let help = "Syntax of the '-select' expressions (all parentheses are important):
(ids <id1> <id2> <id3> ...)
-> the items whose ids are <id1>, <id2>, ...
(and <expr1> <expr2> ...)
-> logical \"and\" between expressions
(or <expr1> <expr2> ...)
-> logical \"or\" between expressions
-> logical negation of an expression
(tags <tag1> <tag2> <tag3>)
-> look for the tags (it is an intersection, i.e. an \"and\")
(matches <field> <regexp>)
-> look if you find <regexp> in <field>
The regexp syntax is Perl-Compatible
-> the field is present
functionally equivalent to (matches (<field> \"\"))
(or (has bibtex) (and (has id) (has authors) (has title) (has how)))
-> selects entries which have a bibtex field, or at least, enough
information to generate a @misc BibTeX entry.
(matches title comp[a-z]+)
-> selects entries whose title field exists and matches the regexp
(e.g. \"The completion\" matches but \"The comp.\" does not).