$substr
Definition
Extracts a substring using UTF-16 indexes.
Expression operator · Text · Studio Visual: available. Studio validates the complete expression separately.
Syntax
{ "$substr": [ <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 — $substr expects (string, int, int) |
| missing | Field absent | Error — $substr expects (string, int, int) |
| decimal | 2.5 | Error — $substr expects (string, int, int) |
| numeric string | "12" | "2" |
| text | "hello" | "ell" |
| boolean | true | Error — $substr expects (string, int, int) |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — $substr expects (string, int, int) |
| empty array | [] | Error — $substr expects (string, int, int) |
| numeric array | [1,2,3] | Error — $substr expects (string, int, int) |
| object | {"x":1} | Error — $substr expects (string, int, int) |
Argument handling
| Invocation | Result |
|---|---|
| {"$substr":[]} | Error — $substr requires [string, start, length] |
| {"$substr":{"unexpected":1}} | Error — $substr requires [string, start, length] |
Examples
Expression:
{
"$substr": [
"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 — $substr expects (string, int, int) |
| missing | Field absent | Error — $substr expects (string, int, int) |
| zero | 0 | Error — $substr expects (string, int, int) |
| negative | -2 | Error — $substr expects (string, int, int) |
| decimal | 2.5 | Error — $substr expects (string, int, int) |
| numeric string | "12" | "2" |
| text | "hello" | "ell" |
| empty string | "" | "" |
| boolean | true | Error — $substr expects (string, int, int) |
| false | false | Error — $substr expects (string, int, int) |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — $substr expects (string, int, int) |
| decimal128 | {"$numberDecimal":"1.25"} | Error — $substr expects (string, int, int) |
| empty array | [] | Error — $substr expects (string, int, int) |
| numeric array | [1,2,3] | Error — $substr expects (string, int, int) |
| mixed array | [1,null,"x",3] | Error — $substr expects (string, int, int) |
| nested array | [[1,2],[3]] | Error — $substr expects (string, int, int) |
| empty object | {} | Error — $substr expects (string, int, int) |
| object | {"x":1} | Error — $substr 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 — $substr expects (string, int, int) |
| missing | Field absent | Error — $substr expects (string, int, int) |
| zero | 0 | "hel" |
| negative | -2 | "" |
| decimal | 2.5 | "llo" |
| numeric string | "12" | Error — $substr expects (string, int, int) |
| text | "hello" | Error — $substr expects (string, int, int) |
| empty string | "" | Error — $substr expects (string, int, int) |
| boolean | true | Error — $substr expects (string, int, int) |
| false | false | Error — $substr expects (string, int, int) |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — $substr expects (string, int, int) |
| decimal128 | {"$numberDecimal":"1.25"} | "ell" |
| empty array | [] | Error — $substr expects (string, int, int) |
| numeric array | [1,2,3] | Error — $substr expects (string, int, int) |
| mixed array | [1,null,"x",3] | Error — $substr expects (string, int, int) |
| nested array | [[1,2],[3]] | Error — $substr expects (string, int, int) |
| empty object | {} | Error — $substr expects (string, int, int) |
| object | {"x":1} | Error — $substr expects (string, int, int) |
| ISO date string | "2026-01-15T15:00:00Z" | Error — $substr expects (string, int, int) |
Argument: length[2]
| Input case | Supplied value | SrotaX result |
|---|---|---|
| null | null | Error — $substr expects (string, int, int) |
| missing | Field absent | Error — $substr expects (string, int, int) |
| zero | 0 | "" |
| negative | -2 | "" |
| decimal | 2.5 | "el" |
| numeric string | "12" | Error — $substr expects (string, int, int) |
| text | "hello" | Error — $substr expects (string, int, int) |
| empty string | "" | Error — $substr expects (string, int, int) |
| boolean | true | Error — $substr expects (string, int, int) |
| false | false | Error — $substr expects (string, int, int) |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — $substr expects (string, int, int) |
| decimal128 | {"$numberDecimal":"1.25"} | "e" |
| empty array | [] | Error — $substr expects (string, int, int) |
| numeric array | [1,2,3] | Error — $substr expects (string, int, int) |
| mixed array | [1,null,"x",3] | Error — $substr expects (string, int, int) |
| nested array | [[1,2],[3]] | Error — $substr expects (string, int, int) |
| empty object | {} | Error — $substr expects (string, int, int) |
| object | {"x":1} | Error — $substr expects (string, int, int) |
| ISO date string | "2026-01-15T15:00:00Z" | Error — $substr 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 — $substr expects (string, int, int) | "" |
| input[0] / missing | Error — $substr expects (string, int, int) | "" |
| input[0] / zero | Error — $substr expects (string, int, int) | "" |
| input[0] / negative | Error — $substr expects (string, int, int) | "2" |
| input[0] / decimal | Error — $substr expects (string, int, int) | ".5" |
| input[0] / typed date | Error — $substr expects (string, int, int) | "026" |
| input[0] / decimal128 | Error — $substr expects (string, int, int) | ".25" |
| start[1] / negative | "" | Error — Executor error during aggregate command on namespace: fluxion_operator_parity_disposable.$cmd.aggregate :: caused by :: $substrBytes: starting index must be non-negative (got: -2) |
| length[2] / negative | "" | "ello" |