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$substrBytes

Definition

Extracts text from a UTF-8 byte range.

Expression operator · Text · Studio Visual: available. Studio validates the complete expression separately.

Syntax

{ "$substrBytes": [ <input>, <start>, <length> ] }

Replace angle-bracket placeholders with expressions or values.

Optional parameters may be omitted.

Parameters

Parameter Required Description
input Yes Expression supplying the value to process. A field reference such as "$price" reads from the current document.
start Yes Starting position or lower bound. See the syntax and examples for this operator.
length Yes Number of bytes or characters to return, depending on the operator.

Arguments are positional: keep the order shown. The authoring signature accepts 3–3 arguments.

Behavior

The following observations use the example expression below. Each row substitutes one argument while retaining the other example arguments. A missing field differs from an explicit null; the result shown is what Core actually returned.

For the input argument:

Input case Supplied value Result
null null Error — $substrBytes expects (string, int, int)
missing Field absent Error — $substrBytes expects (string, int, int)
decimal 2.5 Error — $substrBytes expects (string, int, int)
numeric string "12" "2"
text "hello" "ell"
boolean true Error — $substrBytes expects (string, int, int)
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $substrBytes expects (string, int, int)
empty array [] Error — $substrBytes expects (string, int, int)
numeric array [1,2,3] Error — $substrBytes expects (string, int, int)
object {"x":1} Error — $substrBytes expects (string, int, int)

Argument handling

Invocation Result
{"$substrBytes":[]} Error — $substrBytes requires [string, start, length]
{"$substrBytes":{"unexpected":1}} Error — $substrBytes requires [string, start, length]

Examples

Expression: 

{
  "$substrBytes": [
    "hello",
    1,
    3
  ]
}

Input document: 

{}

Observed SrotaX result: "ell".

Detailed input cases

Expand the complete argument-by-argument test matrix

Each table changes only the indicated argument from the example above. Other arguments retain their baseline values. Values normally come from the input field probe; literal configuration arguments are replaced directly. “Missing” means the field is absent, not explicitly null. Numeric strings are strings, not numbers.

Argument: input[0]

Input case Supplied value SrotaX result
null null Error — $substrBytes expects (string, int, int)
missing Field absent Error — $substrBytes expects (string, int, int)
zero 0 Error — $substrBytes expects (string, int, int)
negative -2 Error — $substrBytes expects (string, int, int)
decimal 2.5 Error — $substrBytes expects (string, int, int)
numeric string "12" "2"
text "hello" "ell"
empty string "" ""
boolean true Error — $substrBytes expects (string, int, int)
false false Error — $substrBytes expects (string, int, int)
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $substrBytes expects (string, int, int)
decimal128 {"$numberDecimal":"1.25"} Error — $substrBytes expects (string, int, int)
empty array [] Error — $substrBytes expects (string, int, int)
numeric array [1,2,3] Error — $substrBytes expects (string, int, int)
mixed array [1,null,"x",3] Error — $substrBytes expects (string, int, int)
nested array [[1,2],[3]] Error — $substrBytes expects (string, int, int)
empty object {} Error — $substrBytes expects (string, int, int)
object {"x":1} Error — $substrBytes expects (string, int, int)
ISO date string "2026-01-15T15:00:00Z" "026"

Argument: start[1]

Input case Supplied value SrotaX result
null null Error — $substrBytes expects (string, int, int)
missing Field absent Error — $substrBytes expects (string, int, int)
zero 0 "hel"
negative -2 ""
decimal 2.5 "llo"
numeric string "12" Error — $substrBytes expects (string, int, int)
text "hello" Error — $substrBytes expects (string, int, int)
empty string "" Error — $substrBytes expects (string, int, int)
boolean true Error — $substrBytes expects (string, int, int)
false false Error — $substrBytes expects (string, int, int)
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $substrBytes expects (string, int, int)
decimal128 {"$numberDecimal":"1.25"} "ell"
empty array [] Error — $substrBytes expects (string, int, int)
numeric array [1,2,3] Error — $substrBytes expects (string, int, int)
mixed array [1,null,"x",3] Error — $substrBytes expects (string, int, int)
nested array [[1,2],[3]] Error — $substrBytes expects (string, int, int)
empty object {} Error — $substrBytes expects (string, int, int)
object {"x":1} Error — $substrBytes expects (string, int, int)
ISO date string "2026-01-15T15:00:00Z" Error — $substrBytes expects (string, int, int)

Argument: length[2]

Input case Supplied value SrotaX result
null null Error — $substrBytes expects (string, int, int)
missing Field absent Error — $substrBytes expects (string, int, int)
zero 0 ""
negative -2 ""
decimal 2.5 "el"
numeric string "12" Error — $substrBytes expects (string, int, int)
text "hello" Error — $substrBytes expects (string, int, int)
empty string "" Error — $substrBytes expects (string, int, int)
boolean true Error — $substrBytes expects (string, int, int)
false false Error — $substrBytes expects (string, int, int)
typed date {"$date":"2026-01-15T15:00:00Z"} Error — $substrBytes expects (string, int, int)
decimal128 {"$numberDecimal":"1.25"} "e"
empty array [] Error — $substrBytes expects (string, int, int)
numeric array [1,2,3] Error — $substrBytes expects (string, int, int)
mixed array [1,null,"x",3] Error — $substrBytes expects (string, int, int)
nested array [[1,2],[3]] Error — $substrBytes expects (string, int, int)
empty object {} Error — $substrBytes expects (string, int, int)
object {"x":1} Error — $substrBytes expects (string, int, int)
ISO date string "2026-01-15T15:00:00Z" Error — $substrBytes expects (string, int, int)

Verification

This page records 60 executed cases. It covers individual argument substitutions, not every cross-product of arguments, locale, daylight-saving transitions, numeric overflow, collation or resource limits. No untested behavior is guaranteed.

Download exact expressions, inputs and SrotaX results. Errors are observed messages, not stable public error codes. The reference generator reads the checked-in execution report; do not edit generated observations by hand.

MongoDB differences

Compared with MongoDB 9.0.4: 11 exact BSON matches, 40 both rejected, 9 value/type/acceptance differences, 0 object-field-order differences.

“Exact” includes numeric BSON types and object key order. A value of 3 as int32 differs from 3.0 as double. Object-field-order differences are reported separately; object key ordering is not enforced by the engine regression baseline. Both rejected does not mean the error codes or messages match. This is finite-case evidence, not full compatibility certification.

Show observed differences
Tested case SrotaX MongoDB
input[0] / null Error — $substrBytes expects (string, int, int) ""
input[0] / missing Error — $substrBytes expects (string, int, int) ""
input[0] / zero Error — $substrBytes expects (string, int, int) ""
input[0] / negative Error — $substrBytes expects (string, int, int) "2"
input[0] / decimal Error — $substrBytes expects (string, int, int) ".5"
input[0] / typed date Error — $substrBytes expects (string, int, int) "026"
input[0] / decimal128 Error — $substrBytes expects (string, int, int) ".25"
start[1] / negative "" Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $substrBytes: starting index must be non-negative (got: -2)
length[2] / negative "" "ello"

Download full comparison evidence.