How can I take a substring from the path to use it elsewhere?

It’s been forever since I edited the HAProxy file. I had been using a GUI for it and I forgot a lot of stuff.

I’m trying to use a section of the path in a request to compose the subdomain string and use it to send it in the Host header.

I though about two 1½ ways of doing it, one is using {I forgot the name just now}, but what I mean is that sort of regex string manipulation shown in the line with set-header in the picture above.

The other is more or less the same, but taking from var(txn.txnpath)^[1] what’s needed, the second (\2) capture group from ^(\/)([^/]*)(.*)$^[2] set that into another variable then call it again from the set-header line, hopefully. I figured that this way although it’s an extra step, I’d also have an extra variable available for use in some other manipulation, right?? IDK it’s probably nonsense.

Of course I could just assemble the strings myself because — yes — I am aware that since what I’m matching is exactly what I am attempting to rewrite, I could just directly write the rule and be done with it since I know the values needed, and wouldn’t likely be creating something close to a wildcard-matching security compromise. But then I wouldn’t learn anything, and as it is I’m breaking the one rule I have which is not to write code (i.e. copy and paste) that I don’t understand. I never thought about what would or should I do if I stopped understanding it and it’s driving me a little insane.

I’m open to any advice/suggestions even if unrelated. Thanks!

1. which BTW I don’t completely remember how are these set; I rescued my config file from the last time I used HAProxy without a GUI hence the existing code syntax. I wasn’t an expert but I felt very natural by the end of it, even the layout looked like more like stats than syntax. ↩︎

2. in other words: whatever’s between the first or leftmost set of /s in the path where the rightmost / is optional ↩︎

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