There are three types of expressions supported:

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JSON Path Expressions

When the expression starts with $, it is treated as a JSON Path expression and will be evaluated as such.

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JSON Path Syntax

JSON PathDescription
$The root object
.Child operator
@The current object
*Wildcard. All elements in an array, or all properties of an object
..Recursive descent
[]Subscript operator
[,]Union operator
[start : end : step]Array slice operator
?(expression)Filter expression
()Script expression
Examples: 
// Given the context: { user: { name: "John", age: 30 }, "info/email": "info@email.com" }
'$.user.name'         // Returns "John"
'$.user.age'          // Returns 30
'$.user[*]'           // Returns ["John", 30]
'$["info/email"]'     // Returns "info@email.com"
For more information on JSON Path syntax, refer to the original JSON Path documentation.

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JEXL Expressions

This kind of expression is enclosed in double brackets {{expression}}. It supports variables and operators.

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Operators

OperatorDescription
!Logical NOT
+Addition, string concatenation
-Subtraction
*Multiplication
/Division
//Floor division
%Modulus
^Exponentiation
&&Logical AND
|Logical OR
==Equal
!=Not equal
>Greater than
>=Greater than or equal
<Less than
<=Less than or equal
inElement of string or array
? :Ternary operator
Examples: 
// Given the context: { x: 10, y: 5 }
'{{x + y}}'                  // Returns 15
'{{x * 2}}'                  // Returns 20
'{{x > y}}'                  // Returns true
'{{x == 10 ? "yes" : "no"}}' // Returns "yes"
'{{x in [1, 2, 3]}}'         // Returns false
'{{x != y}}'                 // Returns true

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Identifiers

Identifiers can be used to reference variables in the context. 
// Given the context:
// { 
//     name: {
//         first: "John",
//         last: "Smith"
//     },
//     jobs: ["Developer", "Designer"]
// }
`{{name.first}}`          // Returns "John"
`{{jobs[1]}}`             // Returns "Designer"

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Collections

Collections, or arrays of objects, can be filtered by including a filter expression in brackets. 
// Given the context:
// {
//     users: [
//         { name: "John", age: 30 },
//         { name: "Jane", age: 25 }
//     ]
// }
`{{users[.name == "John"].age}}` // Returns 30
`{{users[.age > 25].name}}`      // Returns ["John"]

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String Interpolation

To simplify strings usage, a more straightforward syntax is provided for string interpolation of variables using the ${var} syntax. Examples: 
// Given the context: { name: "John", age: 30 }
"Hello ${name}"       // Returns "Hello John"
"User is ${age}"    // Returns "User is 30"
Note: If the expression is a string without any of the patterns described above, it will be returned as is. 
// Given the context: { name: "John", age: 30 }
"Hello world"       // Returns "Hello world"
For more information on the JEXL syntax, refer to the JEXL Syntax documentation.