An unserious BNF of the GNU sort command. Redundant separators are placed deliberately.

sort_bnf.txt

<key>       := <startf> <separator>*
                   (<dot> <separator>* <startc> <separator>*)?
                   (<option> <separator>*)*
               ((<comma> <separator>* <endf> <separator>*)?
                   (<dot> <separator>* <endc> <seprator>*)?
                   (<option> <separator>*)*
               )?

<startf>    := <num>+
<startc>    := <num>+
<endf>      := <num>+
<endc>      := <num>+
<option>    := ('r'|'n')
<num>       := ('0'|'1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9')
<separator> := (' '|'\t')
<dot>       := '.'
<comma>     := ','