Json parsing with iRules

Everyone should have written a JSON parser once in his life, here is mine.

JSON is now the format of choice for most APIs. It's time we were able to parse JSON with F5 iRules too, as simple string matching is not always good enough. That's why I wrote a simple JSON parser for iRules. It is a validating single pass parser that processes the JSON string char by char until the JsonPath expression matches, no recursion or any other fancy stuff. As I do not wanted to reinvent the wheel, it is basically a rewrite of the JSON parser found in the mongoose webserver project in plain TCL.

The usage is very simple:

set token [call json::json_get_tok { $json $path }]
  • $json is the json string to parse
  • $path is a JsonPath expression, following operators are implemented:
Operator Description
$ The root element to query. This starts all path expressions.
.<name> Dot-notated child.
[<number>] Array index.



Simple JSON:

    "aud": "audience \"test\"",
    "iss": "https://issuer.de/issuer/",
    "iat": 1701422123,
    "roles": [
    "obj": {
        "sub": "adcad2b8",
    "ver": "2.0"

JsonPath expression to parse this simple JSON:

JsonPath Return value

"audience \"test\""








To decode the extracted JSON string:

set decoded [call json::json_decode_str { $token }]

This removes the enclosing quotes from a string and decodes JSON escapes.


You can find the code and further documentation in my GitHub repository: https://github.com/JuergenMang/f5-irules-json

Published Apr 24, 2024
Version 1.0

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