AWS Stepfunction: Substring from input - substring

Question

As a general question is it possible do a substring function within step functions?
I receive the following event:
{
"input": {
"version": "0",
"id": "d9c5fec0-d08d-6abd-4ea5-0107fbbce47d",
"detail-type": "EBS Multi-Volume Snapshots Completion Status",
"source": "aws.ec2",
"account": "12345678",
"time": "2021-11-12T12:08:16Z",
"region": "us-east-1",
"resources": [
"arn:aws:ec2::us-east-1:snapshot/snap-0a98c2a42ee266123"
]
}
}
but need the snapshot id as input to DescribeInstances, therefore I need to extract snap-0a98c2a42ee266123 from arn:aws:ec2::us-east-1:snapshot/snap-0a98c2a42ee266123
Is there any simple way to do this within step functions?. That is to say without having to pass it to a lambda or something equally convoluted?

Answer 1

This has recently become possible with the addition of new intrinsic functions. ArrayGetItem gets an item by its index. StringSplit splits a string at a delimeter. Use a Pass state to extract the snapshot name from the resource ARN:
{
"StartAt": "ExtractSnapshotName",
"States": {
"ExtractSnapshotName": {
"Type": "Pass",
"Parameters": {
"input.$": "$.input",
"snapshotName.$": "States.ArrayGetItem(States.StringSplit(States.ArrayGetItem($.input.resources,0), '/'),1)"
},
"ResultPath": "$",
"End": true
}
}
}
output:
{
"input": {
"version": "0",
"id": "d9c5fec0-d08d-6abd-4ea5-0107fbbce47d",
"detail-type": "EBS Multi-Volume Snapshots Completion Status",
"source": "aws.ec2",
"account": "12345678",
"time": "2021-11-12T12:08:16Z",
"region": "us-east-1",
"resources": [
"arn:aws:ec2::us-east-1:snapshot/snap-0a98c2a42ee266123"
]
},
"snapshotName": "snap-0a98c2a42ee266123"
}