$regexFind
Definition
Finds the first regex match and its UTF-16 index.
Expression operator · Text · Studio Visual: available. Studio validates the complete expression separately.
Syntax
{
"$regexFind": {
"input": <input>,
"regex": <regex>,
"options": <options>
}
}
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 "$price" reads from the current document. |
regex |
Yes | Regular-expression pattern to search for. |
options |
No | Regex option string. See Supported values below. |
Supported values
options accepts any combination of i (case-insensitive), m (multiline anchors), s (dot matches newlines), and x (ignore pattern whitespace/comments). Omit it or use "" for no flags. Patterns use Java regular-expression behavior.
Behavior
The following observations use the example expression below. Each row substitutes one argument while retaining the other example arguments. A missing field differs from an explicit null; the result shown is what Core actually returned.
For the input argument:
| Input case | Supplied value | Result |
|---|---|---|
| null | null | Error — Cannot invoke "java.lang.CharSequence.length()" because "this.text" is null |
| missing | Field absent | Error — Cannot invoke "java.lang.CharSequence.length()" because "this.text" is null |
| decimal | 2.5 | Error — class java.lang.Double cannot be cast to class java.lang.String (java.lang.Double and java.lang.String are in module java.base of loader 'bootstrap') |
| numeric string | "12" | null |
| text | "hello" | null |
| boolean | true | Error — class java.lang.Boolean cannot be cast to class java.lang.String (java.lang.Boolean and java.lang.String are in module java.base of loader 'bootstrap') |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — class java.util.Date cannot be cast to class java.lang.String (java.util.Date and java.lang.String are in module java.base of loader 'bootstrap') |
| empty array | [] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| numeric array | [1,2,3] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| object | {"x":1} | Error — class org.bson.Document cannot be cast to class java.lang.String (org.bson.Document is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstr |
Argument handling
| Invocation | Result |
|---|---|
| {"$regexFind":[]} | Error — class java.util.ArrayList cannot be cast to class java.util.Map (java.util.ArrayList and java.util.Map are in module java.base of loader 'bootstrap') |
| {"$regexFind":{"unexpected":1}} | Error — Cannot invoke "String.isEmpty()" because "this.pattern" is null |
Examples
Expression:
{
"$regexFind": {
"input": "abc",
"regex": "ab"
}
}
Input document:
{}
Observed SrotaX result: {"idx":0,"captures":[],"match":"ab"}.
Detailed input cases
Expand the complete argument-by-argument test matrix
Each table changes only the indicated argument from the example above. Other arguments retain their baseline values. Values normally come from the input field probe; literal configuration arguments are replaced directly. “Missing” means the field is absent, not explicitly null. Numeric strings are strings, not numbers.
Argument: input
| Input case | Supplied value | SrotaX result |
|---|---|---|
| null | null | Error — Cannot invoke "java.lang.CharSequence.length()" because "this.text" is null |
| missing | Field absent | Error — Cannot invoke "java.lang.CharSequence.length()" because "this.text" is null |
| zero | 0 | Error — class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap') |
| negative | -2 | Error — class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap') |
| decimal | 2.5 | Error — class java.lang.Double cannot be cast to class java.lang.String (java.lang.Double and java.lang.String are in module java.base of loader 'bootstrap') |
| numeric string | "12" | null |
| text | "hello" | null |
| empty string | "" | null |
| boolean | true | Error — class java.lang.Boolean cannot be cast to class java.lang.String (java.lang.Boolean and java.lang.String are in module java.base of loader 'bootstrap') |
| false | false | Error — class java.lang.Boolean cannot be cast to class java.lang.String (java.lang.Boolean and java.lang.String are in module java.base of loader 'bootstrap') |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — class java.util.Date cannot be cast to class java.lang.String (java.util.Date and java.lang.String are in module java.base of loader 'bootstrap') |
| decimal128 | {"$numberDecimal":"1.25"} | Error — class org.bson.types.Decimal128 cannot be cast to class java.lang.String (org.bson.types.Decimal128 is in unnamed module of loader 'app'; java.lang.String is in module java.base of |
| empty array | [] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| numeric array | [1,2,3] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| mixed array | [1,null,"x",3] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| nested array | [[1,2],[3]] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| empty object | {} | Error — class org.bson.Document cannot be cast to class java.lang.String (org.bson.Document is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstr |
| object | {"x":1} | Error — class org.bson.Document cannot be cast to class java.lang.String (org.bson.Document is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstr |
| ISO date string | "2026-01-15T15:00:00Z" | null |
| omitted named argument | Argument removed | Error — Cannot invoke "java.lang.CharSequence.length()" because "this.text" is null |
Argument: regex
| Input case | Supplied value | SrotaX result |
|---|---|---|
| null | null | Error — Cannot invoke "String.isEmpty()" because "this.pattern" is null |
| missing | Field absent | Error — Cannot invoke "String.isEmpty()" because "this.pattern" is null |
| zero | 0 | Error — class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap') |
| negative | -2 | Error — class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap') |
| decimal | 2.5 | Error — class java.lang.Double cannot be cast to class java.lang.String (java.lang.Double and java.lang.String are in module java.base of loader 'bootstrap') |
| numeric string | "12" | null |
| text | "hello" | null |
| empty string | "" | {"idx":0,"captures":[],"match":""} |
| boolean | true | Error — class java.lang.Boolean cannot be cast to class java.lang.String (java.lang.Boolean and java.lang.String are in module java.base of loader 'bootstrap') |
| false | false | Error — class java.lang.Boolean cannot be cast to class java.lang.String (java.lang.Boolean and java.lang.String are in module java.base of loader 'bootstrap') |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — class java.util.Date cannot be cast to class java.lang.String (java.util.Date and java.lang.String are in module java.base of loader 'bootstrap') |
| decimal128 | {"$numberDecimal":"1.25"} | Error — class org.bson.types.Decimal128 cannot be cast to class java.lang.String (org.bson.types.Decimal128 is in unnamed module of loader 'app'; java.lang.String is in module java.base of |
| empty array | [] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| numeric array | [1,2,3] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| mixed array | [1,null,"x",3] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| nested array | [[1,2],[3]] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| empty object | {} | Error — class org.bson.Document cannot be cast to class java.lang.String (org.bson.Document is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstr |
| object | {"x":1} | Error — class org.bson.Document cannot be cast to class java.lang.String (org.bson.Document is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstr |
| ISO date string | "2026-01-15T15:00:00Z" | null |
| omitted named argument | Argument removed | Error — Cannot invoke "String.isEmpty()" because "this.pattern" is null |
Argument: options
| Input case | Supplied value | SrotaX result |
|---|---|---|
| null | null | Error — Cannot invoke "String.matches(String)" because "options" is null |
| missing | Field absent | Error — Cannot invoke "String.matches(String)" because "options" is null |
| zero | 0 | Error — class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap') |
| negative | -2 | Error — class java.lang.Integer cannot be cast to class java.lang.String (java.lang.Integer and java.lang.String are in module java.base of loader 'bootstrap') |
| decimal | 2.5 | Error — class java.lang.Double cannot be cast to class java.lang.String (java.lang.Double and java.lang.String are in module java.base of loader 'bootstrap') |
| numeric string | "12" | Error — Unsupported regex option |
| text | "hello" | Error — Unsupported regex option |
| empty string | "" | {"idx":0,"captures":[],"match":"ab"} |
| boolean | true | Error — class java.lang.Boolean cannot be cast to class java.lang.String (java.lang.Boolean and java.lang.String are in module java.base of loader 'bootstrap') |
| false | false | Error — class java.lang.Boolean cannot be cast to class java.lang.String (java.lang.Boolean and java.lang.String are in module java.base of loader 'bootstrap') |
| typed date | {"$date":"2026-01-15T15:00:00Z"} | Error — class java.util.Date cannot be cast to class java.lang.String (java.util.Date and java.lang.String are in module java.base of loader 'bootstrap') |
| decimal128 | {"$numberDecimal":"1.25"} | Error — class org.bson.types.Decimal128 cannot be cast to class java.lang.String (org.bson.types.Decimal128 is in unnamed module of loader 'app'; java.lang.String is in module java.base of |
| empty array | [] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| numeric array | [1,2,3] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| mixed array | [1,null,"x",3] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| nested array | [[1,2],[3]] | Error — class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap') |
| empty object | {} | Error — class org.bson.Document cannot be cast to class java.lang.String (org.bson.Document is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstr |
| object | {"x":1} | Error — class org.bson.Document cannot be cast to class java.lang.String (org.bson.Document is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstr |
| ISO date string | "2026-01-15T15:00:00Z" | Error — Unsupported regex option |
| omitted named argument | Argument removed | {"idx":0,"captures":[],"match":"ab"} |
Verification
This page records 63 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: 7 exact BSON matches, 46 both rejected, 6 value/type/acceptance differences, 4 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 |
|---|---|---|
| all / baseline | {"idx":0,"captures":[],"match":"ab"} | {"match":"ab","idx":0,"captures":[]} |
| input / null | Error — Cannot invoke "java.lang.CharSequence.length()" because "this.text" is null | null |
| input / missing | Error — Cannot invoke "java.lang.CharSequence.length()" because "this.text" is null | null |
| regex / null | Error — Cannot invoke "String.isEmpty()" because "this.pattern" is null | null |
| regex / missing | Error — Cannot invoke "String.isEmpty()" because "this.pattern" is null | null |
| regex / empty string | {"idx":0,"captures":[],"match":""} | {"match":"","idx":0,"captures":[]} |
| options / null | Error — Cannot invoke "String.matches(String)" because "options" is null | {"match":"ab","idx":0,"captures":[]} |
| options / missing | Error — Cannot invoke "String.matches(String)" because "options" is null | {"match":"ab","idx":0,"captures":[]} |
| options / empty string | {"idx":0,"captures":[],"match":"ab"} | {"match":"ab","idx":0,"captures":[]} |
| options / omitted named argument | {"idx":0,"captures":[],"match":"ab"} | {"match":"ab","idx":0,"captures":[]} |