!calc PEG

expr            <- subexpr
subexpr         <- operation | subexpr_notop
subexpr_notop   <- function | '(' subexpr ')' | literal
literal         <- decimal | hexadecimal | 'true' | 'false'
decimal         <- [0-9]+ ( '.' [0-9]* )?
hexadecimal     <- '0x' [0-9a-fA-F]+
operation       <- ( unary_operator subexpr ) | ( subexpr_notop binary_operator subexpr )
binary_operator <- '+' | '-' | '*' | '/' | '%' | '^' | '&' | '|' | '==' '='? | '!=' '='? | '<' | '>' | '<=' | '>='
unary_operator  <- '-' | '~'
function        <- function_name '(' ( subexpr ( ',' subexpr )* )? ')'
function_name   <- [a-z][a-z0-9]*