liquid simplified peg from https://github.com/edgurgel/solid/blob/master/src/solid_parser.peg (erlang, works with https://github.com/seancribbs/neotoma)

liquid.peg.erl

text   <- string:blob (object:object text:text
                       / tag:tag text:text
                     % / open_tag:open_tag text:text
                       / open_object:open_object text:text)?

blob   <- (!open_object !open_tag .)*

object <- open_object:open_object space (argument:argument filters:filters / argument:argument)? space close_object:close_object

tag <- cond_if_tag / cond_unless_tag / cond_case_tag / assign_tag / comment_tag

comment_tag <- open_tag space comment space close_tag text:text
               open_tag space endcomment space close_tag

assign_tag <- open_tag space assign space field space "=" space argument space close_tag

cond_case_tag <- case_tag
                 when_tag+
                 else_tag?
                 open_tag space endcase space close_tag

case_tag <- open_tag space case space argument space close_tag text:text

when_tag <- open_tag space when value close_tag text:text

cond_if_tag <- if_tag
               elsif_tag*
               else_tag?
               open_tag space endif space close_tag

if_tag <- open_tag space if expression:boolean_expression close_tag text:text

elsif_tag <- open_tag space elsif expression:boolean_expression close_tag text:text

else_tag <- (open_tag space else space close_tag text:text)

unless_tag <- open_tag space unless expression:boolean_expression close_tag text:text

cond_unless_tag <- unless_tag
                   elsif_tag*
                   else_tag?
                   open_tag space endunless space close_tag

expression <- space (argument space operator space argument / boolean) space

boolean_expression <- expression ((and / or) expression)*

filters <- (space "|" space filter (":" space arguments)? )+

arguments <- argument (space "," space argument)*

argument <- value:value / field

field    <- [0-9a-zA-Z\._]* access*

access <- "[" int "]"

value <- space (string / number / true / false / null) space

string <- single_quoted_string / double_quoted_string

operator       <- "==" / "!=" / ">=" / "<=" / ">" / "<" / "contains"
open_object    <- "{{"
close_object   <- "}}"
open_tag       <-  "{%"
close_tag      <-  "%}"
if             <- "if"
else           <- "else"
elsif          <- "elsif"
endif          <- "endif"
unless         <- "unless"
endunless      <- "endunless"
case           <- "case"
when           <- "when"
endcase        <- "endcase"
assign         <- "assign"
comment        <- "comment"
endcomment     <- "endcomment"
filter         <- [a-zA-Z_]*
space          <- [ \t\n\s\r]*

double_quoted_string <- '"' string:(!'"' ("\\\\" / '\\"' / .))* '"'
single_quoted_string <- "'" string:(!"'" ("\\\\" / "\\'" / .))* "'"