after:         date format is RFC3339, but can be shortened as required.
    before:        i.e. 2011-01-01 is Jan 1 of year 2011 and "2011" means the same.
    attr:          match on attribute. Use attr:foo:bar to match nodes having their foo
                   attribute set to bar.
    format:        file's format (or MIME-type) such as jpg, pdf, tiff.
    has:location   image has a location (GPSLatitude and GPSLongitude can be
                   retrieved from the image's EXIF tags).
    loc:           uses the EXIF GPS fields to match images having a location near
                   the specified location.  Locations are resolved using
                   maps.googleapis.com. For example: loc:"new york, new york" 
    is:image       object is an image
    is:landscape   the image has a landscape aspect
    is:pano        the image's aspect ratio is over 2 - panorama picture.
    is:portrait    the image has a portrait aspect.
    height:        use height:min-max to match images having a height of at least min 
                   and at most max. Use height:min- to specify only an underbound and
                   height:-max to specify only an upperbound. 
                   Exact matches should use height:480
    tag:           match on a tag
    width:         use width:min-max to match images having a width of at least min 
                   and at most max. Use width:min- to specify only an underbound and
                   width:-max to specify only an upperbound.
                   Exact matches should use width:640
    childrenof:    Find child permanodes of a parent permanode (or prefix
                   of a parent permanode): childrenof:sha1-527cf12