Proto ebnf definition from https://developers.google.com/protocol-buffers/docs/reference/proto3-spec

proto3.ebnf

letter = "A" "B" "C" "D" "E" "F" "G" "H" "I" "J" "K" "L" "M" "N" "O" "P" "Q" "R" "S" "T" "U" "V" "W" "X" "Y" "Z"  | "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z"
decimalDigit = "0" "1" "2" "3" "4" "5" "6" "7" "8" "9"
octalDigit   = "0" "1" "2" "3" "4" "5" "6" "7"
hexDigit     = "0" "1" "2" "3" "4" "5" "6" "7" "8" "9" | "A" "B" "C" "D" "E" "F" "G" "H" "I" "J" "K" "L" "M" "N" "O" "P" "Q" "R" "S" "T" "U" "V" "W" "X" "Y" "Z"  | "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z"

ident = letter { letter | decimalDigit | "_" }
fullIdent = ident { "." ident }
messageName = ident
enumName = ident
fieldName = ident
oneofName = ident
mapName = ident
serviceName = ident
rpcName = ident
messageType = [ "." ] { ident "." } messageName
enumType = [ "." ] { ident "." } enumName

intLit     = decimalLit | octalLit | hexLit
decimalLit = ( "1" "2" "3" "4" "5" "6" "7" "8" "9" ) { decimalDigit }
octalLit   = "0" { octalDigit }
hexLit     = "0" ( "x" | "X" ) hexDigit { hexDigit } 

floatLit = ( decimals "." [ decimals ] [ exponent ] | decimals exponent | "."decimals [ exponent ] ) | "inf" | "nan"
decimals  = decimalDigit { decimalDigit }
exponent  = ( "e" | "E" ) [ "+" | "-" ] decimals

boolLit = "true" | "false" 

strLit = ( "'" { charValue } "'" ) |  ( '"' { charValue } '"' )
charValue = hexEscape | octEscape | charEscape |  #"[0-9]"
hexEscape = '\\' ( "x" | "X" ) hexDigit hexDigit
octEscape = '\\' octalDigit octalDigit octalDigit
charEscape = '\\' ( "a" | "b" | "f" | "n" | "r" | "t" | "v" | '\\' | "'" | '"' )
quote = "'" | '"'

emptyStatement = ";"

constant = fullIdent | ( [ "-" | "+" ] intLit ) | ( [ "-" | "+" ] floatLit ) | strLit | boolLit 

syntax = "syntax" "=" quote "proto3" quote ";"

import = "import" [ "weak" | "public" ] strLit ";" 

package = "package" fullIdent ";"

option = "option" optionName  "=" constant ";"
optionName = ( ident | "(" fullIdent ")" ) { "." ident }

type = "double" | "float" | "int32" | "int64" | "uint32" | "uint64"
      | "sint32" | "sint64" | "fixed32" | "fixed64" | "sfixed32" | "sfixed64"
      | "bool" | "string" | "bytes" | messageType | enumType
fieldNumber = intLit;

field = [ "repeated" ] type fieldName "=" fieldNumber [ "[" fieldOptions "]" ] ";"
fieldOptions = fieldOption { ","  fieldOption }
fieldOption = optionName "=" constant


oneof = "oneof" oneofName "{" { oneofField | emptyStatement } "}"
oneofField = type fieldName "=" fieldNumber [ "[" fieldOptions "]" ] ";"


mapField = "map" "<" keyType "," type ">" mapName "=" fieldNumber [ "[" fieldOptions "]" ] ";"
keyType = "int32" | "int64" | "uint32" | "uint64" | "sint32" | "sint64" |
          "fixed32" | "fixed64" | "sfixed32" | "sfixed64" | "bool" | "string"

reserved = "reserved" ( ranges | fieldNames ) ";"
ranges = range { "," range }
range =  intLit [ "to" ( intLit | "max" ) ]
fieldNames = fieldName { "," fieldName }

enum = "enum" enumName enumBody
enumBody = "{" { option | enumField | emptyStatement } "}"
enumField = ident "=" intLit [ "[" enumValueOption { ","  enumValueOption } "]" ]";"
enumValueOption = optionName "=" constant

message = "message" messageName messageBody
messageBody = "{" { field | enum | message | option | oneof | mapField |
reserved | emptyStatement } "}"


service = "service" serviceName "{" { option | rpc | emptyStatement } "}"
rpc = "rpc" rpcName "(" [ "stream" ] messageType ")" "returns" "(" [ "stream" ]
messageType ")" (( "{" {option | emptyStatement } "}" ) | ";")

proto = syntax { import | package | option | topLevelDef | emptyStatement }
topLevelDef = message | enum | service