> ## Documentation Index > Fetch the complete documentation index at: https://docs.fingerprint.com/llms.txt > Use this file to discover all available pages before exploring further. # Expression language reference > Rule expressions are written using a simple expression language. It provides a rich set of operators and builtin functions, and allows you to create flexible conditions and criteria. You can use available data fields with available operators and functions. ## Data fields | Key | Type | Notes | | -------------------------------- | -------------- | ------------------------------------------------------------------------------------------ | | `http.user_agent.is_ai_bot` | `bool` | Whether the UserAgent string matches the criteria for AI Bots. | | `http.user_agent.is_crawler_bot` | `bool` | Whether the UserAgent string matches the criteria for Crawler Bots. | | `http.user_agent.string` | `string` | The full UserAgent string. | | `http.request.headers` | `http_headers` | The map of HTTP request headers. | | `http.request.ip` | `ip` | IP address of the client making the request. | | `sdk.platform.name` | `string` | Name of the SDK platform (i.e. one of `js`, `android`, `ios`, `unknown`). | | `sdk.version` | `string` | Version of the SDK platform. Example value: `1.2.3`. | | `sdk.platform.app_package_name` | `string` | Name of the mobile app bundle, if the request came from a mobile SDK (`android` or `ios`). | | `sdk.platform.is_android` | `bool` | Flag indicating if request came from Android device. | | `sdk.platform.is_browser` | `bool` | Flag indicating if request came from a web browser. | | `sdk.platform.is_ios` | `bool` | Flag indicating if request came from iOS device. | | `sdk.platform.is_unknown` | `bool` | Flag indicating if request came from an unknown platform. | ## Operators `LHS` stands for left-hand side of the operation, `RHS` stands for right-hand side of the operation. | Type | Available operators | | ------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **Equality** | `LHS == RHS` - returns `true` if both sides are equal.`LHS != RHS` - returns `true` if both sides are different. | | **Numerical** | `LHS < RHS` - returns `true` if `LHS` is less than `RHS``LHS > RHS` - returns `true` if `LHS` is more than `RHS``LHS <= RHS` - returns `true` if `LHS` is less than or equal to `RHS``LHS >= RHS` - returns `true` if `LHS` is more than or equal to `RHS`*Requires that `LHS` and `RHS` are numerical values*. | | **Logical** | `!LHS` - returns the opposite boolean value of `LHS`.`LHS && RHS` - returns `true` if both `LHS` and `RHS` are `true``LHS \|\| RHS` - returns `true` if either `LHS` or `RHS` are `true`*Requires that `LHS` and `RHS` are boolean values*. | | **String** | `LHS contains RHS` - returns `true` if `RHS` is a contained in `LHS`.`LHS matches RHS` - returns `true` if `LHS` matches the regular expression in the `RHS`.*Requires that `LHS` and `RHS` are strings. For `matches`, `RHS` needs to be a [regular expression](https://github.com/google/re2/wiki/Syntax), and regex pattern should be enclosed with backticks (`` ` ``).* | | **Array** | `LHS[RHS]` - returns the value at index `RHS` in the array `LHS`. Returns `nil` for missing values.*Requires that `LHS` is an array, and `RHS` is a number*.`LHS in RHS` - returns `true` whether `LHS` is in (or belongs to) `RHS`:`LHS not in RHS` - returns the opposite of `LHS in RHS`.*Requires that `RHS` is an array, and `LHS` can be any type*. | | **Object** | `LHS[RHS]` - returns the value at index `RHS` in the object `LHS`. Returns `nil` for missing values.\`*Requires that `LHS` is an array, and `RHS` is a string*.`LHS in RHS` - returns `true` whether `LHS` is in (or belongs to)`RHS`:`LHS not in RHS` - returns the opposite of `LHS in RHS`.*Requires that `RHS` is an array, and `LHS` can be any type*.`LHS.RHS` is equivalent to `LHS[RHS]` where `LHS` is an `object`, and `RHS` is an unquoted `string` literal that can be used as an identifier.\*Examples:\*1) `http.request` is equivalent to `http["request"]`. | 2. `http.request.headers.origin.0` produces an error because the unquoted `string` literal `0` cannot be used as an identifier. | ## Functions and custom data types Describes additional functions and special data types available during rule evaluation. | Type | Available functions | | ----------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | **String** | `upper(string) -> string` - transforms the string to all uppercase characters`lower(string) -> string` - - transforms the string to all lowercase characters`starts_with(source string, substring string) -> bool``ends_with(source string, substring string) -> bool` | | **Miscellaneous** | `len(string\|array\|map\|http_headers) -> int `returns length of the given value | | **HTTP Headers** | The *Custom Type* `http_headers` behaves similar to a `map`, but it’s keys are normalized to be in MIME Header format in all operations (so usage is case-insensitive), and its values are of type `array` and contain the values for the given header name, preserving the original order.Examples: Check if a header was set: `"cOnTeNt-TyPe" in http.request.headers` Get the first cookie that was set: `http.request.headers["accept"][0]` | | **IP address and CIDR** | The *Custom Type* `ip` represents an IPv4 or IPv6 address. The *Custom Type* `cidr` represents an IPv4 or IPv6 CIDR.`cidr(string) cidr` parses an IPv4 or IPv6 CIDR.The `in` operator can be used to check if an `ip` is in a given `cidr`.Example: `http.request.ip in cidr("1.1.1.1/10")` | | **Semantic Versioning** | `semver_compare(s1, s2) -> int` - compares two semver strings, returns `-1` if `s2 < s1`, `1` if `s2 > s1`, or `0` if they are equal`semver_is_valid(string) -> bool` - checks if given string is a valid semver string | *** [(Beta) Request filtering V2](/docs/request-filtering-v2)