Language: 

CHAR = any character accepted by filesystem and not used in expressions, case sensitive. 
CHARS = CHAR{CHARS} 
WILDCARD = "#?" | "?" | "*" 
SEGMENT = [CHARS] [WILDCARD] [CHARS] | "*" 
PATH = [DOS path | virtual path | assigned path | device path | media path | DB:\ ] 
SIMPLE = [PATH] SEGMENT 

OPERATOR = "-" | "+" | "&" 
NEGATOR = "~" 
COMPLEX = SIMPLE | [NEGATOR] (EXPRESSION {OPERATOR COMPLEX}) 

EXPRESSION = SIMPLE, COMPLEX, or SIMPLE | COMPLEX depending on location. 

where 

"?" = CHAR 
"#?" = [CHARS] 
"*" = [CHARS] 

Examples: 

DB:\* - all files known by M3, regardless of location 
E:\images\* - contents of actual or virtual directory E:\images \
(E:\images\*) - (E:\images\*.gif) - all files not ending with .gif at E:\images\
!(E:\images\*.gif) - all files not ending with .gif at E:\images\