gkanapathy is right about your problem is. I just had a few thoughts that wouldn't fit in a comment.
Just so you know, you can use named capture groups, like (?<emsinstance>\w+) in your transforms.conf file just like you do using rex . When your using named groups, you don't need the FORMAT line anymore. I found that this approach makes it quicker to jump between interactive testing with rex and then copying the regex into my transforms.conf file.
In this case, you also have the option of using an EXTRACT entry in your props.conf file. I rather prefer this approach, since you can setup everything in one place, and you don't to come up with some arbitrary name transformer name. 😉
Here's an example in props.conf :
[source::/app/ems/envs/uat/*/logs/trace.log]
EXTRACT-fields = [/\\]uat[/\\](?[\w_]+)[/\\]logs[/\\] in source
Notes: I use [/\\] because it will match both unix-style and dos-style paths (if that's a concern for you and you can use that in your source config entry too, although it looks ugly: [source::[/\\]app[/\\]ems[/\\]envs[/\\]uat[/\\]*[/\\]logs[/\\]trace.log] ). You may also note that I'm not matching the whole path in the regex, which is because in this case, you really don't need to, your [source::...] stanza is doing that work for you already.
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