Lojban.

text =
    [NAI ...] [CMENE ... # | (indicators & free ...)] [joik-jek] text-1
text-1 =
    [(I [jek | joik] [[stag] BO] #) ... | NIhO ... #] [paragraphs]
paragraphs =
    paragraph [NIhO ... # paragraphs]
paragraph =
    (statement | fragment) [I # [statement | fragment]] ...
statement =
    statement-1
    | prenex statement
statement-1 =
    statement-2 [I joik-jek [statement-2]] ...
statement-2 =
    statement-3 [I [jek | joik] [stag] BO # [statement-2]]
statement-3 =
    sentence
    | [tag] TUhE # text-1 /TUhU#/
fragment =
    ek #
    | gihek #
    | quantifier
    | NA #
    | terms /VAU#/
    | prenex
    | relative-clauses
    | links
    | linkargs
prenex =
    terms ZOhU #
sentence =
    [terms [CU #]] bridi-tail
subsentence =
    sentence
    | prenex subsentence
bridi-tail =
    bridi-tail-1 [gihek [stag] KE # bridi-tail /KEhE#/ tail-terms]
bridi-tail-1 =
    bridi-tail-2 [gihek # bridi-tail-2 tail-terms] ...
bridi-tail-2 =
    bridi-tail-3 [gihek [stag] BO # bridi-tail-2 tail-terms]
bridi-tail-3 =
    selbri tail-terms
    | gek-sentence
gek-sentence =
    gek subsentence gik subsentence tail-terms
    | [tag] KE # gek-sentence /KEhE#/
    | NA # gek-sentence
tail-terms =
    [terms] /VAU#/
terms =
    terms-1 ...
terms-1 =
    terms-2 [PEhE # joik-jek terms-2] ...
terms-2 =
    term [CEhE # term] ...
term =
    sumti
    | (tag | FA #) (sumti | /KU#/)
    | termset
    | NA KU #
termset =
    NUhI # gek terms /NUhU#/ gik terms /NUhU#/
    | NUhI # terms /NUhU#/
sumti =
    sumti-1 [VUhO # relative-clauses]
sumti-1 =
    sumti-2 [(ek | joik) [stag] KE # sumti /KEhE#/]
sumti-2 =
    sumti-3 [joik-ek sumti-3] ...
sumti-3 =
    sumti-4 [(ek | joik) [stag] BO # sumti-3]
sumti-4 =
    sumti-5
    | gek sumti gik sumti-4
sumti-5 =
    [quantifier] sumti-6 [relative-clauses]
    | quantifier selbri /KU#/ [relative-clauses]
sumti-6 =
    (LAhE # | NAhE BO #) [relative-clauses] sumti /LUhU#/
    | KOhA #
    | lerfu-string /BOI#/
    | LA # [relative-clauses] CMENE ... #
    | (LA | LE) # sumti-tail /KU#/
    | LI # mex /LOhO#/
    | ZO any-word #
    | LU text /LIhU#/
    | LOhU any-word ... LEhU #
    | ZOI any-word anything any-word #
sumti-tail =
    [sumti-6 [relative-clauses]] sumti-tail-1
    | relative-clauses sumti-tail-1
sumti-tail-1 =
    [quantifier] selbri [relative-clauses]
    | quantifier sumti
relative-clauses =
    relative-clause [ZIhE # relative-clause] ...
relative-clause =
    GOI # term /GEhU#/
    | NOI # subsentence /KUhO#/
selbri =
    [tag] selbri-1
selbri-1 =
    selbri-2
    | NA # selbri
selbri-2 =
    selbri-3 [CO # selbri-2]
selbri-3 =
    selbri-4 ...
selbri-4 =
    selbri-5 [joik-jek selbri-5 | joik [stag] KE # selbri-3 /KEhE#/] ...
selbri-5 =
    selbri-6 [(jek | joik) [stag] BO # selbri-5]
selbri-6 =
    tanru-unit [BO # selbri-6]
    | [NAhE #] guhek selbri gik selbri-6
tanru-unit =
    tanru-unit-1 [CEI # tanru-unit-1] ...
tanru-unit-1 =
    tanru-unit-2 [linkargs]
tanru-unit-2 =
    BRIVLA #
    | GOhA [RAhO] #
    | KE # selbri-3 /KEhE#/
    | ME # sumti /MEhU#/ [MOI #]
    | (number | lerfu-string) MOI #
    | NUhA # mex-operator
    | SE # tanru-unit-2
    | JAI # [tag] tanru-unit-2
    | any-word (ZEI any-word) ...
    | NAhE # tanru-unit-2
    | NU [NAI] # [joik-jek NU [NAI] #] ... subsentence /KEI#/
linkargs =
    BE # term [links] /BEhO#/
links =
    BEI # term [links]
quantifier =
    number /BOI#/
    | VEI # mex /VEhO#/
mex =
    mex-1 [operator mex-1] ...
    | FUhA # rp-expression
mex-1 =
    mex-2 [BIhE # operator mex-1]
mex-2 =
    operand
    | [PEhO #] operator mex-2 ... /KUhE#/
rp-expression =
    rp-operand rp-operand operator
rp-operand =
    operand
    | rp-expression
operator =
    operator-1 [joik-jek operator-1 | joik [stag] KE # operator /KEhE#/] ...
operator-1 =
    operator-2
    | guhek operator-1 gik operator-2
    | operator-2 (jek | joik) [stag] BO # operator-1
operator-2 =
    mex-operator
    | KE # operator /KEhE#/
mex-operator =
    SE # mex-operator
    | NAhE # mex-operator
    | MAhO # mex /TEhU#/
    | NAhU # selbri /TEhU#/
    | VUhU #
operand =
    operand-1 [(ek | joik) [stag] KE # operand /KEhE#/]
operand-1 =
    operand-2 [joik-ek operand-2] ...
operand-2 =
    operand-3 [(ek | joik) [stag] BO # operand-2]
operand-3 =
    quantifier
    | lerfu-string /BOI#/
    | NIhE # selbri /TEhU#/
    | MOhE # sumti /TEhU#/
    | JOhI # mex-2 ... /TEhU#/
    | gek operand gik operand-3
    | (LAhE # | NAhE BO #) operand /LUhU#/
number =
    PA [PA | lerfu-word] ...
lerfu-string =
    lerfu-word [PA | lerfu-word] ...
lerfu-word =
    BY
    | any-word BU
    | LAU lerfu-word
    | TEI lerfu-string FOI
ek =
    [NA] [SE] A [NAI]
gihek =
    [NA] [SE] GIhA [NAI]
jek =
    [NA] [SE] JA [NAI]
joik =
    [SE] JOI [NAI]
    | interval
    | GAhO interval GAhO
interval =
    [SE] BIhI [NAI]
joik-ek =
    joik #
    | ek #
joik-jek =
    joik #
    | jek #
gek =
    [SE] GA [NAI] #
    | joik GI #
    | stag gik
guhek =
    [SE] GUhA [NAI] #
gik =
    GI [NAI] #
tag =
    tense-modal [joik-jek tense-modal] ...
stag =
    simple-tense-modal [(jek | joik) simple-tense-modal] ...
tense-modal =
    simple-tense-modal #
    | FIhO # selbri /FEhU#/
simple-tense-modal =
    [NAhE] [SE] BAI [NAI] [KI]
    | [NAhE] (time [space] | space [time]) & CAhA [KI]
    | KI
    | CUhE
time =
    ZI & time-offset ... & ZEhA [PU [NAI]] & interval-property ...
time-offset =
    PU [NAI] [ZI]
space =
    VA & space-offset ... & space-interval & (MOhI space-offset)
space-offset =
    FAhA [NAI] [VA]
space-interval =
    ((VEhA & VIhA) [FAhA [NAI]]) & space-int-props
space-int-props =
    (FEhE interval-property) ...
interval-property =
    number ROI [NAI]
    | TAhE [NAI]
    | ZAhO [NAI]
free =
    SEI # [terms [CU #]] selbri /SEhU/
    | SOI # sumti [sumti] /SEhU/
    | vocative [relative-clauses] selbri [relative-clauses] /DOhU/
    | vocative [relative-clauses] CMENE ... # [relative-clauses] /DOhU/
    | vocative [sumti] /DOhU/
    | (number | lerfu-string) MAI
    | TO text /TOI/
    | XI # (number | lerfu-string) /BOI/
    | XI # VEI # mex /VEhO/
vocative =
    (COI [NAI]) ... & DOI
indicators =
    [FUhE] indicator ...
indicator =
    (UI | CAI) [NAI]
    | Y
    | DAhO
    | FUhO