How to extract a substring from the msg object

Published: | by Julian Knight Reading time ~3 min.
📖 Kb | 📎 Development | 🔖 Node-RED

Node-RED passes a msg object between node instances in a flow. It is common to need to extract a sub-string and there are multiple ways to achieve this. Here are some simeple examples.

The two main methods to use are either a “Change” node or a “Function” node.

The examples below simply extract the first 4 characters of the msg.payload property to illustrate the possibilities.

Change Node Examples 🔗︎

This is the simplest and generally the preferred approach unless you need to do a large number of changes to a msg.

Regular Expression Method 🔗︎

Extracting a sub-string is easily done using a regular expression which is one of the methods available to a “Change Rule” in the Change node.

  • Change Rule Type: Change, msg.payload
  • Search For: ^(.{4}).*$
  • Replace With: $1

This regex matches the whole msg.payload string. The () brackets in a regular expression allow the wrapped content specification to be captured to an internal variable which we use in the replace section as $1

[{"id":"4a1695b4.85f22c","type":"inject","z":"20e74d1e.f19692","name":"","topic":"","payload":"21.300000<xml><exec>/tclrega.exe</exec><sessionId></sessionId><httpUserAgent></httpUserAgent><d>BidCos-RF.MEQ0800939:4.ACTUAL_TEMPERATURE</d></xml>","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":270,"y":100,"wires":[["235a041.55c49fc"]]},{"id":"cbbf5f80.6b42a","type":"debug","z":"20e74d1e.f19692","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":630,"y":100,"wires":[]},{"id":"235a041.55c49fc","type":"change","z":"20e74d1e.f19692","name":"","rules":[{"t":"change","p":"payload","pt":"msg","from":"^(.{4}).*$","fromt":"re","to":"$1","tot":"str"}],"action":"","property":"","from":"","to":"","reg":false,"x":440,"y":100,"wires":[["cbbf5f80.6b42a"]]}]

JSONata Methods 🔗︎

There are several ways to use JSONata to extract a sub-string, here I illustrate a couple

$match (regular expression) 🔗︎

  • Change Rule Type: Set, msg.payload
  • To: $match(payload, /^.{4}/)
[{"id":"61aeec2b.95a4f4","type":"change","z":"20e74d1e.f19692","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$match(payload, /^.{4}/)\t","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":420,"y":160,"wires":[["9e768e8.0e2e67"]]},{"id":"ae3d0976.e0a4a8","type":"inject","z":"20e74d1e.f19692","name":"","topic":"","payload":"21.300000<xml><exec>/tclrega.exe</exec><sessionId></sessionId><httpUserAgent></httpUserAgent><d>BidCos-RF.MEQ0800939:4.ACTUAL_TEMPERATURE</d></xml>","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":270,"y":160,"wires":[["61aeec2b.95a4f4"]]},{"id":"9e768e8.0e2e67","type":"debug","z":"20e74d1e.f19692","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":630,"y":160,"wires":[]}]

$substring 🔗︎

  • Change Rule Type: Set, msg.payload
  • To: $substring(payload, 0, 4)
[{"id":"f550ddc8.1a56f","type":"change","z":"20e74d1e.f19692","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$substring(payload, 0, 4)\t","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":420,"y":220,"wires":[["835a813f.698f1"]]},{"id":"1bdcf392.de1a5c","type":"inject","z":"20e74d1e.f19692","name":"","topic":"","payload":"21.300000<xml><exec>/tclrega.exe</exec><sessionId></sessionId><httpUserAgent></httpUserAgent><d>BidCos-RF.MEQ0800939:4.ACTUAL_TEMPERATURE</d></xml>","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":270,"y":220,"wires":[["f550ddc8.1a56f"]]},{"id":"835a813f.698f1","type":"debug","z":"20e74d1e.f19692","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":630,"y":220,"wires":[]}]

Function Node Example 🔗︎

The Function node allows you to use JavaScript to process the msg object. You may wish to use this method if you are familiar with JavaScript and have a fairly complex processing requirement or an existing set of code.

/***
 * This demonstrates a number of ways to extract
 * a sub-string from the payload.
 *
 * Note that you don't really need an intermediate
 * variable but it does make the code easier to read.
 *
 * Also note that I've used `const` which was
 * introduced to JavaScript in ES2015 (AKA ES6).
 ***/

// slice is the easiest method to extract a sub-string
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/slice
let newStr = msg.payload.slice(0, 4);

// ---- Other possible methods ---- {

// Regular Expressions: str.match, str.exec or str.replace
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/match
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace

// str.substr - older method than slice
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr
//let newStr = msg.payload.substr(0,4)
// str.substring - older method than slice
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr
//let newStr = msg.payload.substring(0,3)
//
// The arguments of substring() represent the starting and ending indexes, while the arguments of
// substr() represent the starting index and the length of characters to include in the returned string.

// -------------------------------- }

msg.payload = newStr;

return msg;

[{"id":"da10e728.ae30d8","type":"inject","z":"20e74d1e.f19692","name":"","topic":"","payload":"21.300000<xml><exec>/tclrega.exe</exec><sessionId></sessionId><httpUserAgent></httpUserAgent><d>BidCos-RF.MEQ0800939:4.ACTUAL_TEMPERATURE</d></xml>","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":270,"y":280,"wires":[["7b9217ad.a29e58"]]},{"id":"d49e42b1.f141d","type":"debug","z":"20e74d1e.f19692","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":630,"y":280,"wires":[]},{"id":"7b9217ad.a29e58","type":"function","z":"20e74d1e.f19692","name":"","func":"/***\n * This demonstrates a number of ways to extract\n * a sub-string from the payload.\n * \n * Note that you don't really need an intermediate\n * variable but it does make the code easier to read.\n * \n * Also note that I've used `const` which was \n * introduced to JavaScript in ES2015 (AKA ES6).\n ***/\n\n// slice is the easiest method to extract a sub-string\n// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/slice\nlet newStr = msg.payload.slice(0, 4);\n\n// ---- Other possible methods ---- {\n\n// Regular Expressions: str.match, str.exec or str.replace\n// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/match\n// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec\n// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace\n\n// str.substr - older method than slice\n// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr\n//let newStr = msg.payload.substr(0,4)\n// str.substring - older method than slice\n// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr\n//let newStr = msg.payload.substring(0,3)\n//\n// The arguments of substring() represent the starting and ending indexes, while the arguments of \n// substr() represent the starting index and the length of characters to include in the returned string.\n\n// -------------------------------- }\n\nmsg.payload = newStr;\n\nreturn msg;","outputs":1,"noerr":0,"x":390,"y":280,"wires":[["d49e42b1.f141d"]]}]
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