Specification Version 1.1

BSON is a binary format in which zero or more ordered key/value pairs are stored as a single entity. We call this entity a document.

The following grammar specifies version 1.1 of the BSON standard. We've written the grammar using a pseudo-BNF syntax. Valid BSON data is represented by the document non-terminal.

Basic Types

The following basic types are used as terminals in the rest of the grammar. Each type must be serialized in little-endian format.

byte 1 byte (8-bits)
signed_byte(n) 8-bit, two's complement signed integer for which the value is n
unsigned_byte(n) 8-bit unsigned integer for which the value is n
int32 4 bytes (32-bit signed integer, two's complement)
int64 8 bytes (64-bit signed integer, two's complement)
uint64 8 bytes (64-bit unsigned integer)
double 8 bytes (64-bit IEEE 754-2008 binary floating point)
decimal128 16 bytes (128-bit IEEE 754-2008 decimal floating point)


The following specifies the rest of the BSON grammar. Note that we use the * operator as shorthand for repetition (e.g. (byte*2) is byte byte). When used as a unary operator, * means that the repetition can occur 0 or more times.

document ::= int32 e_list unsigned_byte(0) BSON Document. int32 is the total number of bytes comprising the document.
e_list ::= element e_list
| ""
element ::= signed_byte(1) e_name double 64-bit binary floating point
| signed_byte(2) e_name string UTF-8 string
| signed_byte(3) e_name document Embedded document
| signed_byte(4) e_name document Array
| signed_byte(5) e_name binary Binary data
| signed_byte(6) e_name Undefined (value) — Deprecated
| signed_byte(7) e_name (byte*12) ObjectId
| signed_byte(8) e_name unsigned_byte(0) Boolean - false
| signed_byte(8) e_name unsigned_byte(1) Boolean - true
| signed_byte(9) e_name int64 UTC datetime
| signed_byte(10) e_name Null value
| signed_byte(11) e_name cstring cstring Regular expression - The first cstring is the regex pattern, the second is the regex options string. Options are identified by characters, which must be stored in alphabetical order. Valid options are 'i' for case insensitive matching, 'm' for multiline matching, 'x' for verbose mode, 'l' to make \w, \W, etc. locale dependent, 's' for dotall mode ('.' matches everything), and 'u' to make \w, \W, etc. match unicode.
| signed_byte(12) e_name string (byte*12) DBPointer — Deprecated
| signed_byte(13) e_name string JavaScript code
| signed_byte(14) e_name string Symbol — Deprecated
| signed_byte(15) e_name code_w_s JavaScript code with scope — Deprecated
| signed_byte(16) e_name int32 32-bit integer
| signed_byte(17) e_name uint64 Timestamp
| signed_byte(18) e_name int64 64-bit integer
| signed_byte(19) e_name decimal128 128-bit decimal floating point
| signed_byte(-1) e_name Min key
| signed_byte(127) e_name Max key
e_name ::= cstring Key name
string ::= int32 (byte*) unsigned_byte(0) String - The int32 is the number bytes in the (byte*) plus one for the trailing null byte. The (byte*) is zero or more UTF-8 encoded characters.
cstring ::= (byte*) unsigned_byte(0) Zero or more modified UTF-8 encoded characters followed by the null byte. The (byte*) MUST NOT contain unsigned_byte(0), hence it is not full UTF-8.
binary ::= int32 subtype (byte*) Binary - The int32 is the number of bytes in the (byte*).
subtype ::= unsigned_byte(0) Generic binary subtype
| unsigned_byte(1) Function
| unsigned_byte(2) Binary (Old)
| unsigned_byte(3) UUID (Old)
| unsigned_byte(4) UUID
| unsigned_byte(5) MD5
| unsigned_byte(6) Encrypted BSON value
| unsigned_byte(7) Compressed BSON column
| unsigned_byte(8) Sensitive
| unsigned_byte(128)—unsigned_byte(255) User defined
code_w_s ::= int32 string document Code with scope — Deprecated